UNIT - V

LARGE SIGNAL AMPLIFIERS

Classification Based on Frequencies
Power amplifiers are divided into two categories, based on the frequencies they handle. They are as follows.

 Audio Power Amplifiers − The audio power amplifiers raise the power level of signals that have audio
frequency range (20 Hz to 20 KHz). They are also known as Small signal power amplifiers.

 Radio Power Amplifiers − Radio Power Amplifiers or tuned power amplifiers raise the power level of
signals that have radio frequency range (3 KHz to 300 GHz). They are also known as large signal
power amplifiers.

Classification Based on Mode of Operation

On the basis of the mode of operation, i.e., the portion of the input cycle during which collector current flows,
the power amplifiers may be classified as follows.

 Class A Power amplifier − When the collector current flows at all times during the full cycle of signal,
the power amplifier is known as class A power amplifier.

 Class B Power amplifier − When the collector current flows only during the positive half cycle of the
input signal, the power amplifier is known as class B power amplifier.

 Class C Power amplifier − When the collector current flows for less than half cycle of the input signal,
the power amplifier is known as class C power amplifier.

There forms another amplifier called Class AB amplifier, if we combine the class A and class B amplifiers so
as to utilize the advantages of both.

Before going into the details of these amplifiers, let us have a look at the important terms that have to be
considered to determine the efficiency of an amplifier.

Terms Considering Performance

The primary objective of a power amplifier is to obtain maximum output power. In order to achieve this, the
important factors to be considered are collector efficiency, power dissipation capability and distortion. Let us
go through them in detail.

Collector Efficiency
This explains how well an amplifier converts DC power to AC power. When the DC supply is given by the
battery but no AC signal input is given, the collector output at such a condition is observed as collector
efficiency.

The collector efficiency is defined as

For example, if the battery supplies 15W and AC output power is 3W. Then the transistor efficiency will be
20%.

The main aim of a power amplifier is to obtain maximum collector efficiency. Hence the higher the value of
collector efficiency, the efficient the amplifier will be.

Power Dissipation Capacity

Every transistor gets heated up during its operation. As a power transistor handles large currents, it gets more
heated up. This heat increases the temperature of the transistor, which alters the operating point of the
transistor.

So, in order to maintain the operating point stability, the temperature of the transistor has to be kept in
permissible limits. For this, the heat produced has to be dissipated. Such a capacity is called as Power
dissipation capability.

Power dissipation capability can be defined as the ability of a power transistor to dissipate the heat developed
in it. Metal cases called heat sinks are used in order to dissipate the heat produced in power transistors.

Distortion
A transistor is a non-linear device. When compared with the input, there occur few variations in the output. In
voltage amplifiers, this problem is not pre-dominant as small currents are used. But in power amplifiers, as
large currents are in use, the problem of distortion certainly arises.

Distortion is defined as the change of output wave shape from the input wave shape of the amplifier. An
amplifier that has lesser distortion, produces a better output and hence considered efficient.

Class A Power Amplifiers

We have already come across the details of transistor biasing, which is very important for the operation of a
transistor as an amplifier. Hence to achieve faithful amplification, the biasing of the transistor has to be done
such that the amplifier operates over the linear region.

A Class A power amplifier is one in which the output current flows for the entire cycle of the AC input supply.
Hence the complete signal present at the input is amplified at the output. The following figure shows the circuit
diagram for Class A Power amplifier.
From the above figure, it can be observed that the transformer is present at the collector as a load. The use of
transformer permits the impedance matching, resulting in the transference of maximum power to the load e.g.
loud speaker.

The operating point of this amplifier is present in the linear region. It is so selected that the current flows for
the entire ac input cycle. The below figure explains the selection of operating point.

The output characteristics with operating point Q is shown in the figure above. Here (Ic)Q and (Vce)Q represent
no signal collector current and voltage between collector and emitter respectively. When signal is applied, the
Q-point shifts to Q1 and Q2. The output current increases to (Ic)max and decreases to (Ic)min. Similarly, the
collector-emitter voltage increases to (Vce)max and decreases to (Vce)min.

D.C. Power drawn from collector battery Vcc is given by

  Where I is the R.M.S. value of a.c. output current through load, V is the R.M.S. value of a.c. voltage,
and Vm is the maximum value of V.

