8 - Bipolar Junction Transistor

LECTURE-8
Transistors (CLO-2)
Introduction BJT
BJT Operation
Construction Regions of BJT
Basic BJT
Operation
BJT as an
Amplifier Contents
BJT Transistor
Configurations Currents
Transistor DC BJT VI
Model Characteristics
Introduction
• A transistor is a semiconductor device formed by
adding a third doped element to a crystal diode,
resulting in two PN junctions.
• The name "transistor" is a combination of

"transfer" and "varistor," reflecting its mode of
operation during the early days of electronics
development.
Bipolar Junction Transistor
◦ A Bipolar Junction Transistor (BJT) is a solid-state current-controlled device
with three terminals: emitter, collector, and base.
◦ The flow of current from emitter to collector is regulated by the current applied
to the base.
◦ BJTs consist of two PN junctions formed by sandwiching either P-type or N-
type semiconductors between opposite types, resulting in two types: NPN and
PNP.
◦ The term "bipolar" signifies the utilization of both holes and electrons as
current carriers.
◦ The junction between the base and emitter is called the base-emitter junction,
and the one between the base and collector is the base-collector junction.
◦ The base region is lightly doped and thin compared to the heavily doped
emitter and moderately doped collector regions.
Schematic Symbol of BJT
Operating Regions of BJT
◦ The transistor’s ability to change between two states enables it

to have two basic functions: “switching” (digital electronics)
or “amplification” (analog electronics).
◦ The bipolar transistors have ability to operate within three

different regions:
◦ Active Region – the transistor operates as an amplifier
◦ Saturation – the transistor is “Fully-ON” operating as a
switch and Ic = I(saturation)
◦ Cut-off – the transistor is “Fully-OFF” operating as a switch
and Ic = 0
Basic BJT Operation
◦ To ensure proper operation as an amplifier, a BJT requires correct biasing of its two PN
junctions with external DC voltages. In the depicted arrangement, both NPN and PNP
BJTs are biased with forward bias on the base-emitter (BE) junction and reverse bias on
the base-collector (BC) junction, termed forward-reverse bias.
◦ The heavily doped n-type emitter region contains a high density of free electrons, which
readily diffuse through the forward-biased BE junction into the lightly doped and thin p-
type base region. The base, with a low density of holes, allows a small portion of injected
electrons to recombine with holes, creating hole current as valence electrons traverse the
base and return to the emitter.
◦ The majority of free electrons entering the base region do not recombine due to its
thinness. These electrons move towards the reverse-biased BC junction, where they are
drawn into the collector region by the positive collector supply voltage. Within the
collector region, electrons flow into the external circuit and return to the emitter along
with the base current.
◦ The emitter current slightly exceeds the collector current due to the small amount of base
current that branches off from the total current injected into the base region from the
emitter.
BJT AS AN AMPLIFIER
◦ A transistor functions as an amplifier by increasing the strength of a weak signal.
The weak signal is applied between the emitter-base junction, and the amplified
output is obtained across the load RC connected in the collector circuit.
◦ To ensure faithful amplification, the input circuit must remain forward-biased. This
is achieved by applying a DC bias voltage (VEE) in the input circuit, maintaining
forward bias regardless of the signal polarity.
◦ Small changes in signal voltage cause appreciable changes in emitter current due to
the input circuit's resistance. This, in turn, leads to corresponding changes in
collector current via transistor action. The collector current flowing through the high
load resistance RC generates a significant voltage across it.
◦ Consequently, a weak signal applied to the input circuit emerges as an amplified
signal in the collector circuit. This process illustrates how a transistor acts as an
amplifier.
Bipolar Transistor
Configuration
◦ As the Bipolar Transistor is a three-terminal device, there
are basically three possible ways to connect it within an
electronic circuit with one terminal being common to both
the input and output. Each method of connection
responding differently to its input signal within a circuit as
the static characteristics of the transistor vary with each
circuit arrangement.
◦ Common Base Configuration – has Voltage Gain but no
Current Gain.
◦ Common Emitter Configuration – has both Current and
Voltage Gain.
◦ Common Collector Configuration – has Current Gain
but no Voltage Gain.
TRANSISTOR COMMON BASE
CIRCUIT CONFIGURATION
TRANSISTOR COMMON EMITTER
TRANSISTOR COMMON COLLECTOR
GENERALISED
CHARACTERISTICS
OF THE DIFFERENT
TRANSISTOR
Transistor Currents
◦ The directions of the currents in an NPN transistor and its schematic symbol
are as shown in Figure
In transistor circuits, such as the common emitter (CE) configuration, the relationship
between the currents flowing into and out of the transistor is crucial. When a transistor is
connected to DC bias voltages, the base-emitter junction is forward-biased by a voltage
VBB, while the base-collector junction is reverse-biased by a voltage VCC.
In the CE configuration, the emitter current (IE) is the sum of the collector current (IC)
and the base current (IB), expressed as IE=IC+IB. The load resistance (RL), connected
in series with the collector, results in a large current gain for this configuration, defined
as the ratio IB/IC, represented by the Greek symbol Beta (β). Typical values for Beta
range between 20 and 200 for most general-purpose transistors.
Additionally, the ratio IE/IC, known as Alpha (α), is always less than unity. The
relationship between these parameters allows for a small change in the base current (IB)
to control a much larger change in the collector current (IC). Essentially, for a transistor
with a Beta value of, say, 100, one electron flows from the base terminal for every 100
electrons flowing between the emitter-collector terminals.
Thus, the overall current gain of the transistor can be mathematically expressed by
combining the expressions for Alpha (α) and Beta (β).
Common Collector Current Gain
Example:
Determine the dc current gain βDC and the emitter current IE for a transistor
where IB =50 mA and IC =3.65 mA.
Transistor DC Model
◦ The input circuit is a forward-biased diode through which there is base current.
◦ The output circuit is a dependent current source (diamond- shaped element) with
a value that is dependent on the base current, IB, and equal to βDCIB
BJT Circuit Analysis
Consider the basic transistor bias circuit configuration
in Figure. Three transistor dc currents and three dc
voltages can be identified
◦IB: dc base current

