This is a Class D or switching amplifier circuit. It amplifies the 30 Hz signal to the upper left. First, it compares the input to a high-frequency triangle wave, using a op-amp as a comparator. It uses the output of the comparators to drive two MOSFETs which bring their output to +15 V if the output is higher than the triangle wave, or -15 V if it's lower. Since the triangle wave sweeps up and down at high frequency, we get a series of spikes, which are higher on average when the input is higher. An LC filter smoothes out the spikes to generate the amplified output.

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Generated Wed Dec 7 2016