S&H

[Grant Richter]

A FET works by "pinching off" the channel by applying a highly negative
voltage. Use a 1 meg resistor from the source to the gate. Then use an
op-amp as a comparator with a diode on the output to pull the gate to -15
volts for the "off" state. So "on" = floating "off" = -15 volts

> From: "Paul Maddox" <Paul.Maddox at wavesynth.com>
> Reply-To: owner-synth-diy at node12b53.a2000.nl
> Date: Sat, 30 Dec 2000 15:51:48 -0000
> To: "Synth DIY LIST" <synth-diy at node12b53.a2000.nl>
> Subject: S&H
> 
> Dear all,
> 
> Im trying to get a sample and hold working, useing an OPAMP, CAP and FET..
> 
> signal comes in, goes to Source of FET, Drain of FET goes to Non inverting
> terminal of opamp,
> and inverting goes to output... all straight forward..
> 
> Question, HOW do I gate the damn FET??
> Im useing a 2n3819,
> Im guessing to be OFF (open circuit) the Gate should be at 0v (or should it
> be -VCC?) via resistor (1K in my case)
> And to turn it on, it should go (breifly) to +VCC... am I right?
> 
> My S&H I just built doesnt seem to work, it stays permanently open :-(
> 
> Suggestions?
> Schematics?
> URLs?
> 
> Its for my new module in my modular, I have a LAG processor (cap+opamp+var
> resistor) S&H (above cursed thing)
> and a noise gen which gives PINK, WHITE and BLUE (works like a dream)
> 
> Paul
> 
>