# Beginner question about OP AMP as "DC differential amplifier". Does switching inputs between + and - make any difference?

I'm just starting to learn about OP-AMPS used to control current through a mosfet and I have a few basic questions about the general concept of the schematic below, taken from this thread: op amp+mosfet = current source.Why do we need a feedback resistor?

In the schematic above, is that what's called a "DC differential amplifier"? Are there other names for this type of op amp circuit?

Would it make any difference at all if I switched the inputs around, having the control voltage VC go into the - input on the op amp, and having current sense resistor voltage drop go to the + input? As the op amp regulates it's output to ensure + and - have the same voltage, my assumption is that switching inputs around as I described should make no difference at all. Could that be right?

Could that op amp circuit (with mosfet removed) be used to "convert" a voltage controlled boost IC into a current controlled circuit by replacing the "standard" boost IC feedback divider with the output from the op amp?

Edit: Attempting to clarify this last question. The TPS63020's output is controlled by what voltage is sees on the FB pin.

I'm asking if I can remove that R1 and R2 divider and using an op amp somewhat like in the first schematic instead:

• You need to do some basic reading up on op amp circuits and in particular negative feedback e.g. allaboutcircuits.com/textbook/semiconductors/chpt-8/…. Jan 7, 2021 at 0:52
• This is two quite different questions so please break it up into two postings. Jan 7, 2021 at 0:53
• I didn't realise I couldn't post multiple questions on one posting. Sorry about that. Jan 7, 2021 at 1:07
• I will wait for your new post, but the short answer is yes and no. Yes, you can use a switching power supply to give a regulated current rather than a regulated voltage. No, it is unlikely to be as simple as replacing part of the circuit as you have done. Switching power supplies have "compensation" that tunes them for their purpose. Changing the purpose means calculating what compensation is needed, making appropriate adjustments. Back to yes, it can be done. Jan 7, 2021 at 3:20
• The term "op-amp" is an abbreviation of "operational-amplifier" so it doesn't get capitalised. (It's not an initialisation or acronym.) Jan 7, 2021 at 7:21

is that what's called a "DC differential amplifier"?

No.

Would it make any difference at all if I switched the inputs around

Yes. The circuit would no longer work as designed.

my assumption is that switching inputs around as I described should make no difference at all. Could that be right?

No.

Could that op amp circuit (with mosfet removed) be used to "convert" a voltage controlled boost IC into a current controlled circuit by replacing the "standard" boost IC feedback divider with the output from the op amp?

I don't understand. Could you provide a schematic?

• Thanks for the definitive answers. It certainly looks like I need to read a whole lot more about op amps. As for the last question, I have the updated OP with some schematics, I hope it's clearer. Jan 7, 2021 at 1:57
• Ok, it's not called a DC differential amplifier. What is it called? Jan 7, 2021 at 2:30
• It's a voltage controlled current source. Jan 7, 2021 at 3:09