Input impedance of an inverting amplifier with R1 = 0

[lat901]

Hi,

In a general inverting amplifier built using an ideal op-amp, the input impedance is given by R1 and the gain by -Rf/R1. What would be the input impedance of this circuit if the resistance R1 is shorted ? I am told that the input impedance now would be given by the miller effect and becomes Rf/(1+A).

I am confused. Could any one please tell me what exactly is happening here ?


Thanks
latha

 

[goldsmith]

Hey gentleman!
Hi
i think that in this mode , your input impedance will zero.
Best wishes
Goldsmith

 

[LvW]

Hi Latha and Goldsmith,

it happens not very often - however, both of you are right.
Indeed, the input impedance of this configuration (it is no working amplifier anymore) is Rf/(1+A) with A=opamp open-loop gain (neglecting the finite input iresistance of the inv. terminal itself).
This implies, of course, that for infinite A (ideal opamp) the input impedance tends to zero.