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Lecture 10 transistor as an amplifier in applies physics

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LECTURE TRANSISTOR AS AN AMPLIFIER
For a transistor to act as an amplifier, it should be properly biased. We will discuss the need for proper biasing in the next
chapter. Here, let us focus how a transistor works as an amplifier.
Transistor Amplifier
A transistor acts as an amplifier by raising the strength of a weak signal. The DC bias voltage applied to the emitter base
junction, makes it remain in forward biased condition. This forward bias is maintained regardless of the polarity of the
signal. The below figure shows how a transistor looks like when connected as an amplifier.
The low resistance in input circuit, lets any small change in input signal to result in an appreciable change in the output.
The emitter current caused by the input signal contributes the collector current, which when flows through the load resistor
R
L
, results in a large voltage drop across it. Thus a small input voltage results in a large output voltage, which shows that
the transistor works as an amplifier.
Example
Let there be a change of 0.1v in the input voltage being applied, which further produces a change of 1mA in the emitter
current. This emitter current will obviously produce a change in collector current, which would also be 1mA.
A load resistance of 5kΩ placed in the collector would produce a voltage of
5 kΩ × 1 mA = 5V
Hence it is observed that a change of 0.1v in the input gives a change of 5v in the output, which means the voltage level of
the signal is amplified.
Performance of Amplifier
As the common emitter mode of connection is mostly adopted, let us first understand a few important terms with reference
to this mode of connection.
Input Resistance
As the input circuit is forward biased, the input resistance will be low. The input resistance is the opposition offered by the
base-emitter junction to the signal flow.
By definition, it is the ratio of small change in base-emitter voltage (ΔV
BE
) to the resulting change in base current (ΔI
B
) at
constant collector-emitter voltage.
Input resistance, Ri=ΔVBEΔIBRi=ΔVBEΔIB

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Where R
i
= input resistance, V
BE
= base-emitter voltage, and I
B
= base current.
Output Resistance
The output resistance of a transistor amplifier is very high. The collector current changes very slightly with the change in
collector-emitter voltage.
By definition, it is the ratio of change in collector-emitter voltage (ΔV
CE
) to the resulting change in collector current (ΔI
C
) at
constant base current.
Output resistance = Ro=ΔVCEΔICRo=ΔVCEΔIC
Where R
o
= Output resistance, V
CE
= Collector-emitter voltage, and I
C
= Collector-emitter voltage.
Effective Collector Load
The load is connected at the collector of a transistor and for a single-stage amplifier, the output voltage is taken from the
collector of the transistor and for a multi-stage amplifier, the same is collected from a cascaded stages of transistor circuit.
By definition, it is the total load as seen by the a.c. collector current. In case of single stage amplifiers, the effective
collector load is a parallel combination of R
C
and R
o
.
Effective Collector Load, RAC=RC//RoRAC=RC//Ro
=RC×RoRC+Ro=RAC=RC×RoRC+Ro=RAC
Hence for a single stage amplifier, effective load is equal to collector load R
C
.
In a multi-stage amplifier (i.e. having more than one amplification stage), the input resistance R
i
of the next stage also
comes into picture.
Effective collector load becomes parallel combination of R
C
, R
o
and R
i
i.e,
Effective Collector Load, RAC=RC//Ro//RiRAC=RC//Ro//Ri
RC//Ri=RCRiRC+RiRC//Ri=RCRiRC+Ri
As input resistance R
i
is quite small, therefore effective load is reduced.
Current Gain
The gain in terms of current when the changes in input and output currents are observed, is called as Current gain. By
definition, it is the ratio of change in collector current (ΔI
C
) to the change in base current (ΔI
B
).
Current gain, β=ΔICΔIBβ=ΔICΔIB
The value of β ranges from 20 to 500. The current gain indicates that input current becomes β times in the collector
current.
Voltage Gain
The gain in terms of voltage when the changes in input and output currents are observed, is called as Voltage gain. By
definition, it is the ratio of change in output voltage (ΔV
CE
) to the change in input voltage (ΔV
BE
).
Voltage gain, AV=ΔVCEΔVBEAV=ΔVCEΔVBE
=Changeinoutputcurrent×effectiveloadChangeininputcurrent×inputresistance=Changeinoutputcurrent×effectiveloadCh
angeininputcurrent×inputresistance
IC×RACΔIB×RiICΔIB×RACRi=β×RACRi=ΔIC×RACΔIB×Ri=ΔICΔIB×RACRi=β×RACRi
For a single stage, R
AC
= R
C
.
However, for Multistage,
RAC=RC×RiRC+RiRAC=RC×RiRC+Ri

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LECTURE TRANSISTOR AS AN AMPLIFIER For a transistor to act as an amplifier, it should be properly biased. We will discuss the need for proper biasing in the nex t chapter. Here, let us focus how a transistor works as an amplifier. Transistor Amplifier A transistor acts as an amplifier by raising the strength of a weak signal. The DC bias voltage applied to the emitter base junction, makes it remain in forward biased condition. This forward bias is maintained regardless of the polarity of the signal. The below figure shows how a transistor looks like when connected as an amplifier. The low resistance in input circuit, lets any small change in input signal to result in an appreciable change in the output. The emitter current caused by the input signal contributes the collector current, which when flows through the load resistor RL, results in a large voltage drop across it. Thus a small ...
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