Small Signal Output and Input Resistance

In Lab 2 we showed that the small signal output resistance of an emitter follower circuit is given by

$$R_o=\frac{1}{g_m}+\frac{R_S'}{\beta}\vert\vert R_E$$ (127)

where R_S' is the output or source resistance of the previous stage of the circuit. In our circuit, R_S'=R_C2, so the output resistance of the circuit is given by  

$$R_o=\left(\frac{1}{g_{m4}}+\frac{R_{C2}}{\beta}\right)\vert\vert R_{E4}$$ (128)

The input resistance of the circuit is identical to that of the diff-amp in the previous section of this lab, which is given by  

$$R_{in}=2r_{\pi}$$ (129)

With the small signal input and output resistances, as well as the voltage gain ascertained, we can draw an equivalent circuit for the amplifier.