Chapter:3 Diode and Transistors

INTRODUCTION - IDEAL DIODE

During forward bias, an Ideal Diode acts like a perfect conductor, while when
reverse-biased, it acts like an ideal insulator. The ideal diode properties are resistance,
threshold voltage, breakdown voltage, and current magnitude.

In this article, we will look into the characteristics curve of an ideal diode and see
its behavior for forward and reverse biased conditions. We will also discuss the
properties an ideal diode offers in its different modes of biasing and to conclude our
discussion, we will look into the difference between an ideal and a conventional diode.

WHAT IS AN IDEAL DIODE?

As the name suggests, an ideal diode is a diode that has all of its properties
perfectly without any flaws. Diodes may operate either forwardly or reversely biased.
Thus, these two modes of operation can be analysed separately to determine the
characteristics of the ideal diode.

MODES OF OPERATION OF IDEAL DIODE

The two modes of operation of the ideal diode are

• Forward Bias
• Reverse Bias
Ideal Diode Circuit Symbol

The circuit symbol of an ideal diode is the

simple representation of a diode by a
triangle device. This symbol becomes a
short or open circuit when forward and
reverse-biased, respectively.
Figure 3.1: Circuit symbol of an ideal diode

In forward bias, the current flows from p to n side, and in reverse bias, there is
supposed to be a small current from n to p, but since we are dealing with an ideal diode,
the reverse current would be zero.
 The ideal diode conducts forward current when a forward voltage is applied
across the anode to the cathode. In contrast, it does not conduct reverse current when a
reverse voltage is applied across the anode to the cathode.

EFFECT OF TEMPERATURE ON DIODE CHARACTERISTICS

We have already discussed that, the current that a PN junction diode can
conduct at a given voltage is dependent upon the operating temperature. An increased
temperature will result in a large number of broken covalent bonds increasing the large
number of majority and minority carriers. This amounts to a diode current larger than its
previous diode current. The above phenomenon applies both to forward and reverse
current.

Figure 3.2: Effect of temperature on diode characteristics

The effect of increased temperature on the characteristics curve of a PN junction

diode is as shown in above figure. It may be noted that the forward characteristics shifts
upwards with increase in temperature. On the other hand, the reverse characteristics
shifts downwards with the increase in temperature.

WHAT IS A SEMICONDUCTOR DIODE?

A semiconductor diode is a p-n junction diode. It is a two-terminal device that

conducts current only in one direction. The figure below represents the symbol for the p-
n junction diode, which symbolizes the direction of the current. By applying an external
voltage V we can vary the potential barrier.

Semiconductor Diode Symbol

Figure 3.3: Semiconductor Diode Symbol

A p-n junction is denoted by the symbol shown in the figure above. Here, the
direction of the arrow indicates the permissible direction of the current.

UNBIASED DIODE:

Figure 3.4: Unbiased diode

When an electron diffuses from the n-side to the p-side, an ionised donor is left
behind on the n-side, which is immobile. As the process goes on, a layer of positive
charge is developed on the n-side of the junction.

Similarly, when a hole goes from the p-side to the n-side, an ionized acceptor is
left behind on the p-side, resulting in the formation of a layer of negative charges in the
p-side of the junction. This region of positive charge and negative charge on either side
of the junction is termed as the depletion region.
 Due to this positive space charge region on either side of the junction, an electric
field with the direction from a positive charge towards the negative charge is developed.
Due to this electric field, an electron on the p-side of the junction moves to the n-side of
the junction. This motion is termed the drift. Here, we see that the direction of the drift
current is opposite to that of the diffusion current.

An unbiased condition of a diode is when there is no external energy source.

An unbiased diode sets the electric field across the depletion layer between the
n-type and the p-type material.
This is caused due to the imbalance in free electrons due to doping.
This barrier potential is approximately 0.7V for a silicon diode at room
temperature.
In unbiased conditions, the p-side is positive, and the n- side is negative. So, an
unbiased diode is a diode that is not connected to a battery or is not connected to
any voltage source.
BIASING CONDITIONS FOR THE P-N JUNCTION DIODE

There are three biasing conditions for the P-N junction diode, and this is based on
the voltage applied:

Zero bias: No external voltage is applied to the P-N junction diode.

Forward bias: The positive terminal of the voltage potential is connected to the p-
type while the negative terminal is connected to the n-type.
Reverse bias: The negative terminal of the voltage potential is connected to the
p-type and the positive is connected to the n-type.

P-N JUNCTION DIODE UNDER FORWARD BIAS

Figure 3.5: P-N junction diode under forward bias

 When we apply the external voltage across the semiconductor diode in such a
way that the p-side is connected to the positive terminal of the battery and the n-side is
connected to the negative terminal, then the semiconductor diode is said to be forward-
biased. In this case, the built-in potential of the diode and thus the width of the depletion
region decreases, and the height of the barrier gets reduced. The overall barrier
voltage, in this case, comes out to be V0-V, which is the difference between the built-in
potential and the applied potential.

As we supply a small amount of voltage, the reduction in the barrier voltage from
the above-given formula is very less and thus only a small number of current carriers
cross the junction in this case. Whereas, if the potential is increased by a significant
value, the reduction in the barrier height will be more, thus allowing the passage of more
number of carriers.

P-N JUNCTION DIODE UNDER REVERSE BIAS

Reverse-biased-PN-junction-diode

Figure 3.6: P-N Junction diode under Reverse Bias

When we apply the external voltage across the semiconductor diode in such a
way that the positive terminal of the battery is connected to its n-side and the negative
terminal of the battery is connected to the p-side of the diode, then it is said to be in the
condition of reverse bias. When an external voltage is applied across the diode, as the
direction of the external voltage is the same as that of the barrier potential, the total
voltage barrier sums up to be (V0+V). Also, the width of the depletion region increases.
As a result of this, the motion of carriers from one side of the junction to another
decreases significantly.
Semiconductor Diode Characteristics

Figure 3.7: Semiconductor Diode Characteristics

SURGE CURRENT
WHAT IS SURGE CURRENT IN DIODE?

Surge current in diode is the maximum allowable value of the current that the
diode can conduct in the forward bias condition. The diode gets damaged when the
diode’s forward current is more than the surge current rating of the diode. Therefore, the
diode’s surge current rating is desired to be more than the diode’s inrush current. The
surge current rating is infinite for an ideal diode.

The surge current in the diode is influenced by various factors as follows:

The surge current capability of the diode is highly influenced by the metallization
layer and the bond foot arrangement. The surge current capability of the diode is
more when the metallization layer is thicker and it is less for the diode that has
thinner metallization layers.
The location and size of the contact area of the bond wires influence the surge
current capability of the diode. Diode has better surge current capability when it
has high bond foot area.
 The typical range of surge current rating of the diode is 10 to 12 times of its rated
current.
DIODE AS A SWITCH

Working of Diode as a Switch

Whenever a specified voltage is exceeded, the diode resistance gets

increased, making the diode reverse biased and it acts as an open switch. Whenever
the voltage applied is below the reference voltage, the diode resistance gets
decreased, making the diode forward biased, and it acts as a closed switch.

