Input Resistance with Feedback:

fig. 1, shows a voltage series feedback with the OPAMP equivalent circuit.

In this circuit R_i is the input resistance (open loop) of the OPAMP and R_if is the input resistance of the feedback amplifier. The input resistance with feedback is defined as

Since AB is much larger than 1, which means that R_if is much larger that R_i. Thus R_if approaches infinity and therefore, this amplifier approximates an ideal voltage amplifier.

Output Resistance with Feedback:

Output resistance is the resistance determined looking back into the feedback amplifier from the output terminal. To find output resistance with feedback R_f, input v_in is reduced to zero, an external voltage V_o is applied as shown in fig. 2.

Fig. 2

The output resistance (R_of ) is defined as

This shows that the output resistance of the voltage series feedback amplifier is ( 1 / 1+AB ) times the output resistance R_o of the op-amp. It is very small because (1+AB) is very large. It approaches to zero for an ideal voltage amplifier.

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