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Figure 1

schematic

simulate this circuit – Schematic created using CircuitLab

I am using a TL074CN comparator to compare a fullwave rectified input voltage with a DC voltage of 1.4 V as shown in the above figure. At the output of the comparator, I am receiving a square wave which goes from +12V to -12V.

Figure 2

enter image description here

The figure 2 above shows the square wave generated in microsecond level. As you can see the waveform is very unstable at the moment. So far what I tried was

1) I also added a 100u capacitor at the output of the opmamp but it doesn't stabilize the waveform , it instead curves the waveform at the edges

2) I added a 10K resistor at the output but it doesn't help either.

3) I switched the TL074CN with an LM324N and still did not stabilize the waveform

Is there anything which I can do to make this square wave stable?

Amy
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    What makes you certain that the jitter you're seeing isn't actually in your input source? – Connor Wolf Jul 14 '17 at 01:17
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    What are you using to trigger your oscilloscope? –  Jul 14 '17 at 01:27
  • @duskwuff I have channel 1, 2 and 3.

    Channel 1 is the DC source of +12 and -12 with a potentiometer. Channel 2 is the Fullwave rectifier waveform. Channel 3 is the square wave

    I was using channel 2 which is the full wave rectifier waveform and it displayed an unstable square wave. But when I changed to channel 3, the results were different which I have explained in a separate answer below.

     – Amy Jul 14 '17 at 01:40

2 Answers2

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The TL074 and LM324 are op-amps, not comparators.

If you want a device to operate as a comparator, you should use something designed for that application, like an LM319.

Peter Bennett
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    Op amps generally work fine as comparators, as long as they have plenty of bandwidth. – Connor Wolf Jul 14 '17 at 01:16
  • @ConnorWolf

    So that means if an OPAMP doesn't have much bandwidth, it cannot be used as a comparator? What is the reason for this?

     – Amy Jul 14 '17 at 01:46
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    @Amy It's generally not a good idea to use an op amp as a comparator. You'll get better performance from a comparator than an op amp in a comparator circuit. Unless you only have op amps on hand or you have a spare op amp (e.g. you're only using three op amps in a quad op amp IC) there's little reason to use an op amp instead of a comparator. – Null Jul 14 '17 at 14:41
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Figure 3

enter image description here

My error was that I was not triggering correctly. This time I selected the trigger mode to channel 3 which is the output square wave and selected the "slope" to be falling.

As you can see that when slope was selected to be falling, the falling edge of the square wave is stable while the rising edge shows some instability.

And when experimented vice versa, the rising edge was stable while the falling edge was not.

Amy
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