Comparator as operational amplifier

Question

Using an operational amplifier as a comparator was asked quite a lot times here, but how does it behave the way around?

I've a really cheap one in my circuit with low hysteresis and a MOSFETt push/pull output stage. Using it as a two point regulator with a LPF (fc<100Hz) at the output to drive my actuator should do almost the same job as an operational amplifier? Yes, it will oscillate, leading to higher Iq, and some EMI issues -but the control loop should be stable. Am I missing other huge downsides?

Answer 1

Yes, you can do this, if the bandwidth of the low pass filter is appropriately low to achieve stability.

This will result in a PWM output of variable timebase at the comparator output. As you get closer to 0% and 100% the timebase will get arbitrarily long. The frequency will also be greatly affected by the hysteresis (you need some hysteresis or the frequency will be too high and other effects will likely cause issues).

---

You can also operate some comparators in a linear mode, as Andy says the LM339 is one of the few that allows this because there are not intermediate stages inside that require access in order to add compensation-- in this case just adding a BFC (Big "Fat" Capacitor) to the output as explained in this document.

Limitations include low bandwidth and poor current source capability due to the open-collector output and R3.

Answer 2

There are plenty of comparators that can be unofficially wired as an op-amp without much trouble but, many more will oscillate (go unstable) because they are not designed for linear applications; they are designed for non-negative feedback applications where output transistor switching speed is important. The LM339 is a bit of an exception in that you can get semi-decent op-amp style performance but you have to jump through a few hoops: -

However, speed is usually the big thing needed from comparators and that means the internally circuits are not compensated. And, without compensation, they will be often unstable when using negative feedback (op-amp applications).

But the control loop should be stable.

No, that's a much bigger and broader thing and you cannot make that conclusion at all.

Am I missing other huge downsides?

A lot of comparators don't have low input offset voltage accuracy but they will have (usually) a better common-mode signal handling reliability than op-amps (at the expense of worser input offset voltage specifications).