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Simple Error Amplifier
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eujean:
Hi there,

As I understand it is hardly ever  a good idea to use an op-amp as a comparator, since there are much beter comparators available to complete such a task.

I intend on building a power supply at some point in the future, and was wondering specifically about the error amplifier. Most examples I came across, such as in the good old HP e3610 design, they seem to use an LF411 JFET op-amp to compare the measured voltage to a set voltage(which they achieve by changing the resistor divider  :o). For a dual rail setup, the output voltage should vary between 12V and -12V to turn on a driver transistor which drives the series pass transistor. They also have a resistor and capacitor from the inverting input to the output of the op-amp. I can only presume that this is for some form of frequency compensation or something, but I can't see how there will really be any frequency component when measuring the output voltage? Or am I missing something obvious?

Basically the question is, why use an op-amp as the error amplifier when a comparator is supposedly much faster?
retrolefty:

--- Quote ---Basically the question is, why use an op-amp as the error amplifier when a comparator is supposedly much faster?
--- End quote ---

 Because an op-amp will provide any linear output voltage from rail to rail to drive the output stage(s). A comparator only provides a digital output, either one rail voltage or the other. Different components aimed for different applications. An op-amp can be made a decent comparator with proper external components, but a comparator cannot replace a linear op-amp.



eujean:

--- Quote from: retrolefty on March 24, 2017, 07:24:47 pm --- Because an op-amp will provide any linear output voltage from rail to rail to drive the output stage(s). A comparator only provides a digital output, either one rail voltage or the other. Different components aimed for different applications. An op-amp can be made a decent comparator with proper external components, but a comparator cannot replace a linear op-amp.

--- End quote ---

Since its a closed loop, you would not need a linear output voltage if you have a driver that either turns the pass element on or off. This is assuming the loop is fast enough.
Benta:
Seems you are mixing linear regulators and switched regulators into one thing. They're not.
Linear regulators contain no switching elements and a comparator makes no sense.

eujean:
Maybe I didn't explain entirely, consider this the example circuit I was talking about. This comes directly from the old HP e3610 design with a modification not to adjust the voltage divider, but just the reference voltage and the substitution of the LF411 for the OP07.

Assume the measured voltage is 12 V, then if the set voltage is greater than 1.2 V, the output Vout is the saturated output of the op-amp, i.e 12 V. When the set voltage is less than 1.2V, the output of the op-amp saturates negatively, i.e. -12 V.

Vout is connected to their driving circuit which drives the pass element.

The point I'm trying to make is that, in that setup, the op-amp is essentially operating as a comparator. Why not use a much faster comparator instead?
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