# What is the input impedance of these two?

#### sebholm

Joined Dec 18, 2020
20

We only work with ideal, so thought it was infinite and that no current runs into it.

So why is impedance of the inverting amplifier Zin=Vin/i1=R1(Found it in my book)?

And what is the input impedance of the non-inverting op amp?

#### crutschow

Joined Mar 14, 2008
26,044
In the inverting amp, R1 goes to the virtual ground input (due to the negative feedback from R2), so its input impedance is the same as if R1 was connected to circuit ground, i.e. equal to R1.

The non-inverting amp input goes directly to the op amp plus input, which ideally has what impedance?

#### DickCappels

Joined Aug 21, 2008
6,806
Ideally no current flows into the inverting input as you pointed out but the smallest change in the input voltage results in feedback from the output that results in the virtual ground that crutschow mentioned, so called because the inverting input staying at the same voltage all the time regardless of the current that goes into that node. In the case of ideal opamps anyway.

#### Ian0

Joined Aug 7, 2020
1,172
You can easily work out the current flowing in Rin.
The total voltage across the two resistors is Vin-(-Vout).
The total resistance is R2+R1
So you can calculate the current flowing, both in R1 and in R2.
Then you can calculate the input impedance as Vin/Iin as you said.