Lecture - 24: Cascade Amplifiers

Example - 1

Determine the current and voltage gains for the two-stage capacitor-coupled amplifier shown in fig. 1.

Fig. 1

Solution:

We develop the hybrid equivalent circuit for the multistage amplifier. This equivalent is shown in fig. 2. Primed variables denote output stage quantities and unprimed variables denote input stage quantities.

Fig. 2

Calculations for the output stages are as follows

For the input stage,

The input resistance is determined as:

The current gain, Ai, can be found by applying the equations derived earlier, where the first stage requires using the correct value for Rload derived form the value of Rin to the next stage.

Alternatively, we analyze fig. 2 by extracting four current dividers as shown in fig. 3.

Fig. 3

The current division of the input stage is

The output of the first stage is coupled to the input of he second stage in fig. 3(b). The input resistance of the second stage is

The current in R'in is iload and is given by

Again, i load is current-divided at the input to the second stage. Thus,

The output current is found from fig. 3(c):

The current gain is then

Ai =927

Now using the gain impedance formula, we find the voltage gain:

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