This tutorial actually meant to explain the practical importance of the well known transistor theories, characteristics plots and make use of them to design a high gain amplifier circuit using a single transistor and minimum number of other components.
We will analyze certain transistor characteristics and theories, summaries them, and by the end of this tutorial we will be designing a high gain amplifier by our own.
Common emitter configuration:
Let us consider a common emitter, fixed biased, class A amplifier circuit
Fig. 1: Circuit Diagram of Class A Amplifier
The hybrid equivalent of the same circuit is given below
Fig. 2: Hybrid Equivalent Circuit Diagram of Class A Amplifier
Here hi is the impedance found by any device connected at the input, hoe is the admittance at the output.
( hfe * ib ) forms a current dependent current source where, hfe is the forward current transfer ratio, and ib is the base current. ( hre * Vce ) forms a voltage dependent voltage source, the value is too low that we can ignore it. hre is the reverse voltage ratio.
Hence we can reduce the equivalent circuit as the following:
Fig. 3: Equivalent Circuit Diagram of class A Amplifier
From the figure 2 and 3, we can observe that BJT is a current driven device.
Or, info 1.
Change in input current causes a corresponding change in the output current
We can also observe from the above figure that, the output current ( Collector current ) is directly proportional to the input current ( base current ).
Or we can explain the relation using the equation
Ic = hfe * ib ………………………………………………………………….….. eq 1.
Designing a high voltage gain amplifier
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