# Op amp: Find Vo in terms of Vg and A

1. Oct 10, 2012

### ElijahRockers

1. The problem statement, all variables and given/known data

Find Vo as a function of Vg and the open-loop gain A.

Attempt

I'm not sure what open-loop gain means.

I redrew the circuit so it made more sense to me, but I'm still not getting it. It seems that Vo is the voltage across both RL and the 4k resistor.

The voltage across the 4k resistor is equal the voltages at the input terminals.

Aside from these bits of information I don't really know how this thing operates. Should the current going into the input terminals still be 0? But at the positive terminal the current is 8/50k right? or does this mean there is a -8 voltage source after the 50k resistor?

2. Oct 10, 2012

### Staff: Mentor

The diagram shows neither an RL nor a 4k resistor.

3. Oct 10, 2012

### rude man

It probably means the op amp gain by itself (no feedback or input components). Ground one input, apply a signal to the other input, the OL gain = output/input.

Yes. The op amp is ideal except for finite gain.

4. Oct 10, 2012

### ElijahRockers

Whoops. Sorry guys, wrong picture. But I figured it out with some help, thank you!

5. Oct 11, 2012

### hoangkyem

Vo=5Vg

6. Oct 11, 2012

### rude man

That's incorrect too.

7. Oct 11, 2012

### hoangkyem

Why? Vo=10/2Vg=5Vg
Since it is Inverting amplifier

8. Oct 12, 2012

### rude man

Because you have ignored the finite gain of the amplifier (the "open-loop gain A").

Your expression holds only if the amplifier gain is infinite.

9. Oct 12, 2012

### hoangkyem

I think I'm right. It's is inverting amplifier, so the closed-loop gain (Acl) independent of the op amp's internal open-loop gain (Aol)
Acl=-Rf/Ri where Rf is feedback resistor, Ri is input resistor
→Vo=-10Vg/2=-5Vg

I founded on internet a text book: Electronic Devices, 9th ed, T.Floyd