The following circuit explains the diode acting as a switch.

Figure 3.8: Switching circuit using diode

A switching diode has a PN junction in which P-region is lightly doped and N-

region is heavily doped. The above circuit symbolizes that the diode gets ON when
positive voltage forward biases the diode and it gets OFF when negative voltage
reverse biases the diode.

RECTIFIER
The main application of p-n junction diode is in rectification circuits. These
circuits are used to describe the conversion of a.c signals to d.c in power supplies.
Diode rectifier gives an alternating voltage which pulsates in accordance with time. The
filter smoothes the pulsation in the voltage and to produce d.c voltage, a regulator is
used which removes the ripples.

There are two primary methods of diode rectification:

HALF WAVE RECTIFIER
In a half-wave rectifier, one half of each a.c input cycle is rectified. When the p-n
junction diode is forward biased, it gives little resistance and when it is reversed biased
it provides high resistance. During one-half cycles, the diode is forward biased when the
input voltage is applied and in the opposite half cycle, it is reverse biased. During
alternate half-cycles, the optimum result can be obtained.

Working of Half Wave Rectifier

The half-wave rectifier has both positive and negative cycles. During the positive
half of the input, the current will flow from positive to negative which will generate only a
positive half cycle of the a.c supply. When a.c supply is applied to the transformer, the
voltage will be decreasing at the secondary winding of the diode. All the variations in the
a.c supply will reduce, and we will get the pulsating d.c voltage to the load resistor.

Figure 3.9: Half wave rectifier

In the second half cycle, the current will flow from negative to positive and the
diode will be reverse biased. Thus, at the output side, there will be no current
generated, and we cannot get power at the load resistance. A small amount of reverse
current will flow during reverse bias due to minority carriers.

Characteristics of Half Wave Rectifier

Following are the characteristics of half-wave rectifier:

Ripple Factor

Ripples are the oscillations that are obtained in DC which are corrected by using
filters such as inductors and capacitors. These ripples are measured with the help of the
ripple factor and are denoted by γ. Ripple factor tells us the number of ripples presents
in the output DC. Higher the ripple factor, more is the oscillation at the output DC and
lower is the ripple factor, less is the oscillation at the output DC.

Ripple factor is the ratio of RMS value of the AC component of the output voltage
to the DC component of the output voltage.

DC Current

DC current is given as:

Where,

Imax is the maximum DC load current

DC Output Voltage

The output DC voltage appears at the load resistor RL which is obtained by

multiplying output DC voltage with the load resistor RL. The output DC voltage is given
as:

Where,

VSmax is the maximum secondary voltage

Form Factor

The form factor is the ratio of RMS value to the DC value. For a half-wave
rectifier, the form factor is 1.57.
Rectifier Efficiency

Rectifier efficiency is the ratio of output DC power to the input AC power. For a
half-wave rectifier, rectifier efficiency is 40.6%.

Advantages of Half Wave Rectifier

Affordable
Simple connections
Easy to use as the connections are simple
Number of components used are less
Disadvantages of Half Wave Rectifier

Ripple production is more

Harmonics are generated
Utilization of the transformer is very low
The efficiency of rectification is low
Applications of Half Wave Rectifier

Following are the uses of half-wave rectification:

Power rectification: Half wave rectifier is used along with a transformer for power
rectification as powering equipment.
Signal demodulation: Half wave rectifiers are used for demodulating the AM
signals.
Signal peak detector: Half wave rectifier is used for detecting the peak of the
incoming waveform.
FULL WAVE RECTIFIER

What Is Full Wave Rectifier?

Electric circuits that convert AC to DC are known as rectifiers. Rectifiers are

classified into two types as Half Wave Rectifiers and Full Wave Rectifiers. Significant
power is lost while using a half-wave rectifier and is not feasible for applications that
need a smooth and steady supply. For a more smooth and steady supply, we use the
full wave rectifiers. In this article, we will be looking into the working and characteristics
of a full wave rectifier.

A full wave rectifier is defined as a rectifier that converts the complete cycle of
alternating current into pulsating DC.
 Unlike halfwave rectifiers that utilize only the halfwave of the input AC cycle, full
wave rectifiers utilize the full cycle. The lower efficiency of the half wave rectifier can be
overcome by the full wave rectifier.

Full Wave Rectifier Circuit

The circuit of the full wave rectifier can be constructed in two ways. The first
method uses a centre tapped transformer and two diodes. This arrangement is known
as a centre tapped full wave rectifier. The second method uses a standard transformer
with four diodes arranged as a bridge. This is known as a bridge rectifier.

Figure 3.10: Centre tap full wave rectifier

The circuit of the full wave rectifier consists of a step-down transformer and two
diodes that are connected and centre tapped. The output voltage is obtained across the
connected load resistor.

Working of Full Wave Rectifier

The input AC supplied to the full wave rectifier is very high. The step-down
transformer in the rectifier circuit converts the high voltage AC into low voltage AC. The
anode of the centre tapped diodes is connected to the transformer’s secondary winding
and connected to the load resistor. During the positive half cycle of the alternating
current, the top half of the secondary winding becomes positive while the second half of
the secondary winding becomes negative.

During the positive half cycle, diode D1 is forward biased as it is connected to the
top of the secondary winding while diode D2 is reverse biased as it is connected to the
bottom of the secondary winding. Due to this, diode D1 will conduct acting as a short
circuit and D2 will not conduct acting as an open circuit

During the negative half cycle, the diode D1 is reverse biased and the diode D2
is forward biased because the top half of the secondary circuit becomes negative and
the bottom half of the circuit becomes positive. Thus in a full wave rectifiers, DC voltage
is obtained for both positive and negative half cycle.

Peak Inverse Voltage

Peak inverse voltage is the maximum voltage a diode can withstand in the
reverse-biased direction before breakdown. The peak inverse voltage of the full-wave
rectifier is double that of a half-wave rectifier. The PIV across D1 and D2 is 2Vmax.

DC Output Voltage

The following formula gives the average value of the DC output voltage:

or

DC Current

Currents from both the diodes D1 and D2 are in the same direction when they
flow towards load resistor RL. The current produced by both the diodes is the ratio of
Imax to π, therefore the DC current is given as:

Where, Imax is the maximum DC load current

RMS Value of Current

The RMS value of the current can be calculated using the following formula:
Ripple Factor

Ripple factor for a full-wave rectifier is given as:

Form Factor

The form factor is the ratio of RMS value of current to the output DC voltage. The
form factor of a full-wave rectifier is given as 1.11

The form factor of the full wave rectifier is calculated using the formula:

Peak Factor

The following formula gives the peak factor of the full wave rectifier:

Rectification Efficiency

The rectification efficiency of the full-wave rectifier can be obtained using the
following formula:

The efficiency of the full wave rectifiers is 81.2%.

