**Effect of Negative Feedback on Output Impedance:**

Just as high input impedance is advantageous to an amplifier, so is low output impedance. With lower output impedance, the amplifier is better suited to drive a low impedance load. Such a desirable characteristic can be had by employing negative feedback. The Effect of Negative Feedback on Output Impedance of an amplifier is explained below.

**Voltage-Series Feedback:**

The voltage-series feedback circuit given in Fig. 19.13 provides sufficient circuit detail for determining output impedance with feedback. The input terminals are short circuited so that – V_{f} is now the only input voltage to the amplifier. Now a voltage source V_{out} is applied at the output terminals so that l_{out} current is drawn from the applied source.

Now

Thus, series voltage negative feedback reduces the output impedance of an amplifier by a factor (1 + βA). This is the same factor by which voltage gain is reduced.

**Current-Shunt Feedback:**

The output impedance with current shunt feedback can be determined by disconnecting the load resistance R_{L}, applying a signal V to the output terminals with current I_{s} removed. Now output current I is measured. The ratio of V to I provides the output impedance of circuit [Fig. 19.10 (d)].

Since current in the output circuit due to feedback opposes the current I, due to applied voltage, V

So

and output impedance,

So, output impedance is increased by the factor (1 + βA).

**Current-Series Feedback:**

The output impedance with current-series feedback can be determined by disconnecting load resistance R_{L}, applying a signal V to the output with V_{s} shorted out. The ratio of V to I provides the output impedance of the circuit. Now

or Output impedance,

So, the output impedance is increased by the factor (1 + βA).