simple comparator circuit problem

Thread Starter


Joined May 21, 2012
Hi everyone,

I have an IC OPA227.
At pin 2 i insert from a function generator a sinus signal with peak amplitude 5 volts,50Hz. In between there is a resistance of 1.3kOhm.
Pin 3 and pin 4 are grounded.
At pin 7 i put 5V dc. The not mentioned pins are NC.
When i measure my output(pin6) i have zero when sinus signal is positive and 5 volts when sinus is negative.

Thats all good i think so far.

When i change the connection between pin2 and pin3 alternately, instead of having the opposite result i have 5 volts when the signal is positive and instead of having zero(in my output) when the signal is negative, in some part i have positive voltage again........:confused:

So, i think you got my point of what i am trying to do..Any ideas?!



Joined Jul 17, 2007
The input voltage range (common mode range) is V+ - 2v to V- + 2v. With V+ connected to 5v and V- grounded, your common mode range is then only +2v to +3v, and you must not allow the signal to go outside of those limits.

Furthermore, it sounds like you are attempting to use the opamp in open-loop mode (no feedback path). In this case, you really should be using a comparator, like an LM311 or a single channel of an LM339, as they are designed for that type of thing.

Thread Starter


Joined May 21, 2012
Hi guys, thanks for the answers.

Yes Wayneh this is my pinout and my signal share ground with opamp!

MrChips if i supply with negative voltage i will have negative pulse in my output. I am not very familiar with common mode range but from SgtWookie i think i understood.

I know LM311 suits better but i have not any of them,yet!

So,since my input is a sinus signal from -5 to +5 my common mode range must be at least -5 to +5. But i dont want negative pulse in my output, thats my problem.

I will continue experimenting and i hope soon i will have the LM311,too.


Joined Jan 3, 2012
What you are seeing is called phase reversal. Since you are bringing one of the inputs below the negative rail on the opamp, which is beyond the common mode voltage range for the inputs, circuit opamp can act unpredictably. The are opamps out there that are designed to have no phase reversal when the inputs are taken beyond the supply rails, but the vast majority of opamps are not specified for operation in this manner. It is possible that you may even damage the opamp by doing this. Bottom line, you're operating the opamp beyond where it was designed to operate w.r.t. input voltage range, so all bets are off for normal operation. Also note - that a simulator will often NOT predict what the opamp will do in this condition, since the models are designed to predict performance under normal operating conditions.