Advantages of Full Wave Rectifier

The rectification efficiency of full wave rectifiers is double that of half wave
rectifiers. The efficiency of half wave rectifiers is 40.6% while the rectification
efficiency of full wave rectifiers is 81.2%.
The ripple factor in full wave rectifiers is low hence a simple filter is required. The
value of ripple factor in full wave rectifier is 0.482 while in half wave rectifier it is
about 1.21.
 The output voltage and the output power obtained in full wave rectifiers are
higher than that obtained using half wave rectifiers.
The only disadvantage of the full wave rectifier is that they need more circuit elements
than the half wave rectifier which makes, making it costlier.

Disadvantages of Full Wave Rectifier

Very expensive
Applications of Full Wave Rectifier

Following are the uses of full-wave rectifier:

Full-wave rectifiers are used for supplying polarized voltage in welding and for
this bridge rectifiers are used.
Full-wave rectifiers are used for detecting the amplitude of modulated radio
signals.

BRIDGE RECTIFIER TIFIER

Many electronic circuits require a rectified DC power supply to power various

electronic basic components from the available AC mains supply. Rectifiers are used to
convert an AC power to a DC power. Among the rectifiers, the bridge rectifier is the
most efficient rectifier circuit. We can define bridge rectifiers as a type of full-wave
rectifier that uses four or more diodes in a bridge circuit configuration to efficiently
convert alternating (AC) current to a direct (DC) current. In the next few sections, let us
learn more about its construction, working, and
more.

Construction

The construction of a bridge rectifier is

shown in the figure below. The bridge rectifier
circuit is made of four diodes D1, D2, D3, D4,
and a load resistor RL. The four diodes are
connected in a closed-loop configuration to
efficiently convert the alternating current (AC)
into Direct Current (DC). Figure 3.11: Bridge Rectifier
 The main advantage of this configuration is the absence of the expensive centre-
tapped transformer. Therefore, the size and cost are reduced. The input signal is
applied across the terminals A and B, and the output DC signal is obtained across the
load resistor RL connected between terminals C and D. The four diodes are arranged in
such a way that only two diodes conduct electricity during each half cycle. D1 and D3
are pairs that conduct electric current during the positive half cycle/. Likewise, diodes
D2 and D4 conduct electric current during a negative half cycle.

Working

When an AC signal is applied

across the bridge rectifier, terminal A
becomes positive during the positive
half cycle while terminal B becomes
negative. This results in diodes D1
and D3 becoming forward biased
while D2 and D4 becoming reverse
biased. The current flow during the
positive half-cycle is shown in the
figure 3.12.

Figure 3.12: Current flow during the positive half cycle

During the negative half-cycle, terminal B becomes positive while terminal A becomes
negative. This causes diodes D2 and D4 to become forward biased and diode D1 and
D3 to be reverse biased.The current flow during the negative half cycle is shown in the
figure below:

Figure 3.13: The current flow during the negative half cycle
 From the figures 3.12& 3.13 given above, we notice that the current flow across
load resistor RL is the same during the positive and negative half-cycles. The output DC
signal polarity may be either completely positive or negative. In our case, it is
completely positive. If the diodes’ direction is reversed, we get a complete negative DC
voltage.

Thus, a bridge rectifier allows electric current during both positive and negative
half cycles of the input AC signal.

The output waveforms of the bridge rectifier are shown in the below figure.

Figure 3.14: Output waveforms of the bridge rectifier

Characteristics of Bridge Rectifier

Ripple Factor

The smoothness of the output DC signal is measured by a factor known as the

ripple factor. The output DC signal with fewer ripples is considered a smooth DC
signal while the output with high ripples is considered a high pulsating DC signal.
Mathematically, the ripple factor is defined as the ratio of ripple voltage to pure
DC voltage.
The ripple factor for a bridge rectifier is given by

For bridge rectifiers, the ripple factor is 0.48.

Peak Inverse Voltage

The maximum voltage that a diode can withstand in the reverse bias condition is
known as a peak inverse voltage. During the positive half cycle, the diodes D1 and D3
are in the conducting state while D2 and D4 are in the non-conducting state. Similarly,
during the negative half cycle, diodes D2 and D4 are in the conducting state, and diodes
D1 and D3 are in the non-conducting state.

Efficiency

The rectifier efficiency determines how efficiently the rectifier converts Alternating
Current (AC) into Direct Current (DC). Rectifier efficiency is defined as the ratio of the
DC output power to the AC input power. The maximum efficiency of a bridge rectifier is
81.2%.

Advantages

The efficiency of the bridge rectifier is higher than the efficiency of a half-wave
rectifier. However, the rectifier efficiency of the bridge rectifier and the centre-
tapped full-wave rectifier is the same.

The DC output signal of the bridge rectifier is smoother than the output DC signal
of a half-wave rectifier.

In a half-wave rectifier, only half of the input AC signal is used, and the other half
is blocked. Half of the input signal is wasted in a half-wave rectifier. However, in
a bridge rectifier, the electric current is allowed during both positive and negative
half cycles of the input AC signal. Hence, the output DC signal is almost equal to
the input AC signal.
Disadvantages

The circuit of a bridge rectifier is complex when compared to a half-wave rectifier

and centre-tapped full-wave rectifier. Bridge rectifiers use 4 diodes while half-
wave rectifiers and centre-tapped full wave rectifiers use only two diodes.

When more diodes are used more power loss occurs. In a centre-tapped full-
wave rectifier, only one diode conducts during each half cycle. But in a bridge
rectifier, two diodes connected in series conduct during each half cycle. Hence,
the voltage drop is higher in a bridge rectifier.
 FILTER CIRCUIT
The ripple in the signal denotes the presence of some AC component. This ac
component has to be completely removed in order to get pure dc output. So, we need a
circuit that smoothens the rectified output into a pure dc signal.

A filter circuit is one which removes the ac component present in the rectified
output and allows the dc component to reach the load.

The following figure shows the functionality of a filter circuit.

Figure 3.15: Filter circuits

A filter circuit is constructed using two main components, inductor and capacitor.
We have already studied in Basic Electronics tutorial that

1) An inductor allows dc and blocks ac.

2) A capacitor allows ac and blocks dc.
Let us try to construct a few filters, using these two components.

CHOKE FILTER

Definition: Choke filter consists of an inductor connected in series with rectifier

output circuit and a capacitor connected in parallel with the load resistor. It is also called
L-section filter because the inductor and capacitor are connected in the shape of
inverted L. The output pulsating DC voltage from a rectifier circuit passes through the
inductor or choke coil.

