Creating a 1mA AC Current Source with Noninverting Op-Amp Configuration

[hogrampage]

Homework Statement

Design a circuit based on the noninverting op-amp configuration that functions as a 1mA ac current source.

Homework Equations

Gain = Vout/Vin = 1 + R2/R1

The Attempt at a Solution

I really don't know how to start.

 

[berkeman]

Homework Statement

Design a circuit based on the noninverting op-amp configuration that functions as a 1mA ac current source.

Homework Equations

Gain = Vout/Vin = 1 + R2/R1

The Attempt at a Solution

I really don't know how to start.

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

 

[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.

 

[berkeman]

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.

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

 

[hogrampage]

V+ = v-.

 

[berkeman]

V+ = v-.

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?

 

[hogrampage]

i_out = i_R1 = i_R2 = $\frac{Vin}{R1}$

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

 

[berkeman]

i_out = i_R1 = i_R2 = $\frac{Vin}{R1}$

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

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.

 

[hogrampage]

Ah, okay. That makes sense.

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

 

[berkeman]

All I did was ask questions...