The document discusses different configurations and operating modes of bipolar junction transistors (BJTs). It describes the common emitter (CE), common base (CB), and common collector (CC) configurations. For each configuration, it provides expressions for the collector current (IC) and discusses characteristics like current gain, voltage gain, and input/output resistances. It also covers biasing circuits, operating points, transistor regions of operation, and fixed bias, emitter bias, and voltage divider bias configurations.

1. Bipolar Junction
Transistor:
Different Configurations
Arpan Deyasi
Dept of ECE, RCCIIT, Kolkata, India
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Electrical Characteristics of BJT: Input
IB
VBE
VCE3
VCE2
VCE1 IE
VBE
VBC3
VBC2
VBC1
NPN
PNP

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VCE
IC
IB1
IB2
IB3
Electrical Characteristics of BJT: Output
NPN
VCE (-ve axis)
IC (-ve axis)
-IB1
-IB2
-IB3
PNP

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Operating Modes
The four transistor operation modes are:
Saturation -- The transistor acts like a short circuit.
Current freely flows from collector to emitter.
Cut-off -- The transistor acts like an open circuit. No
current flows from collector to emitter.
Active -- The current from collector to emitter
is proportional to the current flowing into the base.
Reverse-Active -- Like active mode, the current is
proportional to the base current, but it flows in reverse.
Current flows from emitter to collector.

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OperatingModes

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Different types of biasing
Common-Emitter (CE): current gain and voltage gain
Common-Base (CB): no current gain but voltage gain
Common-Collector (CC): current gain but no voltage gain

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CE configuration

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Characteristics of CE configuration
This configuration provides good current gain and voltage gain
Keeping VCE constant, with a small increase in VBE, IB increases
rapidly than in CB configurations
For any value of VCE above knee voltage, IC is approximately equal
to βIB
As the input resistance is of very low value, a small value of VBE is
enough to produce a large current flow of IB
Output resistance of CE circuit is less than that of CB circuit
CE configuration is usually used for bias stabilization methods
and audio frequency applications
current gain in CEconnection is very high. This is the reason this
circuit connection is mostly used in all transistor applications

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E B CI I I= +
Expression of IC in CE configuration
C E CBOI I Iα= +
( )C C B CBOI I I Iα= + +
(1 )C B CBOI I Iα α− = +
1
(1 ) (1 )
C B CBOI I I
α
α α
= +
− −

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Expression of IC in CE configuration
(1 )
C B CEOI I I
α
α
= +
−
C B CEOI I Iβ= +

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In CE configuration, by keeping the base current IB constant,
if VCE is varied, IC increases nearly to 1v of VCE and stays
constant thereafter.
This value of VCE up to which collector current IC changes
with VCE is called the Knee Voltage.
The transistors while operating in CE configuration, they are
operated above this knee voltage.
Knee Voltage

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CB configuration

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Characteristics of CB configuration
This configuration provides voltage gain but no current gain
Being VCB constant, with a small increase in the VEB, IE gets increased
IE is independent of Collector voltage VCB
VCB can affect IC only at low voltages, when VEB is kept constant
As the input resistance is of very low value, a small value of VEB is
enough to produce a large flow of IE
As the output resistance is of very high value, a large change
in VCB produces a very little change in IC
This Configuration provides good stability against increase in
temperature
CB configuration is used for high frequency applications

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E B CI I I= +
Expression of IC in CB configuration
C E CBOI I Iα= +
( )C C B CBOI I I Iα= + +
(1 )C B CBOI I Iα α− = +
1
(1 ) (1 )
C B CBOI I I
α
α α
= +
− −

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CC configuration

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The current gain in CC configuration is same as in CE
configuration
The voltage gain in CC configuration is always less than 1
In CC configuration, the input resistance is high and the output
resistance is low
The input and output signals are in phase
This configuration works as non-inverting amplifier output
This circuit is mostly used for impedance matching. That means,
to drive a low impedance load from a high impedance source
Characteristics of CC configuration

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Expression of IC in CC configuration
E B CI I I= +
C E CBOI I Iα= +
E B E CBOI I I Iα= + +
(1 )E B CBOI I Iα− = +

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Expression of IC in CC configuration
1 1
(1 ) (1 )
E B CBOI I I
α α
= +
− −
( 1) ( 1)E B CBOI I Iβ β= + + +
( 1) ( 1)C E B CBOI I I Iβ β≅ = + + +

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What is biasing circuit?
Application of dc voltages to establish a fixed level of current
and voltage
Purpose of the DC biasing circuit
To turn the device “ON”
To place it in operation in the region of its characteristic
where the device operates most linearly
Proper biasing circuit which it operate in linear region and
circuit have cantered Q-point or midpoint biased
Improper biasing cause Improper biasing cause
[i] „Distortion in the output signal
[ii] „Produce limited or clipped at output signal

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Operating Point Operating point is a fixed point on the
characteristics, it is also called the
quiescent point (abbreviated Q-point)

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1. Linear-region operation
BE junction forward-biased
BC junction reverse-biased
2. Cut-off-region operation
BE junction reverse-biased
BC junction reverse-biased
3.Saturation-region operation
BE junction forward-biased
BC junction forward-biased
Transistor (NPN) Regions Operation

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Load Line

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Load Line

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Load Line

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Load Line

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Fixed -Bias Configuration

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Emitter -Bias Configuration
The addition of the emitter
resistor to the dc bias of the BJT
provides improved stability

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Voltage-Divider Configuration