This is a full-wave rectifier, which is superior to the simple diode version because it lacks the 700 mV diode voltage drop. The lower-left op-amp tries to keep its - input at the same voltage as the + input (which is at ground). When the input signal is negative, the op-amp's output goes positive to hold the - input at ground. No current flows through the 500 Ω resistor since both ends are grounded, so it can be ignored. The second op-amp and the two 1 kΩ resistors act as an inverting amplifier with unity gain, making the output equal to the input, only positive. When the input signal is positive, both op-amps try to hold their input at ground. For this, all four resistors on the left must have a voltage drop equal to the input signal. Twice as much current must flow through the 500 Ω resistor as through the 1 kΩ resistor in the upper left (because it has the same voltage drop but half the resistance), so the upper-right resistor must make up the difference. This requires the circuit's output signal to be at the same level as the input signal, with the same polarity. Thus the output voltage is always positive, but has the same magnitude as the input voltage.
This page is a utility for simulating полноволновой выпрямитель на операционном усилителе online with specified initial values.
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