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WEBENCH® Tools/LM1875: The questions about waveform observed from the power amplifier output(LM1875)

Part Number: LM1875

Tool/software: WEBENCH® Design Tools

Currently, I amusing the power amplifier (LM1875T) that your company sells.

I am trying to amplify the amplitude by about 1.2 times by making inverting amplifier circuit using LM1875T.
We set the waveform which multiplied the signal of 200 Hz and 40 kHz to the input signal to the power amplifier.
However, when observing the waveform output from the LM1875T with an oscilloscope, it is observed that the amplification and extinction of the waveform are alternately repeated

Also, similar phenomena occurred when observed with the A / D converter, and it was also observed that the amplitude was shaken off at the moment when the waveform switched from the amplified state to the disappearing state.

As the picture shows how the waveform is amplified and disappears, you can show it if necessary.

Is there any cause for such a phenomenon?

 

Thank you.