# Resistance input of amplifier

1. Feb 21, 2013

### axcelenator

I have an un ideal OA aplifier(Amax=-9.975).
I have to evaluate its Rinput--> it has to be something like 10*10^10 Ω

I need to put a potenciometr after Vin and to "play" with it when I get half of Vout/vin.
when do I need to put it?
and what is my voltage equations? thanks.

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2. Feb 21, 2013

### Staff: Mentor

Your text does not match your figure, as far as I can see.

What do you mean when you say "Rin has to be something like 10*10^10 Ω"? In the figure, Rin1 is given as 400 kΩ.

3. Feb 22, 2013

### axcelenator

the resistance named Rin is 400K.
But: the input resistance of the circuit is not 400. thats what I have to find

4. Feb 22, 2013

### Staff: Mentor

So you'll redraw the circuit, this time replacing the triangle with a 400k resistor to ground at the input, and show a voltage source with a gain of 20000 and so on. Surround it with the external circuit resistances, then analyze that.

I don't know anything about putting a potentiometer somewhere and playing with it. Maybe that's something you do in the lab?

5. Feb 22, 2013

### rude man

Given the actual gain of the circuit = 9.975 vs. the ideal op amp gain of 10.000 you can compute the open-loop gain of the amplifier.

You then compute the input current for a given inut voltage to get the circuit's input impedance.

You can then put a potentiometer in series with the 1K input resistor and determine the value needed to halve the output.

That is, if I understood what you are trying to say, which is dubious.