 The D.C. power dissipated by the transistor (collector region) in the form of heat, i.e., (PC)dc

We have represented the whole power flow in the following diagram.

This class A power amplifier can amplify small signals with least distortion and the output will be an exact
replica of the input with increased strength.

Let us now try to draw some expressions to represent efficiencies.

Overall Efficiency
The overall efficiency of the amplifier circuit is given by

Advantages of Class A Amplifiers

The advantages of Class A power amplifier are as follows −

 The current flows for complete input cycle

 It can amplify small signals
 The output is same as input
 No distortion is present
Disadvantages of Class A Amplifiers
The advantages of Class A power amplifier are as follows −

 Low power output

 Low collector efficiency

Transformer Coupled Class A Power Amplifier

The class A power amplifier as discussed in the previous chapter, is the circuit in which the output current
flows for the entire cycle of the AC input supply. We also have learnt about the disadvantages it has such as
low output power and efficiency. In order to minimize those effects, the transformer coupled class A power
amplifier has been introduced.

The construction of class A power amplifier can be understood with the help of below figure. This is
similar to the normal amplifier circuit but connected with a transformer in the collector load.

Here R1 and R2 provide potential divider arrangement. The resistor Re provides stabilization, Ce is the bypass
capacitor and Re to prevent a.c. voltage. The transformer used here is a step-down transformer.

The high impedance primary of the transformer is connected to the high impedance collector circuit. The low
impedance secondary is connected to the load (generally loud speaker).
Transformer Action
The transformer used in the collector circuit is for impedance matching. RL is the load connected in the
secondary of a transformer. RL’ is the reflected load in the primary of the transformer.
The number of turns in the primary are n1 and the secondary are n2. Let V1and V2 be the primary and
secondary voltages and I1 and I2 be the primary and secondary currents respectively. The below figure shows
the transformer clearly.
Circuit Operation
If the peak value of the collector current due to signal is equal to zero signal collector current, then the
maximum a.c. power output is obtained. So, in order to achieve complete amplification, the operating point
should lie at the center of the load line.

The operating point obviously varies when the signal is applied. The collector voltage varies in opposite phase
to the collector current. The variation of collector voltage appears across the primary of the transformer.

Circuit Analysis
The power loss in the primary is assumed to be negligible, as its resistance is very small.

The input power under dc condition will be

The efficiency of a class A power amplifier is nearly than 30% whereas it has got improved to 50% by using
the transformer coupled class A power amplifier.
Advantages
The advantages of transformer coupled class A power amplifier are as follows.

 No loss of signal power in the base or collector resistors.

 Excellent impedance matching is achieved.
 Gain is high.
 DC isolation is provided.
Disadvantages
The disadvantages of transformer coupled class A power amplifier are as follows.

 Low frequency signals are less amplified comparatively.

 Hum noise is introduced by transformers.
 Transformers are bulky and costly.
 Poor frequency response.
Applications
The applications of transformer coupled class A power amplifier are as follows.

 This circuit is where impedance matching is the main criterion.

 These are used as driver amplifiers and sometimes as output amplifiers.

Class B Power Amplifier

When the collector current flows only during the positive half cycle of the input signal, the power amplifier is
known as class B power amplifier.

Class B Operation

The biasing of the transistor in class B operation is in such a way that at zero signal condition, there will be no
collector current. The operating point is selected to be at collector cut off voltage. So, when the signal is
applied, only the positive half cycle is amplified at the output.

The figure below shows the input and output waveforms during class B operation.
When the signal is applied, the circuit is forward biased for the positive half cycle of the input and hence the
collector current flows. But during the negative half cycle of the input, the circuit is reverse biased and the
collector current will be absent. Hence only the positive half cycle is amplified at the output.

As the negative half cycle is completely absent, the signal distortion will be high. Also, when the applied
signal increases, the power dissipation will be more. But when compared to class A power amplifier, the
output efficiency is increased.

Well, in order to minimize the disadvantages and achieve low distortion, high efficiency and high output
power, the push-pull configuration is used in this class B amplifier.

