LM358: LM358

Hello, 

You are referring to this thread.

https://e2e.ti.com/support/amplifiers-group/amplifiers/f/amplifiers-forum/1486585/lm358-lm358p/5709970?tisearch=e2e-sitesearch&keymatch=TP4%2520and%2520TP7#5709970

here is how the circuit works in simulation. 

The output TP4 and TP7 are determined by the 50kohm potentiometer's setting. 

Condition 1: Potentiometer at 80% setting.

Condition 2: Potentiometer at 50% setting:

Condition 3: Potentiometer at 10% setting:

Here is the simulation file. I use ideal op amp instead due to convergence issues. 

TL031 LM358 LED 06172025.TSC

Please make sure the LM358's input is operating in a linear or common mode region. If it is not, the LM358 may behave in a phase reversal issue, which it could explain what the issues you may have observed.  

For instance, the following condition that the input of the upper right circuit is outside of Vcm range, and TP7 will go low instead of high due to phase reversal issues. The pre-PCN op amp does not behave this way (if the input is not within the Vcm range, it will not do the phase reversal act. In other words, the TP7 will remain high).   

If I am understanding your issues correctly, this is the scenario where the issue is found. 

If you have other questions, please let me know. 

Best,

Raymond