# What could cause a sin input to turn into a square wave across a non-inverting amplifier?

I've been working on a small non-inverting OPamp circuit with a single supply and have run into a problem. The circuit uses the LT1022 OPamp and is setup like in the image below:

The goal is to amplify a small 20mV signal to about 2 Volts. Ideally more, but the maximum gain I can get out of this circuit is about 20, giving me a 400mV resulting square wave.

Yellow is U1-, Purple is U1+ and Blue is U1 Out. Supply voltage doesn't seem to be the problem here. Dropping PWR down to 10V or bumping it up doesn't have any effect on the waveforms. What strikes me is that U1 Out seems to be limited to a very small gain and quickly bumps against some sort of ceiling. I've tried a few different values for R4 but with very little effect.

What could be causing this behavior?

You potentially have a gain of 480 so it is likely that your output is being limited by the power rails of the op-amp. This makes it look like a square wave.

Another problem is your mid-rail generator - you are using 1Mohm resistors and then R4 (100 ohms) connects to this midpoint - this is not going to work well at all. With such a small value resistor as R4 you need a much "stronger" mid-rail generator.

What you could do temporarily is connect R4 to your most negative supply rail via an electrolytic capacitor and re-check your circuit functionality.

Putting a capacitor in series (maybe 100uF) restricts gain at frequencies below 16 Hz but allows you to proceed and debug any other issues that arise. If you do need a gain of up to 480 at DC then you will likely have to use a different mid-rail generator circuit built around a second op-amp.

I'm also noticing that you are not using capacitor decouplers on the rails of the op-amp - these are nearly always required on any op-amp.

EDIT - maybe you have a x10 probe on the output and you don't realize this!!!

• Power rails were my first consideration but with PWR at 10V, V+ is at 6.8V and V- is at -3.2V. Still a ways away from what U1 Out seems to bump against. I'm not sure what exactly a strong mid-rail generator entails but I will add a few capacitors and play around with that. Thanks for all the considerations though. There's a hell of a lot more to an op amp circuit than I was aware of. Dec 10, 2015 at 10:18
• I can see why the large resistors for the mid-rail generator could be an issue. Current will start flowing through R4 and the Pot, especially if we connect a load. But what are the considerations behind using capacitors on the rails and R4 and ground? Dec 10, 2015 at 10:49
• A stronger mid-rail generator (as I think you may have figured out) just means using lower value resistors for R1 and R2. By having more current flowing through 'down' the potential divider, any currents drawn from the midpoint by the feedback network (R4 etc.) have less of an effect on the stability of the mid-point reference (because they become less significant compared to the quiescent current of the divider).
– Ted
Dec 10, 2015 at 11:05
• Op-amps need capacitors directly on the power rails Dec 10, 2015 at 11:37
• Using a cap to -Ve instead of R4 directly connecting to midrail stops that current thru R4 upsetting the weak midrail. Dec 10, 2015 at 11:50