Effect of the Load

The analysis so far has considered an unconnected or open-circuit amplifier. We now add in the load resistance R_L and see what effect this has. Remember that R_L models the load that is driven by the amplifier.

The load resistor R_L has no effect on the DC bias because of the capacitor C_C, however it does effect the AC signal path and should be added in parallel to R_C. In fact we define

$\begin{displaymath}R_{CL} = R_C \parallel R_L \end{displaymath}$

and here this is R_CL=1.21 k $\Omega$ . See Figure 114.

**Figure:** AC equivalent circuit with hybrid- $\pi$ model for the circuit of Figure 110 including R_L.
$\begin{figure} \begin{center} \epsfig{file=images/bjtimg20a.eps}\end{center}\end{figure}$

The calculations continues as before, with the change

v_out = v_c = -i_c R_CL .

Consequently the gain is now given by

$\begin{displaymath}A_{in,out} = - \frac{\beta R_{CL}}{r_\pi} = - \frac{\beta R_{CL}}{(\beta + 1)r_e} \approx - \frac{R_{CL}}{r_e} . \end{displaymath}$

(106)

In this example we have

A_in,out = - 177 .

Consequently the load can have a significant effect on the amplifier gain, and should be taken into account unless R_L is very large relative to r_out.

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