# Sine wave signal at resonant frequency gets distorted

I am currently at lost on what I need to do to solve my problem. I have a sine wave signal as my input, and the output is sine wave as well. At resonant frequency, the output of the signal gets high. I was able to get good signal output using function generator, but I am not getting a similar result using a circuitry. The input signal gets distorted when it is in resonant frequency. From the image below, purple is the output and the yellow is the input. I am currently using an inverting amplifier to output a gain of 10. DDS module outputs a positive sine wave with 1Vpk-pk. With the inverting gain of 10, it outputs 10Vpk-pk. After using AC coupling in oscilloscope, it goes -5V to +5V (yellow) as shown in the image below. At resonant, the yellow signal gets distorted. What can I do to not have distortion when it is in resonant frequency. Any help is appreciated. Distorted signal when in-phase No distortion when out of phase • Please label the output on the schematic. Apr 20, 2021 at 6:04
• @Mattman944, added output label. Thanks for letting me know
– Sam
Apr 20, 2021 at 6:15
• You are probably exceeding the capability of the second opamp. Reduce your input amplitude. Probe the top of the inductor to learn more. Apr 20, 2021 at 6:32
• @Mattman944, you are spot on with the fact that if I do reduce the amplitude down to 3V, I see a little distortion. The opamp I am using is TSH82 from sparkfun. I measured the voltage after the inductor, and it does go up to 53V; I believe this an inductive kick. What opamp should I look for to deal with this issue?
– Sam
Apr 20, 2021 at 6:38
• Put values on your components and explain what you are attempting to accomplish. Apr 20, 2021 at 6:44

Assuming that R =20 ohms, at resonance, the peak value of Vout is more than 4 volts.

I(R1) = V/R = 4/20 = 200 mA!

This is a series circuit, so the opamp output current is the same. This is way beyond the capability of this opamp. simulate this circuit – Schematic created using CircuitLab Or, by analysis ...

At resonance, the LC impedance is zero, so the opamp must be able to drive the resistor. https://en.wikipedia.org/wiki/LC_circuit#Series_circuit

• So the distortion is not related to the Gain Bandwidth product but due to the lack of power that causes distortion.Hmm
– Sam
Apr 20, 2021 at 21:31
• If you are just experimenting, change the resistor to 1k. If you need this to work at low resistances, there are high-power opamps (expensive). Apr 20, 2021 at 21:50
• I increased the resistor value to 200ohm and I didn't see distortion anymore. What's interesting is when I put in 5K ohms, the output is always in phase with the input. Do you by any chance know why the input and the output are always in phase when the resistance is high?
– Sam
Apr 21, 2021 at 2:53
• With a larger value resistor the resonant peak will be less sharp. There will still be phase shift if you go far enough from the center frequency. Apr 21, 2021 at 7:47

LM358 has unity gain at 1.1MHz. Lets look at LM358 datasheet. At 200kHz voltage gain is between 6 and 15 and has quite big hysteresis (thats why you have more disortion on negative half). You are just limited by bandwith of opamp. At 161kHz max gain is better and you see no disiortion.

• I am currently using TSH82, and it has a Gain bandwidth product of 10Mhz, and -3dB at 100MHz per datasheet. Isn't that more than sufficient?
– Sam
Apr 20, 2021 at 18:00