Hi, apologies in advance for what is probably a very simple question. I'm curious to know the function of a series RC in positive feedback in an op-amp comparator config as below. I'm familiar with the hysteresis effect of adding resistance in positive feedback, but does the RC allow for some delay as well as adjusting the threshold for a comparator type application? Many Thanks in advance, Dave

 

Hi, apologies in advance for what is probably a very simple question. I'm curious to know the function of a series RC in positive feedback in an op-amp comparator config as below. I'm familiar with the hysteresis effect of adding resistance in positive feedback, but does the RC allow for some delay as well as adjusting the threshold for a comparator type application? Many Thanks in advance, Dave

Hard to tell without seeing the rest of the circuit. It could also be to phase shift - even make an Oscillator.

 

For a comparator, it could provide a high hysteresis voltage initially after a change in comparator state, but then allow the hysteresis voltage to go to a low value after some period of time for more sensitivity to the change of state in the opposite direction.

 

Think of it as "temporary hysteresis".

 

Seems to be part of a Filter circuit;

or an OpAmp Monoshot;

or a Switch Debounce....

 

Hi,

As others have nicely pointed out, the capacitor provides a temporary increase in hysteresis because after it charges (or discharges) the hysteresis level becomes much lower. This idea is often used in a higher noise environment where for example something else of higher power switches at the same time as the comparator switches (and may actually be caused by the comparator switching states) which generates a lot of noise and would interfere with the comparator normal action, and the comparator sense level can not be allowed to change too much (or else a smaller resistor with no cap could be used). That configuration though is usually used with a larger resistor to provide for some constant hysteresis so it's a little unusual to see it all by itself with no second resistor.

A second possibility is if the comparator is actually being used as an op amp (which is sometimes done to keep component count lower) then the capacitor provides for enough AC feedback to set the lower frequency gain but the resistor allows for some other lower gain setting at higher frequencies. So the amp would have some normal gain but as the frequency reaches a certain point the gain levels off at some constant value.

I guess a third possibility could be an oscillator of some type, but it would help to see the rest of the circuit and the application as well.