# Op Amp as AC Current Source

1. Feb 5, 2013

### hogrampage

1. The problem statement, all variables and given/known data
Design a circuit based on the noninverting op-amp configuration that functions as a 1mA ac current source.

2. Relevant equations
Gain = Vout/Vin = 1 + R2/R1

3. The attempt at a solution
I really don't know how to start.

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Last edited: Feb 5, 2013
2. Feb 5, 2013

### Staff: Mentor

What is the "virtual ground" property of opamps? And how can you use that to help in this problem?

3. Feb 5, 2013

### hogrampage

Well, the current flowing into the +/- inputs is essentially 0. I understand the basics of the op amp (how to derive gain, vout, etc.), but I have never encountered a problem asking to use one as a current source. And, the book (Microelectronic Circuits & Devices, 2nd, Horenstein) is very vague, unfortunately.

If I have a different resistance connected to Vout (not R1 or R2), then it will change the current, won't it? That's where I am getting lost. I don't know how to make the current independent of whatever load resistance is connected.

4. Feb 5, 2013

### Staff: Mentor

The property you mention is not the "virtual ground" property of opamps. Try again?

5. Feb 5, 2013

V+ = v-.

6. Feb 5, 2013

### Staff: Mentor

Bingo!

So what will the voltage at the - input always be? And so what will the current be through R1? And since the current into the - input is zero as you already pointed out, what will the output current be?

7. Feb 5, 2013

### hogrampage

iout = iR1 = iR2 = $\frac{Vin}{R1}$

So, I could choose Vin = 1V, R1 = 1kΩ, and R2 = whatever?

Last edited: Feb 5, 2013
8. Feb 5, 2013

### Staff: Mentor

Yep!

The Vout will depend on the load impedance. If the load impedance is too large, the output of the opamp will saturate (like if Rload = 10kOhms, and the supplies to the opamp are +/-5V). But as long as the opamp doesn't saturate, the output current will be 1mApp.

9. Feb 5, 2013

### hogrampage

Ah, okay. That makes sense.

Thank you so much for your help! I really appreciate it.

10. Feb 5, 2013

### Staff: Mentor

All I did was ask questions...