Equal Input Voltages on Op-Amps

  • Thread starter Thread starter Bill48
  • Start date Start date
  • Tags Tags
    Input
Click For Summary

Discussion Overview

The discussion revolves around the behavior of operational amplifiers (op-amps) configured as voltage comparators, specifically focusing on the output when the inverting and non-inverting inputs are equal. The scope includes theoretical considerations and practical implications of op-amp behavior in comparison applications.

Discussion Character

  • Technical explanation
  • Debate/contested

Main Points Raised

  • Some participants assert that when the inverting input is higher than the non-inverting input, the output saturates to the highest voltage, while it outputs zero when the inverting input is lower.
  • One participant questions the behavior of the output when the inverting and non-inverting voltages are equal, suggesting it may not be zero or saturated.
  • Another participant proposes that if the inputs are equal, the output should be halfway between the supply rails, but notes that finite gain and hysteresis prevent the output from settling at this intermediate point.
  • It is mentioned that without hysteresis, the op-amp may amplify input noise, potentially resulting in a square wave output if one input is a low amplitude sine wave.

Areas of Agreement / Disagreement

Participants express differing views on the output behavior of the op-amp when the inputs are equal, with some suggesting it could be an intermediate voltage while others emphasize the role of hysteresis in preventing this outcome. The discussion remains unresolved regarding the exact output in this scenario.

Contextual Notes

There are assumptions regarding the behavior of op-amps under different conditions, including the effects of finite gain and feedback mechanisms that are not fully explored. The discussion also highlights the dependence on specific configurations and input conditions.

Bill48
Messages
7
Reaction score
0
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:
Engineering news on Phys.org
Bill48 said:
When the inverting input is higher than the non-inverting input, an the 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?

hi there

have a read here and see if it answers your questions

http://www.electronics-tutorials.ws/opamp/op-amp-comparator.html
 
Bill48 said:
When the inverting input is higher than the non-inverting input, an the 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?
The part I have put in bold seems backwards to me.
 
  • Like
Likes   Reactions: sophiecentaur and 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.
 
Baluncore said:
and so can never settle at an intermediate point
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.
 

Similar threads

Replies
12
Views
3K
Replies
2
Views
2K
Replies
18
Views
3K
  • · Replies 21 ·
Replies
21
Views
3K
  • · Replies 21 ·
Replies
21
Views
3K
  • · Replies 9 ·
Replies
9
Views
5K
  • · Replies 33 ·
2
Replies
33
Views
6K
  • · Replies 15 ·
Replies
15
Views
3K
  • · Replies 6 ·
Replies
6
Views
3K
Replies
15
Views
3K