Electronics and Communication Engineering - Analog Electronics - Discussion


The input impedance of op-amp circuit of figure is

[A]. 120 k ohm
[B]. 110 k ohm
[C]. infinity
[D]. 10 k ohm

Answer: Option D


Due to the presence of virtual ground at input, the resistance in the series path of input of inverting amplifier is input impedance.

Chenelyn Cedron said: (Aug 3, 2016)  
Let's try some math and the Miller's Theorem to solve this one.

First, the closed loop gain A{cl} of a negative feedback is:

A{cl} = (V{out})/(V{in}) = (-Rf)/(Ri)

Where Rf is the feedback resistance and Ri is the input resistance. It should be clear that Rf is equal to Z.

Using Miller's theorem,
Z{equivalent_input} = (Z)/(1 - A) = (Rf)/(1 - ((-Rf)/(Ri)).
Z{equivalent_input} = ((Ri)(Rf))/(Ri + Rf).
Z{equivalent_input} = ((10k)(100k))/(100k + 10k).
Z{equivalent_input} = 9.09k.

Letter D is the nearest choice.

Jyothi R said: (Oct 4, 2016)  
By virtual ground concept.

vs - (10k * is) = 0;.

So, vs/is = input impedence = 10k ohm.

Hindosh said: (Aug 2, 2017)  
Thank you both for the given explanation.

Laksh said: (Mar 21, 2018)  
Wonderful explanation, thank you all.

Pankhuri Saxena said: (Apr 4, 2018)  
Well said, thanks @ChenelynCedron.

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