S&H

[Magnus Danielson]

From: "Paul Maddox" <Paul.Maddox at wavesynth.com>
Subject: S&H
Date: Sat, 30 Dec 2000 15:51:48 -0000

> Dear all,

Paul,

>   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 :-(

Well, I guess you need to bias it.

> Suggestions?

Take a peak at the ETI/Powertran vocoder (I just happends to own one,
so it is natural to come to think of something you got, right?) where
each channel has a slewrate control/hold function built by a PWM
generator and sample and hold. Now if you look at that sample and hold
(fig5a1.tif in Anders scan) you would find that the FET is bias from
the source end by feeding the signal through a 2k2 and a common bias
at 120 mV through a 15 k resistor into the drain. The source of the
FET is hooked to the 100 nF cap. The cap is hooked up as you
described, into the non-inverted input of an op-amp setup for
buffering. The gate is then feeded from the op-amp output through a 1M
resistor. A diode is then used to pull current from the gate.

> Schematics?

Figure 5 of the ETI/Powertran vocoder, as found on Anders Spontóns site.

> URLs?

Sure.

http://omega.tellus.vallentuna.se/anders/

Go for vocoders, ETI Vocoder, pull the 1 M zip file down. Dig up
fig5a1.tif and look at the "Slewing Rate Controller" section.

>  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)

Well, then Anders site should have a lot of goodies for you.

Cheers,
Magnus