When the switch is closed or the FET is conducting, the circuit behaves as an inverting amplifier with a gain of R2/Rl. Since as the inverting tenninal of the op amp is a virtual ground, the capacitor is kept charged to the output voltage by the op amp. When the switch is opened and the FET is nonconducting, the voltage at the output is held constant by the capacitor, the current demands of the next stage are met by the op amp.
Signal-track-and-hold - schematic

The value of C should be chosen so that its impedance at the operating frequency is large compared to Rl and R2.

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