Op-amp resister circuit analysis

[joel amos]

Homework Statement

Given the ideal op-amp, determine the resistor R₂ and the current I₂ where V_in=500mV, V_out = 2.5 V, and R₁=5kV.

Homework Equations

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

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=V_out/R_eq = 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.

 

[psparky]

You are 100% correct sir.

V+ = V- in ideal opamps...

Nice job.

 

[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.