Sample and Hold
There is not much to say to this, a S&H is frequently used to create random voltages, or to sample LFOs resulting in repeating voltage patterns. When constructing this take care to deflux the board. The S&H can handle the "full" +/-10V scale. Operation is very simple, when the Gate of the FET is at -15 the FET is completely off. Thus no current can flow out of the capacitor (exept some leakage). When the Gate is turned towards +15V the FET is turned on and can charge the capacitor to the input voltage. The circuit on the right converts 5V triggers to the +/-15V triggers needed by the S&H
Jörgen Bergfors has pointed out that with a BF245A and the gate resistor R1 is altered to 47k, the circuit can reach -10V input. (Which isn't always the case in the above version.)
Yet another Sample and Hold
My newer S&Hs are build arround the LF398 which contains all that is needed to make a fine S&H. Additionally there is an oscillator that can be used for free running sampling, without an external trigger signal.