# Op-amp input voltage gets clipped off

I'm trying to build a simple voltage comparator using LM324N. Here is the circuit:

simulate this circuit – Schematic created using CircuitLab

The strange thing that happens is that after apply the input voltage, a small part of it gets clipped off. Here are the pictures of the circuit and the resulted voltages:

I've connected all of the grounds and negative terminals (i.e., CH1, CH2 and the negative terminals of the input voltage and power supply) together. Also using another LM324N and applying input voltage with a resistor didn't solve the problem. As soon as I disconnect the input voltage from LM324N, it gets back to its normal shape which is a sine wave with 3V amplitude.

How do I stop the voltage from clipping on the opamp input?

• You are feeding a signal to the non inverting input that exceeds your opamp supply. As a result, the high input impedance property of an opamp is ruined. You need to have a DC bias on that input if you ground the negative supply of your opamp. Since you have a 3V amplitude sine wave, you'll also need to raise the supply voltage of the opamp to at least 6V Commented Feb 20, 2021 at 22:32
• @LetterSized Thank you so much. That solved the problem. Commented Feb 20, 2021 at 22:44

On many opamps, they have input protection diodes that prevent the input from exceeding -0.3V. The input goes to -3V so this is a problem and the diode is turning on to protect the input. Either make a bidirectional rail (so the v- terminal is -3V) or change the function generator to have a DC offset so the 3V sine wave has a DC offset of 1.5V and centered around that value.

Source: LM124 Datashet

• Thanks. So when do these protection diodes turn on? How this depends on the voltage of the v-? Commented Feb 20, 2021 at 22:52
• When you go below or above the rails by 0.3V Commented Feb 20, 2021 at 22:57