sample + hold with single supply?

[lightningfingers]

I'm designing a "random" filter for a friend today....the wah filter I have sounds really nice, but it is a transistor design and will only work *well* with a single supply. I'm wondering if a sample and hold module will run off a single supply, all the schematics I've ever seen use a bipolar supply; is there a reason for this? Or will it work perfectly well with a single supply?

TIA,

Martin

[Doug_H]

I did a Pspice model of an S&H circuit a while ago. Here's what I discovered, YMMV: (I may not have all the details correct here, I'll have to double-check monday at work.) IIRC, there's a FET that does the sampling, triggered by the clock, I believe. From what I remember, the input impedance to that FET ckt had to be really high for it to work properly (I don't remember why, I'll have to check the sim and get back to you.) Well a FET does have a really high input Z. But if you run it off a single-supply, you compromise the input Z with the biasing network required because of the single supply. By using a bipolar supply, you don't need the biasing network (biasing the gate at 1/2VCC, etc) so the input Z of that portion of the circuit remains really high. IIRC, I even tried the sim with 10M bias resistors and it did not work very well. I *think* it had something to do with the sharpness of the pulse or something, but don't quote me on that.

My guess is there may be a different way of clocking in the sample than using a FET. ()

I'll check monday and let you know what I found out.

Doug

[lightningfingers]

Right, I see now, Thanks Doug . I don't mind using a bipolar supply for the s+h module but, as the filter is running from a single supply i'm guessing the control voltage (from the s+h module) will always need to be +ve...anyone have any ideas how to acheive that?

[gez]

You might be able to get away with a 4016 switch followed by a MOSFET input opamp follower with a small cap at its input.  Cap charges up to 'new' voltage every time the switch opens and high input impedance of MOSFET input prevents cap discharging when switch is closed.

Worth a try...

[puretube]

4066

[Paul Perry (Frostwave)]

There is no reason why you can't have a 9v single sided noise generator and a single sided S&H. But since most of the old guitar S&H stuff has been lifted (I suspect) from analog synths (which are traditionally bipolar 15V supplies) we're stuck with it, unless we make new designs. (as we should :roll: ).
FWIW, I once had a tantalum coupling cap in an audio stage & had a slight reverse bias on it (various resistors prevented it from exploding) and got a wonderful low frequency random voltage from it..... I don't know how repeatable or consistent this would be! it certainly led to a lot of puzzled troubleshooting.

[lightningfingers]

thanks fellas---