Determine the input and output resistance of the following circuit, assuming that the operational amplifier is a 741.
From the material in Section 16.3.1, we know that this circuit represents a noninverting amplifier with a gain of 20.
The open-loop gain of a 741 is typically 2 × 10^{5} and the closed-loop gain of this circuit is 20. Therefore (1 + AB) = (2 × 10^{5})/20 = 10^{4} .
The output resistance of a 741 is typically about 75 Ω and in this circuit the output voltage is fed back. Thus the feedback reduces the output resistance by a factor of (1 + AB) which becomes 75/10^{4} = 7.5 mΩ .
The input resistance of a 741 is typically about 2 MΩ and in this circuit a feedback voltage is subtracted from the input voltage. Thus, the feedback increases the input resistance by a factor of (1 + AB), which becomes 2 × 10^{6} × 10^{4} = 2 × 10^{10} = 20 GΩ .