# Negative voltage on both inverting and non-ínverting inputs of a differential amplifier gives non-zero output

I used an LM324 op-amp for my differential amplifier, in which the inverting input voltage is constant and the non-inverting voltage varies between the range -1.30 V to 1.30 V.

I am having an issue when I give -1.30 V to the non-inverting input. The op-amp produces a very high output voltage, instead of 0 V. Is there any mistake with the sample circuit below? Does anyone know how to rectify the error?

simulate this circuit – Schematic created using CircuitLab

PARAMETERS
V1 = -1.30 V
V2 = 1.30 V to -1.30 V
Vcc and Vdd connected to 12 V and Gnd

• You want to use the opamp in feedback mode so the opamp tries to make the voltage difference at the inputs zero. Do the analysis, when V2 = -1.3 V, what is the voltage at the opamp's + input? The opamp wants to make the voltage at the - input the same. What would the voltage at the opamp's output need to be? Is that possible when the opamp has a supply of 0 V and +12 V? Note that the opamp can only output voltages within that range (actually a bit less depending on the opamp). Feb 4, 2020 at 13:32
• The absolute maximum rating for the inputs is no more negative than -0.3V below the negative power rail which you are apparently violating. Feb 4, 2020 at 13:38

V2 = 1.30 V to -1.30 V

That means you may have -0.65 volts on the non-inverting input (R3 = R4 = 1 kohm).

Vcc and Vdd connected to 12 V and Gnd

The absolute maximum ratings indicate that the lowest acceptable input voltage is -0.3 volts. This means -0.65 is too excessive.

The recommended operating conditions specify that the lowest voltage any any input is 0 volts.

There may be other issues but this stands out as problematic for your design.

• thank you Andy aka for the correction. but since i am using dual source can i use -1.3v as inverting input voltage. i.e V1 = -1.3v? Feb 5, 2020 at 11:17
• In your question you state: Vcc and Vdd connected to 12 V and Gnd. That is not a dual power supply (if that's what you meant to say). That is a single power supply of 12 volts. Feb 5, 2020 at 11:22
• okay. so i cannot use -1.3v on both inverting and non inverting inputs thank u once again. Feb 5, 2020 at 11:27
• hi, does anyone know how to change the negative input voltage range of lm324n from -0.3v to -1.3v as i am trying to get a certain output which is possible with a voltage -1.3v to -1,5v Feb 24, 2020 at 11:48
• You would need a negative supply voltage to pin 11 of the LM324. Feb 24, 2020 at 11:49