Question 16.6: 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 16.3.2, we know that this circuit represents an inverting amplifier with a gain of -20.
The open-loop gain of a 741 is typically 2 × 10^{5} and the magnitude of the closedloop 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 current is subtracted from the input current. Thus, the feedback decreases the input resistance. In this case, the input is connected to a virtual earth point by the resistance R_{2} and so the input resistance is equal to R_{2} , which is 1 kΩ.