Class B Push-Pull Amplifier

Though the efficiency of class B power amplifier is higher than class A, as only one half cycle of the input is
used, the distortion is high. Also, the input power is not completely utilized. In order to compensate these
problems, the push-pull configuration is introduced in class B amplifier.

Construction
The circuit of a push-pull class B power amplifier consists of two identical transistors T1 and T2 whose bases
are connected to the secondary of the center-tapped input transformer Tr1. The emitters are shorted and the
collectors are given the VCC supply through the primary of the output transformer Tr2.

The circuit arrangement of class B push-pull amplifier, is same as that of class A push-pull amplifier except
that the transistors are biased at cut off, instead of using the biasing resistors. The figure below gives the
detailing of the construction of a push-pull class B power amplifier.
The circuit operation of class B push pull amplifier is detailed below.

Operation
The circuit of class B push-pull amplifier shown in the above figure clears that both the transformers are
center-tapped. When no signal is applied at the input, the transistors T1 and T2 are in cut off condition and
hence no collector currents flow. As no current is drawn from VCC, no power is wasted.

When input signal is given, it is applied to the input transformer Tr1 which splits the signal into two signals that
are 180o out of phase with each other. These two signals are given to the two identical transistors T 1 and T2.
For the positive half cycle, the base of the transistor T1 becomes positive and collector current flows. At the
same time, the transistor T2 has negative half cycle, which throws the transistor T2 into cutoff condition and
hence no collector current flows. The waveform is produced as shown in the following figure.

For the next half cycle, the transistor T1 gets into cut off condition and the transistor T2 gets into conduction, to
contribute the output. Hence for both the cycles, each transistor conducts alternately. The output transformer
Tr3 serves to join the two currents producing an almost undistorted output waveform.

Power Efficiency of Class B Push-Pull Amplifier

The current in each transistor is the average value of half sine loop.

For half sine loop, Idc is given by

The collector efficiency would be the same.

Hence the class B push-pull amplifier improves the efficiency than the class A push-pull amplifier.
Complementary Symmetry Push-Pull Class B Amplifier
The push pull amplifier which was just discussed improves efficiency but the usage of center-tapped
transformers makes the circuit bulky, heavy and costly. To make the circuit simple and to improve the
efficiency, the transistors used can be complemented, as shown in the following circuit diagram.

The above circuit employs a NPN transistor and a PNP transistor connected in push pull configuration. When
the input signal is applied, during the positive half cycle of the input signal, the NPN transistor conducts and
the PNP transistor cuts off. During the negative half cycle, the NPN transistor cuts off and the PNP transistor
conducts.

In this way, the NPN transistor amplifies during positive half cycle of the input, while PNP transistor amplifies
during negative half cycle of the input. As the transistors are both complement to each other, yet act
symmetrically while being connected in push pull configuration of class B, this circuit is termed
as Complementary symmetry push pull class B amplifier.

Advantages
The advantages of Complementary symmetry push pull class B amplifier are as follows.

 As there is no need of center tapped transformers, the weight and cost are reduced.

 Equal and opposite input signal voltages are not required.

Disadvantages
The disadvantages of Complementary symmetry push pull class B amplifier are as follows.

 It is difficult to get a pair of transistors (NPN and PNP) that have similar characteristics.

 We require both positive and negative supply voltages.

Cross-over Distortion

In the push-pull configuration, the two identical transistors get into conduction, one after the other and the
output produced will be the combination of both.

When the signal changes or crosses over from one transistor to the other at the zero voltage point, it produces
an amount of distortion to the output wave shape. For a transistor in order to conduct, the base emitter junction
should cross 0.7v, the cut off voltage. The time taken for a transistor to get ON from OFF or to get OFF from
ON state is called the transition period.

At the zero voltage point, the transition period of switching over the transistors from one to the other, has its
effect which leads to the instances where both the transistors are OFF at a time. Such instances can be called as
Flat spot or Dead band on the output wave shape.

The above figure clearly shows the cross over distortion which is prominent in the output waveform. This is the
main disadvantage. This cross over distortion effect also reduces the overall peak to peak value of the output
waveform which in turn reduces the maximum power output. This can be more clearly understood through the
non-linear characteristic of the waveform as shown below.
It is understood that this cross-over distortion is less pronounced for large input signals, where as it causes
severe disturbance for small input signals. This cross over distortion can be eliminated if the conduction of the
amplifier is more than one half cycle, so that both the transistors won’t be OFF at the same time. This idea
leads to the invention of class AB amplifier, which is the combination of both class A and class B amplifiers, as
discussed below.

