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Operational Amplifiers - Technical MCQs

Q1:

Input impedance [Zin(I)] of an inverting amplifier is approximately equal to:

A Ri

B Rf + Ri

C
∞

D
Rf – Ri
 

ANS:A - Ri

The input impedance, as always, is the impedance to ground for an input signal. The - input is at the same voltage as the + input which is ground. We can thus consider the - input to be at what is referred to as virtual ground. The input impedance for the overall amplifier will be simply Ri.

Zin = Ri.