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Effect of the Bypass Capacitor

 

The emitter resistor R_E is used to enhance the stability of the DC bias with respect to variations in the transistor parameters, and if it is not shorted out to AC signals it can have an effect on the AC gain. So let's now remove the bypass capacitor C_E, and reconsider the above analysis.

The main change is that r_e gets replaced by r_e+R_E, and $r_\pi$ gets replaced by $r_\pi + (\beta+1)R_E$.

So the gains become

$$A_{in,out} = - \frac{\beta R_C}{(\beta + 1)(r_e+R_E)} \approx - \frac{R_C}{r_e+R_E}$$ (108)

and

$$A_{s,out} = \frac{r_{in} \beta R_{CL}}{r_\pi(r_{in} + R_S)}$$ (109)

with r_in now given by

$$r_{in} = R_{B1} \parallel R_{B2} \parallel (\beta+1)(r_e+R_E)$$

These calculate to be r_in = 11.81 k$\Omega$ and

$$A_{in,out} = -1.2, \ \ \ A_{s,out} = -0.65 .$$

Input resistance has increased, but the gain has decreased.

It can be seen that the bypass capacitor has a very significant effect on gain.

Intermediate situations are possible with R_E split into two R_E1+R_E2, with only one of these bypased, see Hardware Lab 6.

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