# Op Amp help needed

#### lolol

Joined Mar 6, 2007
3
From what I understand, since Vs=1, the node connected to the (-) terminal of the opamp has to be at 1V as well. I also know that no current flows through th e opamp. I'm really in the dark on this, as all of my professor's material is of the basic principles and has nothing on the level of this homework. Any help would be appreciated...

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#### Distort10n

Joined Dec 25, 2006
429
Correct. Ideally, the summing junction (inverting input or '-' pin) will equal the voltage presented at the non-inverting input or '+' pin by virtue of negative feedback.

If Vs = 1V then your gain should be no more than 2 if you can only source 200mA.

#### JoeJester

Joined Apr 26, 2005
4,390
Verify this for me ...

The op-amp in the amplifier in the figure below operates with a +/- 15 V supplies and can output no more than 200 mA.
What is the maximm gain allowable for the amplifier if the maximum values of Vs is 1V?

R1 + R2 = 40k
Is that correct?

#### JoeJester

Joined Apr 26, 2005
4,390
If Vs = 1V then your gain should be no more than 2 if you can only source 200mA.
Knight,

You might want to rethink your solution.

#### lolol

Joined Mar 6, 2007
3
Verify this for me ...

Is that correct?
Yeah, that's correct.. I'm not sure why the picture resized on me..

#### lolol

Joined Mar 6, 2007
3
Using KCL at the (-) terminal, I got this:

1/R1 + (1-Vo)/R2 =0 ...Rewrote it to this...

Vo = (R2/R1) +1

So I basically want to maximize R2 and minimize R1...

But the output can only be 200mA, so (Vo-Vs)/R2 = 200?

Subbing in for Vo and Vs gives...

(((R2/R1) +1)-1)/R2 = 200
(R2/R1)/R2 = 200
1/R1=200
R1=.005 kOhm

Therefore, since R1 + R2 = 40, R2 =39.995

So... Vo = (R2/R1) +1 = 8000

But the question asks for the gain... Is that just the ratio between Vo and Vs? So the answer would be 8000/1? I'd appreciate any further commenting or pointing out of errors in my work.

#### JoeJester

Joined Apr 26, 2005
4,390
You can only source 200 mA through the Load resistor.

That will tell you the value of Vo. You already know what Vs is and Vo can be calculated.

Draw the circuit ... leave the values for R1 and R2 blank. Annotate on your drawing the known values ... Vs ... I load.

You know the total for R1 and R2.

You should be able to work this out.

The gain is the ratio of Vo and Vs.

#### Distort10n

Joined Dec 25, 2006
429
Ah, you're right. What in the world was I thinking? My mind was on a given input voltage and a given gain that would allow 200 mA through a 20 ohm resistor. Would have made sense if Vs = 2V.
Let's see if I can redeem myself.

Av = 4 if Vs = 1V
Rg = 10k
Rf = 30k