Class AB Power Amplifier

As the name implies, class AB is a combination of class A and class B type of amplifiers. As class A has the
problem of low efficiency and class B has distortion problem, this class AB is emerged to eliminate these two
problems, by utilizing the advantages of both the classes. The cross over distortion is the problem that occurs
when both the transistors are OFF at the same instant, during the transition period. In order to eliminate this, the
condition has to be chosen for more than one half cycle. Hence, the other transistor gets into conduction, before
the operating transistor switches to cut off state. This is achieved only by using class AB configuration, as
shown in the following circuit diagram.

Therefore, in class AB amplifier design, each of the push-pull transistors is conducting for slightly more than
the half cycle of conduction in class B, but much less than the full cycle of conduction of class A. The
conduction angle of class AB amplifier is somewhere between 180o to 360o depending upon the operating
point selected. This is understood with the help of below figure.
The small bias voltage given using diodes D1 and D2, as shown in the above figure, helps the operating point
to be above the cutoff point. Hence the output waveform of class AB results as seen in the above figure. The
crossover distortion created by class B is overcome by this class AB, as well the inefficiencies of class A and B
don’t affect the circuit.

So, the class AB is a good compromise between class A and class B in terms of efficiency and linearity having
the efficiency reaching about 50% to 60%. The class A, B and AB amplifiers are called as linear amplifiers
because the output signal amplitude and phase are linearly related to the input signal amplitude and phase.

AMPLIFIER DISTORTION

For a signal amplifier to operate correctly without any distortion to the output signal, it requires some
form of DC Bias on its Base or Gate terminal. A DC bias is required so that the amplifier can amplify the input
signal over its entire cycle with the bias “Q-point” set as near to the middle of the load line as possible.

The bias Q-point setting will give us a “Class-A” type amplification configuration with the most common
arrangement being the “Common Emitter” for Bipolar transistors or the “Common Source” configuration for
unipolar FET transistors.

The Power, Voltage or Current Gain, (amplification) provided by the amplifier is the ratio of the peak output
value to its peak input value (Output ÷ Input).
However, if we incorrectly design our amplifier circuit and set the biasing Q-point at the wrong position on the
load line or apply too large an input signal to the amplifier, the resultant output signal may not be an exact
reproduction of the original input signal waveform. In other words the amplifier will suffer from what is
commonly called Amplifier Distortion. Consider the common emitter amplifier circuit below.

Common Emitter Amplifier

Distortion of the output signal waveform may occur because:

 Amplification may not be taking place over the whole signal cycle due to incorrect biasing levels.
 The input signal may be too large, causing the amplifiers transistors to be limited by the supply voltage.
 The amplification may not be a linear signal over the entire frequency range of inputs.

This means then that during the amplification process of the signal waveform, some form of Amplifier
Distortion has occurred.

Amplifiers are basically designed to amplify small voltage input signals into much larger output signals and
this means that the output signal is constantly changing by some factor or value, called gain, multiplied by the
input signal for all input frequencies. We saw previously that this multiplication factor is called the Beta, β
value of the transistor.

Common emitter or even common source type transistor circuits work fine for small AC input signals but
suffer from one major disadvantage, the calculated position of the bias Q-point of a bipolar amplifier depends
on the same Beta value for all transistors. However, this Beta value will vary from transistors of the same type,
in other words, the Q-point for one transistor is not necessarily the same as the Q-point for another transistor of
the same type due to the inherent manufacturing tolerances.

Then amplifier distortion occurs because the amplifier is not linear and a type of amplifier distortion called
Amplitude Distortion will result. Careful choice of the transistor and biasing components can help minimise
the effect of amplifier distortion.

Amplitude Distortion

Amplitude distortion occurs when the peak values of the frequency waveform are attenuated causing distortion
due to a shift in the Q-point and amplification may not take place over the whole signal cycle. This non-
linearity of the output waveform is shown below.

