Op-amp resister circuit analysis

1. Mar 9, 2015

joel amos

1. The problem statement, all variables and given/known data
Given the ideal op-amp, determine the resistor R2 and the current I2 where Vin=500mV, Vout = 2.5 V, and R1=5kV.

2. Relevant equations
No current passes through op-amps, the op-amp tries to make V+ and V- equal.
Ohm's law, voltage divider equation

3. The attempt at a solution
Since V+ is 500mV, I assumed that the node between R1 and R2 must also be 500mV, as it is connected to V-. So 500mV = 2.5V(R2/(5000Ω+R2)). I got R2 to be 1,250Ω.
Then I found I2=Vout/Req = 2.5V/(1250Ω + 5000Ω) = .4mA.
However, I am unsure because I am not sure if I did the part with the op-amp right; namely, the node between the resistors being set to 500mV.

2. Mar 9, 2015

psparky

You are 100% correct sir.

V+ = V- in ideal opamps...

Nice job.

3. Mar 9, 2015

LvW

Yes - and for the future you can use the general closed-loop non-inverting gain formula (assuming an ideal opamp): Acl=1+R1/R2.