# Equal Input Voltages on Op-Amps

1. Jan 22, 2017

### Bill48

When the inverting input is higher than the non-inverting input, an op-amp voltage comparator will cause the output to saturate to the highest possible voltage. When the inverting input is lower that the non-inverting input, it outputs zero.

Question: what is the output when the inverting and non-inverting voltages are the same?

Last edited: Jan 22, 2017
2. Jan 22, 2017

### davenn

hi there

http://www.electronics-tutorials.ws/opamp/op-amp-comparator.html

3. Jan 22, 2017

### Averagesupernova

The part I have put in bold seems backwards to me.

4. Jan 23, 2017

### Baluncore

The op-amp comparator compares the two input voltages V+ and V–.
If it is true that V+ is greater than V–, then the output rises towards the positive supply voltage.
If it is false that V+ is greater than V–, then the output falls towards the negative supply voltage.

If the inputs are equal then the output should be half way between the supply rails. BUT,
the finite gain makes the comparator linear, so a small amount of positive feedback is used to create hysteresis. That makes sure the output will always be going towards a stable high or a stable low output voltage and so can never settle at an intermediate point.

5. Jan 23, 2017

### sophiecentaur

IN the limit, the amp will just be amplifying the input noise. If one of your input waveforms is a very low amplitude sine wave, your output can be a square wave with a short burst of 'shash' at each zero crossing - that is if you don't use some positive (Schmitt Trigger) feedback to suppress it.