Amplitude Distortion due to Incorrect Biasing

if the transistors biasing point is correct, the output waveform should have the same shape as that of the input
waveform only bigger, (amplified). If there is insufficient bias and the Q-point lies in the lower half of the load
line, then the output waveform will look like the one on the right with the negative half of the output waveform
“cut-off” or clipped. Likewise, if there is too much bias and the Q-point lies in the upper half of the load line,
then the output waveform will look like the one on the left with the positive half “cut-off” or clipped.

Also, when the bias voltage is set too small, during the negative half of the cycle the transistor does not fully
conduct so the output is set by the supply voltage. When the bias is too great the positive half of the cycle
saturates the transistor and the output drops almost to zero.

Even with the correct biasing voltage level set, it is still possible for the output waveform to become distorted
due to a large input signal being amplified by the circuits gain. The output voltage signal becomes clipped in
both the positive and negative parts of the waveform an no longer resembles a sine wave, even when the bias is
correct. This type of amplitude distortion is called Clipping and is the result of “over-driving” the input of the
amplifier.
When the input amplitude becomes too large, the clipping becomes substantial and forces the output waveform
signal to exceed the power supply voltage rails with the peak (+ve half) and the trough (-ve half) parts of the
waveform signal becoming flattened or “Clipped-off”. To avoid this the maximum value of the input signal
must be limited to a level that will prevent this clipping effect as shown above.

Amplitude Distortion due to Clipping

`Amplitude Distortion greatly reduces the efficiency of an amplifier circuit. These “flat tops” of the
distorted output waveform either due to incorrect biasing or over driving the input do not contribute anything
to the strength of the output signal at the desired frequency.

Having said all that, some well known guitarist and rock bands actually prefer that their distinctive sound is
highly distorted or “overdriven” by heavily clipping the output waveform to both the +ve and -ve power supply
rails. Also, increasing the amounts of clipping on a sinusoid will produce so much amplifier distortion that it
will eventually produce an output waveform which resembles that of a “square wave” shape which can then be
used in electronic or digital synthesizer circuits.

We have seen that with a DC signal the level of gain of the amplifier can vary with signal amplitude, but as
well as Amplitude Distortion, other types of amplifier distortion can occur with AC signals in amplifier
circuits, such as Frequency Distortion and Phase Distortion.
Frequency Distortion

Frequency Distortion is another type of amplifier distortion which occurs in a transistor amplifier when the
level of amplification varies with frequency. Many of the input signals that a practical amplifier will amplify
consist of the required signal waveform called the “Fundamental Frequency” plus a number of different
frequencies called “Harmonics” superimposed onto it.

Normally, the amplitude of these harmonics are a fraction of the fundamental amplitude and therefore have
very little or no effect on the output waveform. However, the output waveform can become distorted if these
harmonic frequencies increase in amplitude with regards to the fundamental frequency. For example, consider
the waveform below:

Frequency Distortion due to Harmonics

In the example above, the input waveform consists a the fundamental frequency plus a second harmonic signal.
The resultant output waveform is shown on the right hand side. The frequency distortion occurs when the
fundamental frequency combines with the second harmonic to distort the output signal. Harmonics are
therefore multiples of the fundamental frequency and in our simple example a second harmonic was used.

Therefore, the frequency of the harmonic is twice the fundamental, 2*ƒ or 2ƒ. Then a third harmonic would be
3ƒ, a fourth, 4ƒ, and so on. Frequency distortion due to harmonics is always a possibility in amplifier circuits
containing reactive elements such as capacitance or inductance.
Phase Distortion

Phase Distortion or Delay Distortion is a type of amplifier distortion which occurs in a non-linear transistor
amplifier when there is a time delay between the input signal and its appearance at the output.

If we say that the phase change between the input and the output is zero at the fundamental frequency, the
resultant phase angle delay will be the difference between the harmonic and the fundamental. This time delay
will depend on the construction of the amplifier and will increase progressively with frequency within the
bandwidth of the amplifier. For example, consider the waveform below:

Phase Distortion due to Delay

Other than high end audio amplifiers, most practical amplifiers will have some form of Amplifier Distortion
being a combination of both “Frequency Distortion” and “Phase Distortion”, together with amplitude
distortion. In most applications such as in audio amplifiers or power amplifiers, unless the amplifiers distortion
is excessive or severe it will not generally affect the operation or output sound of the amplifier.