The inductor has low DC resistance and extremely high AC reactance. Thus,
ripples get filtered through choke coil. Some of the residual ripples if present in filtered
signal from inductor coil will get bypassed through the capacitor. The reason behind this
is that capacitor allow AC and block DC.
Significance of Choke Filter or L-section filter

Choke filter came into existence due to shortcomings of the series inductor and
shunt capacitor filter. A series inductor filter filters the output current but reduces the
output current (RMS value and Peak value) up to a large extent. And the shunt
capacitor filter performs filtering efficiently but increases the diode current. The excess
of current in a diode may lead to its destruction.

Moreover, the ripple factor of series inductor filter is directly proportional to the
load resistance it means as the load resistance increases, ripple factor also starts
increasing. And in the case of shunt capacitor, the ripple factor is inversely proportional
to the value of load resistance. It implies that in shunt capacitor filter the ripple factor
decreases with increase in load resistance and increases with the decrease in load
resistance.

Thus, for better performance, we need a filter circuit in which ripple factor is low
and do not vary with the variation in load resistance. This can be achieved by using the
combination of series inductor filter and shunt capacitor filter. The voltage stabilization
property of shunt capacitor filter and current smoothing property of series inductor filter
is utilized for the formation of choke filter or L-section filter.

The combination of series inductor filter and shunt capacitor filter is generally
used for most of the applications. The combination results in two types, i.e. L-section
filter and Pi filter. In this article, we will discuss the working of L-section or choke filter
and in next article, we will discuss Pi filter in detail.

Working of Choke Filter or L-section filter

When the pulsating DC signal from the output of the rectifier circuit is feed into
choke filter, the AC ripples present in the output DC voltage gets filtered by choke coil.
The inductor has the property to block AC and pass DC. This is because DC resistance
of an inductor is low and AC impedance of inductor coil is high. Thus, the AC ripples get
blocked by inductor coil.

Although the inductor efficiently removes AC ripples, a small percentage of AC

ripples is still present in the filtered signal. These ripples are then removed by the
capacitor connected in parallel to the load resistor. Now, the DC output signal is free
from AC components, and this regulated DC can be used in any application.

If the inductor of high inductive reactance (XL), greater than the capacitive
reactance at ripple frequency is used than filtering efficiency gets improved.
 Figure 3.16: Choke input or L section filter

Waveform of Choke Filter or L-section Filter

The waveform of DC output signal with a filter and without filter is shown in the
below diagram.

Figure 3.17: Choke filter output voltage waveform

Advantages of Choke Filter or L-section Filter

It provides better voltage regulation.The ripple factor can be varied according to

the need.

Disadvantages of Choke Filter or L-Section filter

Bulky Size:

These kinds of filters were popular in ancient time but it has become obsolete
now due to bulky size of inductors and capacitors.

Not suitable for low voltage power Supplies:

These are not suitable for low voltage power supplies. IC regulators or active
filters are used in such devices.
 It is the combination of series inductor filter and shunt capacitor filter. The
advantages of both these filters are utilized to form Choke input filters. And the
disadvantages of both of these filters are removed in choke filter.

Capacitor input filter or Shunt Capacitor Filter

The Shunt capacitor filters comprise of capacitor along with the load resistor. In
this, the capacitor is connected in parallel with respect to the output of rectifier circuit
and also in parallel with the load resistor. During conduction, the capacitor starts
charging and stores energy in the form of the electrostatic field. The capacitor will
charge to its peak value because the charging time constant is almost zero.

Figure 3.18: Capacitor input filter

During non-conduction, the capacitor will discharge through the load resistor.
Thus, in this way, the capacitor will maintain constant output voltage and provide the
regulated output. The shunt capacitor filters use the property of capacitor which blocks
DC and provides low resistance to AC. Thus, AC ripples can bypass through the
capacitor.

Figure 3.19 : Output voltage waveform

If the value of capacitance of the capacitor is high, then it will offer very low
impedance to AC and extremely high impedance to DC. Thus, the AC ripples in the DC
output voltage gets bypassed through parallel capacitor circuit, and DC voltage is
obtained across the load resistor.

Advantages of capacitor-filters are:

1. Cheaper
2. smaller in size
3. readily available
Disadvantages of the filter-capacitor are:

1. It is sensitive to temperature change

2. Its capacitance reduces with time.

CLIPPER CIRCUITS

A clipper is a device which limits, remove or prevents some portion of the wave
form (input signal voltage) above or below a certain level In other words the circuit
which limits positive or negative amplitude, or both is called chipping circuit. The clipper
circuits are of the following types.

1. Series positive clipper

2. Series negative clipper
3. Shunt or parallel positive clipper
4. Shunt or parallel negative clipper
5. Dual (combination)Diode clipper

SERIES POSITIVE CLIPPER

In a series positive clipper, a diode is connected in series with the output, as

shown in Fig 3.20. During the positive half of the input voltage, the terminal A is positive
with respect to B. These reverse biases the diode and it acts as an open switch
Therefore all the applied voltage drops across the diode and none across the resistor
As a result of this there is no output voltage during the positive half cycle of the input
voltage.
 Figure 3.20: Series Positive Clipper

During the negative half cycle of the input voltage the terminal B is positive with
respect to A. Therefore it forward biases the diode and it acts as a closed switch. Thus
there is on voltage drop across diode during the negative half cycle of the input voltage.
All the input voltage is dropped across the resistor as shown in the output wave form.
Clippers prevent either or both polarities of a wave form exceeding a specific amplitude
level. However a positive Clipper is that which removes or clips the positive half
completely. Hence the circuit of the Fig 3.20 is called a positive Clipper Here it may he
noted the diode acts a series switch between the source and load. Due to this reason
the circuit is called series positive clipper.

SERIES-POSITIVE CLIPPER WITH BIAS

Sometimes it is desired to remove a Small portion of positive or apposite halt cycle of

the signal voltage (input signal). For this purpose a biased clipper is used Fig 3.21
shows the circuit of a biased series positive clipper.

Figure 3.21: Biased series positive clipper

 It may be observed that the clipping takes place during the positive cycle only
when the input voltage is greater thence battery voltage (i.e. Vi > VB). The chipping
level can be shifted up or down by varying the bias voltage (VB)

SERIES NEGATIVE CLIPPER

In a series negative clipper a diode is connected in a direction appositive to that

of a positive clipper Fig 3.20 shows the circuit of a negative clipper.

Figure 3.22: Series Negative Clipper

During the positive half cycle of the voltage, the terminal A is positive with
respect to the terminal B Therefore the diode is forward biased and it acts it as a closed
switch as a result, all the input voltage appears across the resistor as shown in Fig 3.22.
During the negative half cycle of the input voltage, the terminal B is positive with respect
to the terminal A. Therefore the diode is reverse biased and it acts as an open switch,
Thus there is no voltage drop across the resistor during the negative half cycle as
shown in the output waveform. It may be observed that if it is desired to remove or clip
the negative half-cycle of the input, the only thing is to be done is to reverse the
polarities of the diode in the circuit.

