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Effect of the Source

The signal applied to the input of the amplifier comes from some source, which we model by a Thevenin equivalent, as in Figure 110. So we now consider the gain from v_s to v_out:

$$A_{s,out} = \frac{v_{out}}{v_s} .$$

Now

$$v_{in} = \frac{r_{in} v_s}{r_{in} + R_S}$$

and so

$$A_{s,out} = \frac{r_{in} }{r_{in} + R_S} A_{in,out} = - \frac{r_{in} \beta R_{CL}}{r_\pi(r_{in} + R_S)}$$ (107)

which evaluates to be

A_s,out = -26.5 .

This substantial change in gain is due to the fact that R_S is much higher than r_in in this example (think of voltage divider action). A large r_in is desirable.

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