HEAT SINKS USED IN POWER TRANSISTORS

Heat sinks are used for power transistors as the power dissipated at their collector junction is large. If heat
dissipation is not done, this will cause large increases in junction temperature.

In a transistor, the collector to base junction temperature (temperature of surrounding air) rises or because of
self-heating. The self-heating is due to the power dissipated at collector junction
This power dissipation at junction causes the junction temperature to rise, and this in turn increases the
collector current which causes further increase in power dissipation. If the phenomenon continues then it may
result in permanent damage of the transistor. This is known as thermal runaway.

In power transistor or large signal transistors, the power to be dissipated at the collector causes junction
temperature to rise to a high level. It is possible to increase the power handling capacity of the transistor if a
device that can cause rapid conduction of heat away from the junction is used. Such a device is called a heat
sink. A heat sink is a mechanical device. It is connected to the case of the semiconductor device. So it is
providing a path for the heat transfer. The heat flows through the heat sink and is radiated to surrounding air. If
a heat sink is not used then all the heat has to he transferred from a transistor case to surrounding air causing
case temperature to increase. If the power handled by the transistor is higher, then the case temperature will he
higher. The temperature of the two types of power transistor is

Germanium: 100°C to 110°C Silicon : 150°C to 200°C

Heat sinks increase the power rating (ie. power handling capacity) of a transistor by getting rid of the heat
developed quickly. It is in the form of a sheet of metal. Since the power dissipation within a transistor is
mainly due to power dissipated at collector junction, the collector (connected to the case of the transistor) is
bolted on to metal sheet for faster radiation of heat.

In this case, to prevent the collector from shorting to metal sheet, a thin mica washer is used between the two.

Fig shows a heat sink. The heat now radiate more quickly because of in creased surface area.

Sometimes the transistor is connected to a large heat sink with fins causing more efficient removal of heat from
the transistor.

When heat flows out of a transistor, it passes through the case transistor and into the heat sink, which then
radiates the heat into the surrounding air.

The temperature of the transistor case T will be slightly higher than the temperature of the heat sink which in
turn is slightly higher than the ambient temperature TA.

Ambient Temperature: The heat produced at the junction passed through the transistor case (metal or plastic
housing) are radiates to the surrounding air. The temperature of this air is known as the ambient temperature.
Class C power amplifier.

Class C power amplifier is a type of amplifier where the active element (transistor) conduct for less than one
half cycle of the input signal. Less than one half cycle means the conduction angle is less than 180° and its
typical value is 80° to 120°. The reduced conduction angle improves the efficiency to a great extend but causes
a lot of distortion. Theoretical maximum efficiency of a Class C amplifier is around 90%.

Due to the huge amounts of distortion, the Class C configurations are not used in audio applications. The most
common application of the Class C amplifier is the RF (radio frequency) circuits like RF oscillator, RF
amplifier etc where there are additional tuned circuits for retrieving the original input signal from the pulsed
output of the Class C amplifier and so the distortion caused by the amplifier has little effect on the final output.
Input and output waveforms of a typical Class C power amplifier is shown in the figure below.

From the above figure it is clear that more than half of the input signal is missing in the output and the output is
in the form of some sort of a pulse.

Output characteristics of Class C power amplifier.

 In the above figure you can see that the operating point is placed some way below the cut-off point in the DC
load-line and so only a fraction of the input waveform is available at the output.

Biasing resistor Rb pulls the base of Q1 further downwards and the Q-point will be set some way below the cut-
off point in the DC load line. As a result the transistor will start conducting only after the input signal amplitude
has risen above the base emitter voltage (Vbe~0.7V) plus the downward bias voltage caused by Rb. That is the
reason why the major portion of the input signal is absent in the output signal.

Inductor L1 and capacitor C1 forms a tank circuit which aids in the extraction of the required signal from the
pulsed output of the transistor. Actual job of the active element (transistor) here is to produce a series of current
pulses according to the input and make it flow through the resonant circuit.