SERIES-NEGATIVE CLIPPER WITH BIAS

The Fig 3.23 shows the circuit of a biased series negative diver. In this circuit
clipping take place during the negative half cycle only when the input voltage Vi > VB
she clipping level can be shifted up or down by varying the bias voltage (-VB)
 Figure 3.23: Biased series negative clipper

SHUNT OR PARALLEL POSITIVE CLIPPER

A parallel clipper circuit uses the same diode theory and circuit operation a
resistor and diode are connected in series with the input signal and the output signal is
developed across the diode. The output is in parallel with the diode hence the circuit
name parallel clipper the parallel clipper can limit either the positive or negative
alternation of the input signal Fig.3.24 shows the circuit of a shunt positive clipper. In
this circuit, the diode acts as a closed switch when the input voltage is positive (i.e. Vi >
0 and as an open switch when the input voltage is negative (i.e. Vi< 0) the output
waveform is the same as that of a series positive clipper in the parallel clippers the alp
will develop when the diode is cut off.

Figure 3.24: Shunt or parallel positive clipper

SHUNT OR PARALLEL POSITIVE CLIPPER WITH BIAS

As is in Fig 3.25, positive terminal of the battery is connected to the cathode of

the diode. This causes the diode to be reversed biased at all times except when the
input signal is more positive the bias voltage(i e Vi > VB). it will be interesting to know
that if the polarity of the bias voltage is reversed , the resulting circuits will be as shown
in Fig 3.25(b) Here the input signal lying above the voltage —VB is clipped the
waveforms of the of the output voltage are also shown with figures

Figure 3.25 : Biased shunt /parallel positive clipper

SHUNT OR PARALLEL NEGATIVE CLIPPER

The negative clipper has allowed to pass the positive half cycle of the input
voltage and clipped the negative half cycle completely Fig 3.26 shows the shunt
(parallel) negative clipper.

Figure 3.26: Shunt or Parallel negative clipper

 In such a circuit the diode acts as a closed switch for a negative input voltage
(i.e. Vi < O) and as an open switch for a positive input voltage (i.e. Vi O) the output
waveform of the Circuit is the same as that of series negative clipper.

SHUNT OR PARALLEL NEGATIVE CLIPPER WITH BIAS

In such a circuit clipping take place during the negative half cycle only when the
input voltage (Vi < VB) the clipping level can be shifted up or down by varying the bias
voltage (—VB). It will be interesting to know that if the polarity of the bias voltage is
reversed, then the resulting circuits will be as shown in Fig 3.27 (b) Here the entire
signal below the voltage level VII has been clipped off .

Figure 3.27: Biased shunt /parallel negative clipper

DUAL (COMBINATION) DIODE CLIPPER

The type of clipper combines a parallel negative clipper with negative bias (D1
and B2) and a parallel positive bias (D1 and B1). Hence the combination of a biased
positive clipper and a biased negative clipper is called combination or dual diode
clipper. Such a clipper circuit can clip at both two in dependent levels depending upon
the bias voltages. Fig 3.28(a) show the circuit of a dual (combination) clipper.
 Figure 3.28: Dual diode clipper

Let us suppose a sinusoidal ac voltage is applied at the input terminals of the

circuit. Then during the positive half cycle, the diode D1 is forward biased, while diode
D2 is reverse. biased. Therefore the diode D1 will conduct and will acts as a short
circuit. On the other hand, diode D2 will acts as an open circuit. However, the value of
output voltage cannot exceed the voltage level of VB1 as Shown in Fig 3.28.

Similarly during the negative input half cycle the diode D2 acts as a short circuit
while the diode D1 as an open circuit However the value of output voltage cannot
exceed the voltage level of VB2 It may be noted that the clipping levels of the circuit be
varied by changing the values of VB1 and VB2 If the values of VB1 and VB2 are equal,
the circuit will clip both the positive and negative half cycles at the same voltage level.
Such a circuit is known as a symmetrical clipper

CLAMPING CIRCUITS

Certain applications in electronics require that the upper or lower extremity of a

wave be fixed at a specific value In such applications ,a clamping/clamper circuits are
used.

A circuit that places either the positive or negative peak of a signal at a desired
D.C level is known as a clamping circuit. A clamping circuit introduces (or restores) a
D.C level to an A.C signal. Thus a clamping circuit is also known as D.C restorer, or D.C
reinserted or a baseline stabilizer. The following are two general types of clamping.

Positive clamping occurs when negative peaks raised or clamped to ground or on

the zero level In other words, it pushes the signal upwards so that negative peaks fall on
the zero level.
 Negative clamping occurs when positive peaks raised or clamped to ground or
on the zero level In other words, it pushes the signal downwards so that the positive
peaks fall on the zero level.

In both cases the shape of the original signal has not changed, only there is
vertical shift in the signal Fig. 3.29 shows the clamping wave form

Figure 3.29: clamper circuits demonstration

DIODE CLAMPERS

POSITIVE CLAMPER

The Fig 3.30 shows the circuit of a positive clamper It consists of a diode and a
capacitor the clamper output is taken across the load resistance R.

Figure 3.30 : Positive clamper

POSITIVE CLAMPER WITH BIAS

Biased clamper circuit operates in exactly the same manner as unbiased

clampers. The different is only that a dc bias voltage is add in series with the diode and
resistor. A biased clamper means that the clamping can be done at any voltage level
other than zero.

The Fig 3.31(a) shows the circuit of positive clamper with positive biased Here a
battery of 10 V is added in such a way that the clamping take place positively at 10V.
Similarly, it is possible to clamp the input wave form positively at -10V by reversing the
battery connections as shown in Fig 3.31(b).

Figure 3.31: Positive Clamper with Bias

NEGATIVE CLAMPER

The Fig 3.32 shows the circuit of a negative clamper.During the positive half
cycle of the input signal, the capacitor is charged to Vm, with the polarity shown in Fig
3.32. Observe that voltage across the capacitor is opposing the input voltage V. This
gives negative clamped voltage and is called negative clamper circuit.
 Figure 3.32: Negative Clamper

NEGATIVE CLAMPER WITH BIAS

The Fig 3.33 (a) shows the circuit of negative clamper with positive bias. With no
input signal the capacitor charges to the battery voltage and the output is positive
because the negative side of the batter is grounded. The output waveform is clamped to
+10V, the value of the battery. Since this is a negative clamper (cathode to ground), the
top of the output wave touch the +10V reference line.

