Question 16.7: Determine the input and output resistance of the following c......
Determine the input and output resistance of the following circuit, assuming that the operational amplifier is a 741.
From the material in Section 16.4.1, we know that this circuit represents a noninverting amplifier with a gain of 1 (a unity gain buffer amplifier).
The open-loop gain of a 741 is typically 2 × 10^{5} and the closed-loop gain of this circuit is 1. Therefore (1 + AB) = (2 × 10^{5})/1 = 2 × 10^{5} .
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/(2 × 10^{5}) ≈ 400 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}) × (2 × 10^{5}) = 4 × 10^{11} = 400 GΩ .