Re: [sdiy] comparator feedback resistor

[Roman]

It does both.
Without any feedback opamp's output voltage equals its gain times differential input voltage (assuming offset is trimmed), and the gain has limited value, so for slow changing input signal, output  changes slowly too, not what you'd like to see in comparator operation. When you make positive feedback, output voltage adds to this slow signal, so opamp is treated with slightly faster transition. This translates directly to output slew rate. I'm not talking here about IC limit tho.

How to calculate hysteresis:
Uh = (Ri/Rf)*Uspan
where Ri is input resistor, Rf - feedback resistor, Uspan is opamp's output voltage range, in first aproximation we can assume it's total power supply (30V usually).

hope that helps

Roman

---- Wiadomość Oryginalna ----
Od: Michael Ruberto <frankentron at hotmail.com>
Do: synth-diy at dropmix.xs4all.nl
Data: Fri, 27 Jun 2003 14:05:14 -0400
Temat: [sdiy] comparator feedback resistor

>most schematics I see with comparators do not have a feedback resistor, but 
>sometimes they do. one I am looking at now uses a TL07X and has a resistor 
>of 2.2M between the + input and the output. now from what I have read it 
>seems that the resistor is there for either hysteresis or to improve 
>reaction speed. since one text claims one explanation and the 2nd text the 
>other explanation could somebody please tell me which is correct? or are 
>they both true? I guess I need a better explanation.
>
>also, if it's an advantage to include this feedback resistor how do you go 
>about calculating the proper value for it?
>
>Thanks!
>
>~Mike
>
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