Values of L1 and C1 are so selected that the resonant circuit oscillates in the frequency of the input signal.
Since the resonant circuit oscillates in one frequency (generally the carrier frequency) all other frequencies are
attenuated and the required frequency can be squeezed out using a suitably tuned load.

Harmonics or noise present in the output signal can be eliminated using additional filters. A coupling
transformer can be used for transferring the power to the load.

Advantages of Class C power amplifier.

 High efficiency.
 Excellent in RF applications.
 Lowest physical size for a given power output.
 Disadvantages of Class C power amplifier.

 Lowest linearity.
 Not suitable in audio applications.
 Creates a lot of RF interference.
 It is difficult to obtain ideal inductors and coupling transformers.
 Reduced dynamic range.

Applications of Class C power amplifier.

 RF oscillators.
 RF amplifier.
 FM transmitters.
 Booster amplifiers.
 High frequency repeaters.
 Tuned amplifiers etc.

CLASS D POWER AMPLIFIER.

Class D power amplifier is a type of audio amplifier were the power handling devices are operated as binary
switches. Since the power handling devices (MOSFETS) works as perfect binary switches, no time is wasted in
between the transition of stages and no power is wasted in the zero input condition. Class D power amplifiers
are much power efficient when compared to its predecessors like Class A, Class B and Class AB. Out of the list
the most efficient Class AB only have a maximum theoretical efficiency of 78.5%. In practical scenario with
real speakers as load, the efficiency of Class AB amplifiers can drop as low as 50%. At the same time a well
designed Class D amplifier with real speakers as load will never go below 90% in terms of efficiency. The
theoretical efficiency of a Class D amplifier is the ideal 100%.

An ideal binary switch will pass all current through it with no voltage across it when it is ON. When it is OFF
the entire voltage remains across it and no current will flow through it. This means no power is wasted across
the switching element which does the amplification, and it accounts for the unbelievable efficiency of the Class
D amplifier. Conversely, the class AB amplifier will always have some current passing through and some
voltage remaining across the switching element.

Higher efficiency means low thermal dissipation and it means it dissipates less power when compared to the
predecessors (The Class A, Class B, Class AB and Class D). Since Class D amplifiers are highly power
efficient, they require a smaller heatsink and a smaller power supply. Smaller heatsink and smaller power
supply reduces the size and it is the main advantage of a Class D amplifier. Class D amplifiers have become
very popular in applications like hand held audio devices, portable home theaters, mobile phones etc where all
in these cases output must be decent (in terms of power and fidelity) and the size must be as small as possible.
Class D is the only option for combining all these requirements together.

A typical Class D power amplifier consists of a sawtooth waveform generator, comparator (based on an
OPAMP), switching circuit, and a low pass filter. The block diagram of a Class D amplifier is shown in the
figure below.
Sawtooth waveform generator.

The sawtooth waveform generator generates a high frequency sawtooth waveform for sampling the input audio
signal. The frequency of the sawtooth waveform is usually selected 10 times the maximum frequency of interest
in the input audio signal.

Comparator.

The main job of the comparator is to digitize the input audio signal by mixing it with the chopping sawtooth
waveform. The result of this mixing will be a digital copy of the analog input signal. The low frequency
components of the digital signal will represent the input audio signal and the high frequency components of the
digital signal are of no interest. Input and output waveforms of the comparator are shown in the figure below.
 Switching circuit.

Even though the output of the comparator is a digital representation of the input audio signal, it doesn’t have the
power to drive the load (speaker). The task of the switching circuit is to provide enough current and voltage
gain which is essential for an amplifier. The switching circuit is generally designed around MOSFETs. Input
and output waveforms of the switching circuit are shown in the figure below.

Low pass filter.

The task of the low pass filter is to filter out useful low frequency components from the output of the switching
circuit. The output of the low pass filter will be a scaled replica of the input audio signal. Negative feedback
loops are often included in between the low pass filter output and the comparators audio input in order to fight
the errors.

Advantages of Class D amplifier.

 Low heat dissipation.

 Reduced size and weight.
 High power conversion efficiency. Almost all power drawn is supplied to the load.

Disadvantages of Class D amplifier.

 Requires a very clean and stable power supply.

 The high frequency response is dependent on the loudspeaker impedance.