Why would you want Comparators outputs to Oscillate?

[danny davis]

Re: Comparators outputs Oscillate

Why is there a swing of bias voltage around steady point?
where does this bias voltage come from?
Why does a comparator need to be compensated?
to prevent oscillation?
because of the GAIN?
I thought a comparator was a DC amplifier or DC switching function block , it's not an amplifier function block or has gain

Can you please help me out on this, it will help me out a lot

 

[Audioguru]

Re: Comparators outputs Oscillate

What is the biasing circuit of an oscillator? what do u mean?

If you do not properly bias the input of an opamp then it WILL NOT WORK! It will not amplify and it will not oscillate.

Why is there a swing of bias voltage around steady point?

I think there is a translation problem from German to English.

Why does a comparator need to be compensated? to prevent oscillation? because of the GAIN?

A comparator is rarely compensated (only when it is used as a very low frequency amplifier). It compares two input voltages and produces an output that is either high or is low. It is rarely a linear amplifier. If it produces a high frequency oscillation when its inputs are near the threshold voltage then hysteresis is added to stop the oscillation and it then has a fast "snap" switching action.

An opamp has a compensation capacitor to make it stable (so it does not oscillate at a high frequency) when negative feedback is added.

I thought a comparator was a DC amplifier or DC switching function block , it's not an amplifier function block or has gain

One datasheet shows a comparator with an output capacitor so it is a stable very low frequency amplifier with any amount of gain but it is rarely used like that. Its gain as a comparator is about 200,000 so it can compare tiny voltages.

 

[danny davis]

Re: Comparators outputs Oscillate

If it produces a high frequency oscillation when its inputs are near the threshold voltage then hysteresis is added to stop the oscillation and it then has a fast "snap" switching action.

How can a DC input voltage to a comparator has a High frequency oscillation near the threshold voltage? the threshold voltage is a DC voltage level, DC = no frequency , so where is high frequency oscillation coming from on the input? it's from the op amps GAIN? from the comparators negative feedback? so it's the Op amp internally that is output a high frequency oscillation feedback to the input you're saying right?

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If you do not properly bias the input of an opamp then it WILL NOT WORK! It will not amplify and it will not oscillate.

So if you don't bias the input of an op amp the output of the amp will have a positive or negative clipping?

If you don't bias the input of an op amp it's not in it's operating range

When not biasing the input of an op amp how is it different than not biasing a transistor

When not biasing a transistor in it's Q point range, the positive or negative cycle with be clipped or not symmetrical

What happens with an op amp when it's not biased right?

 

[Audioguru]

Re: Comparators outputs Oscillate

Since a comparator has a voltage gain of about 200,000 then the tiny amount of capacitance from its output to its +input is positive feedback and causes oscillation when the inputs are near the threshold voltage.

The inputs of an opamp are usually biased at half the supply voltage (0V if the supply has dual polarity) so its output can swing equally up and down.
If the inputs are not biased then its output voltage will be completely wrong. The design of the inputs will tell you what happens at the output.

An opamp contains transistors so it is silly to compare an opamp to a transistor.

 

[danny davis]

Re: Comparators outputs Oscillate

then the tiny amount of capacitance from its output to its +input is positive feedback and causes oscillation when the inputs are near the threshold voltage.

I think this is called stray capacitance , coming from the PCB traces

I still don't get how a Steady DC input voltage crossing a threshold voltage can cause Oscillation, because Oscillation is a frequency, DC = No frequency

Oscillation= frequency = AC signal
DC = No frequency = No oscillation

So what you're saying is that the stray capacitances from the output pin to the input pin there is Capacitance which converts the DC output of the comparator into AC oscillation because that's what a capacitor does creating an AC oscillation

 

[Audioguru]

Re: Comparators outputs Oscillate

What is a steady DC voltage when every electronic circuit produces some noise.
When the noise at the input is amplified 200,000 times then do you think the output is a steady DC voltage? Of course not, the output is bouncing up and down by the amplified noise. Then the output is capacitive-coupled back to the input and is amplified 200,000 times again and the output is capacitive-coupled back to the input and is amplified 200,000 times again and the output is capacitive-coupled back to the input and is amplified 200,000 times again and the output is capacitive-coupled back to the input and is amplified 200,000 times again and .... it is an oscillator. Almost EVERY oscillator is started with noise.

Quote: "So what you're saying is that the stray capacitances from the output pin to the input pin there is Capacitance which converts the DC output of the comparator into AC oscillation?"
No. The stray capacitance from the output pin to the input pin couples Amplified Noise. The tiny capacitance passes only high frequencies so the oscillation is a high frequency.

 

[danny davis]

Re: Comparators outputs Oscillate

oh ok yes true makes sense now