Figure 3.33: Negative Clamper with Bias

 Similarly it is possible to clamp the input waveform negatively at by reversing the
battery connections as shown in Fig 3.33(b)

USES OF CLAMPING CIRCUITS

Clamping circuit are used to shift any part of the input signal waveform and can
be maintained at a specified voltage level Such circuit are used in television receivers to
restore the original d.c reference signal (corresponding to the brightness level of the
picture) to the video Signal The clamping of peak (i.e. 2Vm, 3Vm, 4Vm etc.,) Such to
circuit are known as voltage multipliers These circuit are used to supply power to thigh
voltage/low current devices like cathode ray tubes used in Television receivers,
oscilloscopes and computer displays.

APPLICATIONS OF CLIPPERS AND CLAMPERS

The applications of clippers are:

• They are frequently used for the separation of synchronizing signals from the
composite picture signals.
• The excessive noise spikes above a certain level can be limited or clipped in FM
transmitters by using the series clippers.
• For the generation of new waveforms or shaping the existing waveform, clippers
are used.
• The typical application of a diode clipper is for the protection of transistors from
transients, as a freewheeling diode connected in parallel across the inductive
load.
• A frequently used half-wave rectifier in power supply kits is a typical example of a
clipper. It clips either positive or negative half-wave of the input.
• Clippers can be used as voltage limiters and amplitude selectors.
The applications of clampers are:

• The complex transmitter and receiver circuitry of the television clamper is used
as a baseline stabilizer to define sections of the luminance signals to preset
levels.
• Clampers are also called direct current restorers as they clamp the waveforms to
a fixed DC potential.
• These are frequently used in test equipment, sonar, and radar systems.
 • For the protection of the amplifiers from large errant signals, clampers are used.
• Clampers can be used for removing the distortions
• For improving the overdrive recovery time clampers are used.
• Clampers can be used as voltage doublers or voltage multipliers.
These are all the detailed applications of both clippers and clampers. Clippers
and clampers circuits are used for molding a waveform to a required shape and
specified range.

VOLTAGE MULTIPLIERS
There are applications where the voltage needs to be multiplied in some cases.
This can be done easily with the help of a simple circuit using diodes and capacitors.
The voltage if doubled, such a circuit is called as a Voltage Doubler. This can be
extended to make a Voltage Tripler or a Voltage Quadrupler or so on to obtain high DC
voltages.

To get a better understanding, let us consider a circuit that multiplies the voltage
by a factor of 2. This circuit can be called as a Voltage Doubler. The figure15 shows
the circuit of a voltage doubler. The input voltage applied will be an AC signal which is in
the form of a sine wave as shown in the figure below.

Figure 3.34: Voltage multiplier

Working:

The voltage multiplier circuit can be understood by analysing each half cycle of
the input signal. Each cycle makes the diodes and the capacitors work in different
fashion. Let us try to understand this.
 Figure 3.35: Input / output waveform

During the first positive half cycle − When the input signal is applied, the
capacitor C1 is charged and the diode D1is forward biased. While the diode D2 is
reverse biased and the capacitor C2 doesn’t get any charge. This makes the output V0
to be Vm. This can be understood from the following figure.

Hence, during 0 to π, the output voltage produced will be Vmax. The capacitor
C1 gets charged through the forward biased diode D1 to give the output, while C2
doesn’t charge. This voltage appears at the output.

Figure 3.36: During the first positive half cycle

During the negative half cycle − After that, when the negative half cycle arrives,
the diode D1 gets reverse biased and the diode D2gets forward biased. The diode D2
gets the charge through the capacitor C2 which gets charged during this process. The
current then flows through the capacitor C1 which discharges. It can be understood
from the following figure.

Figure 3.37: During the negative half cycle

Hence during π to 2π, the voltage across the capacitor C2 will be Vmax. While
the capacitor C1 which is fully charged, tends to discharge. Now the voltages from both
the capacitors together appear at the output, which is 2Vmax. So, the output voltage V0
during this cycle is 2Vmax

During the next positive half cycle − the capacitor C1 gets charged from the
supply and the diode D1gets forward biased. The capacitor C2 holds the charge as it
will not find a way to discharge and the diode D2 gets reverse biased. Now, the output
voltage V0 of this cycle gets the voltages from both the capacitors that together appear
at the output, which is 2Vmax.

During the next negative half cycle − the next negative half cycle makes the
capacitor C1 to again discharge from its full charge and the diode D1 to reverse bias
while D2 forward and capacitor C2 to charge further to maintain its voltage. Now, the
output voltage V0 of this cycle gets the voltages from both the capacitors that together
appear at the output, which is 2Vmax.

Hence, the output voltage V0 is maintained to be 2Vmax throughout its

operation, which makes the circuit a voltage doubler.

Application

Voltage multipliers are mostly used where high DC voltages are required. For
example, cathode ray tubes and computer display.
Bipolar Junction Transistors (BJT)

General configuration and definitions

The transistor is the main building block “element” of electronics. It is a semiconductor

device and it comes in two general types: the Bipolar Junction Transistor (BJT) and the
Field Effect Transistor (FET). Here we will describe the system characteristics of the
BJT configuration and explore its use in fundamental signal shaping and amplifier
circuits. The BJT is a three terminal device and it comes in two different types. The npn
BJT and the pnp BJT. The BJT symbols and their corresponding block diagrams are
shown on Figure 3.38.

The BJT is fabricated with three separately doped regions. The npn device has one p
region between two n regions and the pnp device has one n region between two p
regions. The BJT has two junctions (boundaries between the n and the p regions).
These junctions are similar to the junctions we saw in the diodes and thus they may be
forward biased or reverse biased. By relating these junctions to a diode model the pnp
BJT may be modelled as shown on Figure 3.39.

The three terminals of the BJT are called the Base (B), the Collector (C) and the Emitter
(E).

Fig 3.38. BJT schematics and structures. (a) npn transistor, (b) pnp transistor

Fig 3.39. BJT schematics modelling structures. (a) npn transistor, (b) pnp transistor
Since each junction has two possible states of operation (forward or reverse bias) the
BJT with its two junctions has four possible states of operation.

Here it is sufficient to say that the structure as shown on Figure 3.38 is not symmetric.
The n and p regions are different both geometrically and in terms of the doping
concentration of the regions. For example, the doping concentrations in the collector,
base and emitter may be 1015, 1017 and 1019 respectively. Therefore, the behavior of the
device is not electrically symmetric and the two ends cannot be interchanged. Before
proceeding let’s consider the BJT npn structure shown on Figure 3.40.

Fig. 40 BJT npn structure

With the voltage VBE and VCB as shown, the Base-Emitter (B-E) junction is forward
biased and the Base-Collector (B-C) junction is reverse biased.

The current through the B-E junction is related to the B-E voltage as

(1)

Due to the large differences in the doping concentrations of the emitter and the base
regions the electrons injected into the base region (from the emitter region) results in
the emitter current IE. Furthermore, the number of electrons injected into the collector
region is directly related to the electrons injected into the base region from the emitter
region.

Therefore, the collector current is related to the emitter current which is in turn a
function of the B-E voltage.

