Use opamp or comparator

Hello,
Have you failure when using opamp instead of comparator? and when we can use when we can not use.
Anyone can make it clear.

Re: use opamp or comparator

The first question is very vague but................in general terms, the failure rate is not related whether it is an opamp or a comparator.

To your second question: You can use an opamp as a comparator, and the web is filled with circuits doing so. But if you are using a generic plain vainilla opamp, it will be slower and the output voltage may not swing all the way up to the rail. My advice is when you have a "compare voltage" function, is to use a proper comparator device.

On the other hand, you CANNOT EVER use a comparator as an opamp.

Re: use opamp or comparator

My question is if replace an opamp in place of comparator might failure? opamp can cover comparator works but vice versus is not correct. Mostly comparator has opend drain output and work with digital circuit.

Re: use opamp or comparator

Hi,

for me the answer is very clear:

OPAMP:
It is designed to amplify analog input signals in a way that there is an analog signal on the output, too.
There always is a negtive feedback.
This means the voltage between both inputs is about zero.
The output is not saturated at the supply rails.
The output always is push-pull.
It may suffer form phase reversal if the input stage is overdriven.
It may suffer form high recovery time when input was overdriven.

Comparator:
It is designed for the output to switch, it is a digital output.
There is no negative feedback.
The voltage between both input may be large.
The output usually saturates at one of the supply rails.
The output may be open collector or push-pull.
A comparator used as an amplifier may suffer from instability.

Klaus

Re: use opamp or comparator

noisepic;
I can see that English is not your mother language.....and neither it is mine.

But please make an effort to understand the difference between the words "circuit failure" and "circuit not working".

They have significantly different meanings, which made me provide an incorrect answer.

Re: use opamp or comparator

Yes, english is not my mother english. The circuit not working and circuit failure has the same meaning in my question.

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Opamp use amplify small signal,with large signal at the input it behave like the function of comparator. Opamp has finite gain in range of input, but comparator always has infinite gain.

Re: use opamp or comparator

Both an opamp and a comparator have circuits with very high voltage gain. The circuit produces phase shift at high frequencies which will cause the circuit to oscillate when negative feedback is used with an opamp so the opamp has a "frequency compensation" capacitor inside to reduce its gain at frequencies where phase shift causes the negative feedback to become positive feedback. A comparator is never used with negative feedback so it does not need a "frequency compensation" capacitor which would seriously slow down its switching. An opamp switches very slowly, a comparator switches very quickly.

A comparator is usually used with hysteresis which is a little added positive feedback so it has a '"snap action". With the positive feedback then its gain is infinite.

Re: use opamp or comparator

Op amps are for clean amplification at controlled gain (closed loop).

Comparators are for fast decisions, open loop.

An op amp in a comparator application will be slow (delay and
transition) and in many technologies, offer a poor logic level
(inappropriate altogether, or low noise margin).

A comparator in an op amp application will likely be unstable,
lacking any internal compensation. If you could stabilize it
you might still not like the crossover and slew rate asymmetry
distortion. Some comparators will draw high current and/or
chatter when you try to hold them linear (like ones with
CMOS output stages that have decent logic drive strength
in both directions).