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Simple Error Amplifier
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eujean:

--- Quote from: Benta on March 24, 2017, 10:07:37 pm ---
Which begs the question: "what on earth are you trying to achieve?" An OP07 is not a cheap amp, and I'm sure it's in the design for a reason...

--- End quote ---

The original schematic calls for a LF411 Low offset Low Drift op-amp. I simply chose the OP07 because it was something I had on hand and was easy enough to simulate quickly. The specific op-amp is not as relevant.

I'm just curious why use an op-amp which is not as suited for the task it is ultimately performing?
Thanks for all your advice so far! Much appreciated!
Benta:

--- Quote ---I'm just curious why use an op-amp which is not as suited for the task it is ultimately performing?
--- End quote ---

You're still not understanding the circuit. It is not operating as a comparator, it is operating as an amplifier in its linear region.

You need to include the rest of the circuit to fully see this.

MagicSmoker:

--- Quote from: eujean on March 25, 2017, 12:15:11 pm ---...I'm just curious why use an op-amp which is not as suited for the task it is ultimately performing?
Thanks for all your advice so far! Much appreciated!
--- End quote ---

I know it's hard to imagine, but the error amplifier in a linear regulator really is operating in the linear region most of the time. This is because during steady state conditions the error-amp output will be at the voltage necessary to ensure the pass element delivers the desired output voltage. The extremely high DC gain (effectively the open-loop gain of the op-amp) ensures that the two inputs will be at nearly the same voltage during steady state operation.

It might help with your understanding if you lowered the DC gain of the error amp to, say, x10, and simulated it in LTSpice or the like. Note how the performance of the regulator changes, both in the steady state output voltage error and in response to abrupt changes in load (both small and large changes).

eujean:
The circuit below illustrates the loop for constant voltage.

The LT1037 is much faster/more accurate than the OP07 according to simulation, and thus was used instead.
If the op-amp is replaced with a comparator, such as the LT1018 for example, and the feedback capacitor removed, the output is exactly the same (even for node Vea), hence the question.

Benta:

--- Quote ---the output is exactly the same
--- End quote ---

I'm not surprised. As you've drawn it, the opamp is not doing anything, your regulator is open loop.
I see a 100/900 kohm combination at the output. Shouldn't that go to the opamp?
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