I need help explaining why I am getting such a large DC offset across the output for my op amp.
I am using an OPA548 to amplify an AC signal and then drive a solenoid shaker. I have the circuit designed to have a gain of 10 and I am not current limiting the op amp (the OPA548 has that capability). Before I input any signal into the op amp, I connected the output, with respect to ground on the circuit, to an oscilloscope and I saw that there was a DC offset around 13.3V.
To clarify more, I am powering the op amp with +-15V and amplifying a 1V pk-pk input to 10V pk-pk. I then adjusted the voltage that I am powering the op amp with (+-15V) and I saw that the offset is completely dependent upon the +15V. Why is that?
I understand that op amps can have DC offset because of the internal circuitry but I never have had it where the DC offset was dependent upon the input voltage to the op amp. Then, when I applied an input signal and then looked at the output voltage signal on an oscilloscope, the DC offset seemed to "go away" because the output signal was oscillating around 0V. I don't understand why that is. The signal still looked clean at that point.
Lastly, I connected the output across the load (solenoid shaker). The picture below is what I saw on the oscilloscope. My guess as to why that is is as follows: The DC offset noticed before still must be having an affect. When I connected the shaker across the output before applying a signal (the time when I noticed the DC offset) I could feel that the shaker was energized. It felt like the weight inside of it was off to one side and operating at a high frequency. My reasoning as to why the oscilloscope graph looked the way it does is because the DC offset must be holding the weight off to one side before enough negative voltage can override and then it goes back to where it was before.
I would like your thoughts on this and potentially how I could fix this issue. I hope I was detailed enough. Please ask for more details if you think that would help. I attached pictures of both the solenoid shaker and the oscilloscope graph that I am trying to interpret. Thanks in advance for your help