Author Topic: Useful circuit- Low parts count Sample/hold that works well with 9vdc supply.  (Read 6580 times)

Brian Marshall

For a long time, I've tried to get a sample hold circuit to work well at 9 volts, that didnt require large IC's, exotic parts, and that would work over a large voltage range.  CD4016/4066's are cool IC's but using one when you only need one switch is just a wast of real estate.  This one is especially simple, especially considering that some of the parts could actually be left out, and still work.

Here's the schematic.
http://www.subdecay.com/samplehold.jpg

Essentially anything you put in that is within the opamps voltage range will come out the other side in sample form.
The first opamp could actually be turned in to a noise amplifying circuit with reverse biased zener diode, and some heavy amplification... may require a trimmer somewhere since noise per diode can vary quite a bit.

R10 is intended to be a fairly large resistor like 1M or so... you dont want low impedance square edged voltage signals running all the way across your circuit board, but depending on layout this could be left out.  C2 is intended to work with R10 to keep the sample voltages from changing too abrupty and causing ticking... of course you could do this after the ouput as well, and for some circuits it may not be needed at all.  R5 and C1 already do this to a point as well.

IC1 can be just about any Jfet input dual opamp.

IC2 should be a low power single opamp, and should be well decoupled from the power supply to avoid ticking.

The mosfet i used was a BS170.  I tested it with about 10 different ones, and it worked with all of them.

Due to the narrow power supply voltage, the signal is reduced at the input, so that the drain to source voltage at the mosfet is never too great, and it can turn off and on all the way.  The input should be centered at ~ half the supply.

All 4.5 volt connections should be low impedance.  A pair of 10K resistors and a 100uF cap for a voltage divider should work, althoug I was actually buffering my bias voltage with an opamp.


MartyMart

Looks great Brian and nice and simple too set up as well  :D
V-cool ! .... much appreciated ...
Marty.
"Success is the ability to go from one failure to another with no loss of enthusiasm"
My Website www.martinlister.com

slacker

Very cool :) I've tried to do a single supply sample and hold a couple of times and never had any success, I'll have to give this a try.
I guess if you added input and output caps and fed it a guitar signal you could also use it as an aliaser/bitcrusher effect :icon_twisted:

soggybag

I had been trying to get something like this to work for a while and gave up. This looks very cool, I will have to give it a try. The low parts count is a great and makes it even more useful. Good work!

Brian Marshall

Hopefully some people find it useful...  Of course I should do the standard disclaimer that it's for personal use only, blah blah blah... but really using a mosfet for sample/hold isn't exactly original.  Just getting it to work with a narrow supply voltage requires a little work.  Mainly reducing the signal. 

Really all the work is done by the clock and the mosfet section.  IC1 is simply used to reduce, and recover gain.  If you already have a low impedance signal centered around mid supply, and don't need a large voltage swing at the output, you could essentially lose IC1 completely, although the output would probably need to be buffered or end up going somewhere that has very high impedance, otherwise it will discharge C1.

swt

HI. I can't see the whole picture...just the first part of it. Any ideas?. Can someone email it to me?

Brian Marshall

HI. I can't see the whole picture...just the first part of it. Any ideas?. Can someone email it to me?

the file is somewhat large.. you may just have to wait

or right click and save the pic.

calculating_infinity

Wow awesome!  I haven't built anything in a long time, I definitely will give this a shot!  Thanks a lot Brian.  Got any samples? 

~Jonathan