This is an inverting half-wave rectifier, superior to the simple diode version because there is no 700 mV diode drop. Even though the input signal only swings 500 mV, it can still be rectified.
The op-amp attempts to keep its – input at the same voltage as the + input (which is at ground). When the input signal is positive, the op-amp output goes negative to keep the – input at ground. No current flows through the upper resistor or diode, so the output of the circuit is at ground. When the input signal is negative, the op-amp output goes positive, and current flows through the upper resistor and diode to bring the – input to ground. The two 10k resistors act as a voltage divider, with one end at the input, the other at the output, and the middle at ground, so the output must be the negative of the input.
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