# op-amp outputs supply voltage on GND Signal

Electrical Engineering Asked by RIJIK on December 12, 2020

I am trying to use an op-amp (first time).
I know the basic theory of its amplification circuits.
To test I am using an Arduino Nano Microcontroller to create a Signal (0-5V). I want to amplify the signal by a Factor of 3. I measured the input and the output of the opamp relative to GND with an oscilloscope.
The amplification works (5V to 15V –> factor 3), however when the signal goes to ground the opamp always reaches the supply voltage (tried with different supply voltages).

I know that an op amp amplifies to the supply voltage if there is no feedback, but i cannot explain how it can happen in my case.
How do I prevent the opamp from switching to supply voltage once the signal raches GND?

EDIT: The Op-Amp is a MC33078 (datasheet)

The circuit you draw is the right one for a gain of 3, but the waveforms you show indicate a negative gain and an offset. The equation for Vout would be something like Vout=15V-A*Vin where A is your gain.

I think your circuit you've built is not what you've drawn. Double check your connections. 5Vin should make 15Vout. If the opamp is rail to rail, you'll also get 0Vin makes 0Vout.

Answered by miles60 on December 12, 2020

Looks like you driving the output tri-state (open circuit) rather than pulling it to ground. You can add a resistor from the non-inverting input to ground or change your firmware. When it's open the op-amp input will float up or down, predictably if it's a bipolar op-amp to one supply or the other, and unpredictably if it's a MOS input op-amp. For example, an LM324 input will float upward because the bias current flows out of the op-amp input.

You should also be using a "single supply" (or rail-to-rail I/O) op-amp - the output should swing to near ground and the input common mode range should include the negative rail.

Edit: The MC33708 is not 'single supply', as you know. Some op-amps (not sure about the MC33708, I've used them, but have not knowingly violated the input common mode range that way) may behave the way you describe. LM358 is the the same pinout and should work okay.

Answered by Spehro Pefhany on December 12, 2020