G'day,

I'm trying to calculate the input resistance of an op-amp circuit. I already know that an op-amp by itself has an input resistance of around 2 mega-ohms.

I have a 50 kilo-ohm resistor, hooked up to a voltage follower (LM741 with the output fed back to the negative input - giving no voltage gain), with the output from this hooked up to an inverting amplifier that looks like this:

Where R1 = 22 kilo-ohms and R2 = 220 kilo-ohms, this gives a voltage gain of -0.1 (-R1/R2).

I can't find any info in my text books aside from that the value itself is around 2 Mega-ohms, and that inverting amplifiers can reduce the input impedance. However I'm not sure how to go about calculating this.

Thanks.

 

Gain=-R2/R1

Thus, if R2=220K and R1=22K then the gain is -10.

The inverting input is very close to 0V.

Thus the input current is:

Iin=Vin/R1

The input resistance is:

Rin=Vin/Iin=Vin/(Vin/R1)=R1