Electronic – Feedback capacitor in comparator circuit with an OP-AMP

comparatorfeedbackhysteresisoperational-amplifier

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Greetings,

This is a scheme of a comparator with dynamic hysteresis.

Green line is the output
Red line is the input
Blue line is the positive feedback.

I want to ask for the explanation why C1 does not start charging up at 5.3V (like the green line) but has its peak at 4.3V and goes down from there?

Best Answer

With lots of practice, you may be able to do dynamic KVL in your head.

let's state my assumptions without any of your graphs.

  • Output swing = 4.6+0.7 = 5.3V with Rout=2700 Ohms and Iout= (15-5.3)/2k7=3.6mA
  • feedback loop resistance = (R5+R3+R1 )//R2 (neglect R2 for now)
  • the peak is defined when the voltage output is at peak and input across R1 is near 0 due to R1/R2 ratio
  • initial voltage drop across C is thus determine by KVL ratio of R5 to the rest of the loop.
  • lowering both R3 R5 to 0 would restore full amplitude across C1 with a 10.6v transient from -5.3 to +5.3

Also note relevant to your previous question on same circuit, R5C1 reduces the slew rate, not the Op Amp but only after the zero crossing.

Conclusion: To make V(c1) = 50% peak of total output swing of 10.6V choose R5=R1+R3=3.5k (not 2700)