The collector current and the base current are related by

IC = β IB (2)

And by applying KCL we obtain

IE = IC + IB (3)

And thus from equations 2 and 3 the relationship between the emitter and the base
currents is

IE = (1+ β)IB (4)

And equivalently

IC = (β/(1+β)) IE (5)

The fraction (β/(1+β)) is called α.

For the transistors of interest β = 100 which corresponds to α = 0.99 and

The direction of the currents and the voltage polarities for the npn and the pnp BJTs are
shown on Figure 3.41.

Figure 3.41 Current directions and voltage polarities for npn (a) and pnp (b) BJTs

Modes of operation

The two junctions of BJT can be either forward or reverse-biased.

The BJT can operate in different modes depending on the junction bias.
The BJT can operate in different modes depending on the junction bias.
Switching applications utilize both the cutoff and saturation modes.

Mode EBJ CBJ

Cutoff Reverse Reverse
Active Forward Reverse
Saturation Forward Forward
Operation of the npn transistor in the active mode:
Electrons in emitter regions are injected into base due to the forward bias at EBJ .
Most of the injected electrons reach the edge of CBJ before being recombined if the
base is narrow.
Electrons at the edge of CBJ will be swept into collector due to the reverse bias at CBJ.
iEn
Emitter injection efficiency =
iEn + iEp

Fig 3.42: Operation in Active mode

Transistor i-v characteristics

A. Transistor Voltages
Three different types of voltages are involved in the description of transistors and
transistor circuits. They are:
Transistor supply voltages: VCC , VBB .
Transistor terminal voltages: VC , VC , VE
Voltages across transistor junctions: VBE , VCE , VCB
All of these voltages and their polarities are shown on Figure 3.43 for the npn BJT.

Vo

Figure 3.43. Voltages and their polarities

Transistor Operation and Characteristic i-v curves

The three terminals of the transistors and the two junctions, present us with multiple
operating regimes. In order to distinguish these regimes we have to look at the i-v
characteristics of the device. The most important characteristic of the BJT is the plot of
the collector current, IC, versus the collector-emitter voltage, VCE, for various values of
the base current, I B as shown on the circuit of Figure 3.44.

Figure 3.44. Common emitter BJT circuit for determining output characteristics

Figure 3.45 shows the qualitative characteristic curves of a BJT. The plot indicates the
four regions of operation: the saturation, the cutoff, the active and the breakdown. Each
family of curves is drawn for a different base current and in this plot IB 4 > IB3 > IB2 > I B1
Figure 3.45. BJT characteristic curve

The characteristics of each region of operation are summarized below.

1. cutoff region:

Base-emitter junction is reverse biased. No current flow.

2. saturation region:

Base-emitter junction forward biased, Collector-base junction is forward biased Ic

reaches a maximum which is independent of IB and β. No control.

VCE < VBE

3. active region:

Base-emitter junction forward biased Collector-base junction is reverse biased Control,

IC = β IB (as can be seen from Figure 3.45 there is a small slope of IC with VCE.

VBE < VCE < VCC

4. breakdown region:

IC and VCE exceed specifications damage to the transistor.

BJT as a switch:

Consider the circuit shown on Figure 3.46. If the voltage vi is less than the voltage
required to forward bias the base-emitter junction then the current IB =0 and thus the
transistor is in the cutoff region and IC = 0 Since IC = 0 the voltage drop across Rc is zero
and so Vo=Vcc.

If the voltage vi increases so that forward biases the base-emitter junction the transistor
will turn on and

(6)

Once the transistor is on we still do not know if it is operating in the active region or in
the saturation region. However, KVL around the C-E loop gives

VCC = ICRC +VCE (7)

And so

VCE = VCC – ICRC (8)

Note that VCE = V0 as shown on Figure 3.46.

Figure 3.46. npn BJT switch circuit

Equation (8) is the load line equation for this circuit. In graphical form it is shown on
Figure 3.47.

Figure 3.47. Graphical form of load equation

As the base current increases the transistor may operate at points along the load line
(thick dashed line on Figure 3.47). In the limit, the base current I B3 results in the largest
current IC. This is the saturation current and when the transistor operates at this point it
is said to be biased in the saturation mode. In saturation, the base-collector junction is
forward biased and the relationship between the base and the collector current is not
linear.
Therefore the collector current at saturation is

(9)

In saturation the collector-emitter voltage, VCE, is less than the VBE. Typically, the VCE at
saturation is about 0.2 Volts.

The transistor as an amplifier

A BJT circuit with a collector resistor RC can be used as a simple voltage amplifier. Base
terminal is used the amplifier input and the collector is considered the amplifier output.

Cutoff mode:

The voltage transfer characteristic (VTC) is obtained by solving the circuit from low to
high VBE.

Active mode:

Saturation mode:

VBE further increases.

Fig 3.48: BJT as an amplifier

Gain and Bandwidth

Gain and bandwidth in an amplifier are inversely proportional to each other and their
relationship is summarized as the unity-gain bandwidth. Unity-gain bandwidth defines
the frequency at which the gain of an amplifier is equal to 1. If the GBWP of an
operational amplifier is 1 MHz, it means that the gain of the device falls to unity at 1
MHz. Hence, when the device is wired for unity gain, it will work up to 1 MHz (GBWP =
gain × bandwidth, therefore if BW = 1 MHz, then gain = 1) without excessively distorting
the signal.

Transistor Configuration
Depending upon the terminals which are used as a common terminal to the input and
output terminals, the transistors can be connected in the following three different
configuration.
1. common base configuration
2. common emitter configuration
3. common collector configuration

Common base configuration

In this configuration base terminal is connected as a common terminal.

The input is applied between the emitter and base terminals. The output is taken
between the collector and base terminals.

Input characteristics: The output (CB) voltage is maintained constant and the input
voltage (EB) is set at several convenient levels. For each level of input voltage, the input
current IE is recorded. IE is then plotted versus VEB to give the common-base input
characteristics.

Fig 3.49: BJT common base connection

Output characteristics: The emitter current IE is held constant at each of several fixed
levels. For each fixed value of IE, the output voltage VCB is adjusted in convenient steps
and the corresponding levels of collector current IC are recorded. For each fixed value of
IE, IC is almost equal to IE and appears to remain constant when VCB is increased.

Fig 3.50: BJT common base characteristics and collector characteristics

Common emitter configuration:

In this configuration emitter terminal is connected as a common terminal.

The input is applied between the emitter and base terminals. The output is taken
between the collector and base terminals.

Input characteristics: The output voltage VCE is maintained constant and the input
voltage VBE is set at several convenient levels. For each level of input voltage, the input
current IB is recorded. IB is then plotted versus VBE to give the common-base input
characteristics.