◦IE: dc emitter current
◦IC: dc collector current
◦VBE: dc voltage at base with respect to emitter
◦VCB: dc voltage at collector with respect to base
◦VCE: dc voltage at collector with respect to emitter
The base-bias voltage source, VBB, forward-biases the base-
emitter junction, and the collector-bias voltage source,VCC,
reverse-biases the base-collector junction.
When the base-emitter junction is forward-biased, it is like a

forward-biased diode and has a nominal forward voltage
drop of
Since the emitter is at ground (0 V), by Kirchhoff’s voltage

law, the voltage across RB is
Example:
A transistor is connected in common emitter (CE) configuration in which collector

supply is 8V and the voltage drop across resistance RC connected in the collector
circuit is 0.5V. The value of RC=800Ω. If α =0.96, determine:
i. Collector-Emitter voltage
ii. Base current
Transistor VI
characteristics
Three different types of voltages are involved in the
description of transistors and transistor circuits.
◦Transistor supply voltages: VCC ,VBB
◦Transistor terminal voltages: VB ,VC ,VE
◦Voltages across transistor junctions: VBE ,VCE , VCB
◦All these voltages and their polarities are shown in the
next slide
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(IB) as
shown on the circuit
Cutoff region
Base-emitter junction is reverse biased. No current flow
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(sat) is normally assumed 0.2V

Active region:
• Base-emitter junction forward biased
• Collector-base junction Reverse biased
• Control, IC = βIB (can be seen from Figure there is a
small slope of IC with VCE VBE <VCE<VCC
Breakdown region:
IC and VCE exceed specifications damage to the transistor
Chapter-8:Transistors
Principles of Electronics by VK Mehta,
11th Ed.
http://fourier.eng.hmc.edu/e84/lectures/ch
4/node3.html
Related
Readings and https://www.electronics-tutorials.ws/trans
References istor/tran_1.html
https://components101.com/articles/unde
rstanding-bjt-transistor-and-how-to-use-it
-in-your-circuit-designs

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LECTURE-8

• The name "transistor" is a combination of

◦ The transistor’s ability to change between two states enables it

◦ The bipolar transistors have ability to operate within three

◦IB: dc base current

When the base-emitter junction is forward-biased, it is like a

Since the emitter is at ground (0 V), by Kirchhoff’s voltage

A transistor is connected in common emitter (CE) configuration in which collector

VCE(sat) is normally assumed 0.2V

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