Fig 3.51: BJT common emitter connection

 Fig 3.52: BJT base emitter voltage and collector emitter voltage

Output characteristics: The Base current IB is held constant at each of several fixed
levels. For each fixed value of IB, the output voltage VCE is adjusted in convenient steps
and the corresponding levels of collector current IC are recorded. For each fixed value of
IB, IC level is Recorded at each VCE step. For each IB level, IC is plotted versus VCE to give
a family of characteristics

Common collector configuration:

In this configuration collector terminal is connected as a common terminal.

The input is applied between the base and collector terminals. The output is taken
between the emitter and collector terminals.

Input characteristics: The common-collector input characteristics are quite different from
either common base or common-emitter input characteristics. The difference is due to
the fact that the input voltage (VBC) is largely determined by (VEC) level.

VEC = VEB + VBC, VEB = VEC – VBC

Output characteristics: The operation is much similar to that of C-E configuration. When
the base current is ICO, the emitter current will be zero and consequently no current will
flow in the load. When the base current is increased, the transistor passes through
active region and eventually reaches saturation. Under the saturation conditions all the
supply voltage, except for a very small drop across the transistor will appear across the
load resistor.
Fig 3.53: BJT common collector connection

Fig 3.54: BJT common collector voltages and base current

Biased and Unbiased BJT

Unbiased transistor is a transistor with its terminals not connected to any source.
Biasing a transistor is applying a suitable DC voltage across the transistor terminals to
operate the transistor in the desired region
Transistor Biasing is the process of setting a transistors DC operating voltage or current
conditions to the correct level so that any AC input signal can be amplified correctly by the
transistor.

Necessary of transistor biasing:

• To active an transistor, biasing is essential. For proper working it is essential to apply to

apply voltages of correct polarity across its two junctions.
• If it is not biased correctly, it would work inefficiently and produce distortion in the output
signal
• Q-point is not middle Output signal is distorted & the signal is clipped.
• Further for various applications, BJT is biased as shown in table
Region of operation Base Emitter Junction Collector base junction Application
Cut off Reverse bias Reverse bias As a switch
Active Forward bias Reverse bias As amplifier
Saturation Forward bias Forward bias As a switch

In order to have these applications, we need to connect external DC power supplies

with correct polarities & magnitude. This process is called as biasing of transistor.
Stability Factor
The stability of Q point of transistor amplifier depends on the following three
parameters:
1. Leakage current ICO 2. βdc 3. Base to emitter voltage
The effect of these parameters can be expressed mathematically by defining the
stability factors
1. Stability factor

This represents the change in collector current due to change in reverse saturation
current ICO. The other two parameters that means VBE & βdc are assumed to be
constant.
2. Stability factor

S‟ represents the change in IC due to change in VBE at constant ICO & βdc
3. Stability factor

Total change in collector current

Δ𝐼𝐶= S. Δ𝐼𝐶𝑂 + S‟. Δ𝑉𝐵𝐸 + S”. βdc
• Ideally the values of all the stability factors should be zero and practically they should
be as small as possible.

• Practically the value of S is significantly higher than the other two stability factor.
Hence while comparing the biasing circuits, the values of S is more significant.

Voltage Divider Bias:

The most famous circuit based on -the prototype of emitter bias is called the voltage
divider bias (VDB).
Recall the steps of analyzing the emitter bias circuit:

1. VE
2. IE
3. IC
4. Voltage drop across RC
5. VC
6. VCE

The three most important steps are:

1. IE
2. VC
3. VCE

Fig 3.55. Emitter Biased Circuit

Problem: Sometimes the voltage from the VCC power supply is too large to apply directly
at the base.

Solution:

- extra power supply for the base

- or ==> VDB

Fig 3.56. VDB Circuit

The voltage drop across R2 is applied directly to the base, which means:

V2 = V B
1. step: find voltage drop across R2
2. step: subtract 0.7V to get VE

VDB analysis

Design errors of 5% or less are acceptable, because of resistor tolerances.

Fig 3.57. VDB Example Circuit

Find the base voltage:

Assumption: Base current is so small that it has no effect on the voltage divider.

5% error - > base current is 20 times smaller than the divider current.

VB = I * R2 = 0.82 mA * 2.2KW = 1.8V

VE = VB - VBE = 1.8V - 0.7V = 1.1V

VC = VCC -(RC * IC) = 10V - (3.6KW * 1.1 mA) = 6.04V

VCE = VC - VE = 6.04V - 1.1V = 4.94V

Checking the assumption:

5% error -->

The current gain can vary from 30 to 300.

Even under the worst-case condition the calculation is within the 5% limit, hence the
assumption can be done.

Summary of Process and Formulas

Divider current

Base voltage V B = I * R2
Emitter voltage VE = VB - VBE
Emitter current

Collector voltage VC = VCC - (IC * RC)

Collector emitter voltage VCE = VC - VE

HO: What will change if the emitter resistor increases to 2KW? (Unchanged voltage
divider)

Fig 3.58. VDB Circuit

Solution:

I = 0.82 mA

VB = 1.8V

VE = 1.1V

VC = VCC - (RC * IC) = 8.02V

VCE = VC - VE = 6.92V

VDB Load-Line and Q-Point

Saturation point:

Visualize short between collector and emitter Fig 3.59. VDB Circuit

VRC = VCC - VE = 10V - 1.1V = 8.9V

Cutoff point:

Visualize open between collector and emitter

-VCE (cut) = VCC - VE = 8.9V

Q-point:

VC = VCC - (IC * RC) = 10V - (1.1 mA * 1KW) = 6.04V

VCE = VC - VE = 6.04V - 1.1V = 4.94V

Now we plot these values and get the load line and the Q-point:

Fig 3.60. Output curve with load line and Q-point

The values VCC, RC, R1, and R2 are controlling saturation current and cutoff voltage. To
move the Q-point is possible by varying the emitter resistance (RC).

Get the Q-point in the Middle of the Load Line

To set the Q-point is a important preparation as you will see later on.

Effect of RE:

RE too large -- > Q-point moves into cutoff

RE too small --> Q-point moves into saturation

Q - point in the middle of the load line:

Half the value of IC (sat) and redesign RE

IC (sat) = 2.47 mA ==> 1.23 mA

Look for the nearest standard value:

=910 ohm
 Fig 3.61. Output curve, Q-point in the middle

REFERENCES

1) Electronic Principles by A. P. Malvino, Tata McGraw Hill Publication New

Delhi.
2) Electronic Devices and Circuits by Jacob Millman and Halkias, Tata
McGraw Hill Publication New Delhi.
3) Electronic Devices and Circuits by Robert L. Boylestad and Louis
Nashelesky,Pearson, Prentice Hall.
4) Electronic Devices by Thomas L. Floyd, Pearson, Prentice Hall.
5) Linear Electronic Circuits and Devices by James Cox,Delmar Publication.
6) Electronic Devices and Circuits by David A. Bell, Oxford Publication.