Author Topic: Jfet power filtering and running op amp at rails  (Read 588 times)

Eddododo

Jfet power filtering and running op amp at rails
« on: July 31, 2022, 09:21:38 PM »
Got back into circuits when a need arose, and I have a lot of cobwebs..
Glad to be back

So I have a mostly fet circuit with an op amp envelope detector (sort of a flat liner setup but Jfet gain block.) I have another half an op amp to use up, so I thought I’d use it to buffer my power and keep noise down.. I’ve used buffers to supply 1/2 V+, but can I do the same thing for the +9v to supply the Jfet stages?

Something like this





I don’t know; it seems overkill, but I figured it would help, and would maybe protect the transistors better.
« Last Edit: August 01, 2022, 03:50:36 AM by Eddododo »

Eddododo

Re: Jfet power filtering and running op amp at rails
« Reply #1 on: July 31, 2022, 09:29:27 PM »
On a related note.. what’s going on with U5 in the pigtronics compared to the envelope detector in the flatliner… is it actually amplifying an inverted envelope signal, or is it just something for it to ‘push and pull’ on as a kind of ground?




anotherjim

Re: Jfet power filtering and running op amp at rails
« Reply #2 on: August 01, 2022, 05:07:12 AM »
In principle, you may use an opamp like that. In practice, you need the opamp +V supply higher than the +input voltage. For one thing, as drawn, the opamp input voltage range will probably be exceeded and for another, the opamp output will be at the + limit and be uncontrolled. It's a pick yourself up by your bootstraps situation.
I suspect that to use any opamp this way would need the circuit output to run off about 6v with a suitable 6v reference on the +input. Expensive full rail-to-rail opamps might be ok at 8v.
You've already got polarity protection with a diode. Overvolts for the JFET is over 40v, do you expect that to happen?
Extra RC filtering in the power for the audio input circuit is probably the only extra "clean-up" necessary.

Croeso i Diystompboxes.

I have no April 1st project

Eddododo

Re: Jfet power filtering and running op amp at rails
« Reply #3 on: August 01, 2022, 09:00:16 AM »
Thanks, that’s what I suspected.. i’m in a really awkward balance right now between finding a bunch of old notes and remembering a lot of things, but all of it being hampered by 1 billion little forgotten things

amptramp

Re: Jfet power filtering and running op amp at rails
« Reply #4 on: August 02, 2022, 07:50:28 AM »
Usually people buffer the Vb line with a unity-gain amplifier so the amplifier is well within its own limits.  The only way to get an amplifier to buffer the rail voltage is if the op amp is powered by a higher voltage.  If that is available, use a regulator rather than a buffer because a JFET stage has almost no power supply rejection.  The rejection is from a voltage divider between the drain resistor and the JFET output impedance.  If you have a 10K drain resistor and a 490K output impedance (typical values), 98% of the supply ripple appears in the signal.  With a regulator you get rid of the ripple.

puretube

Re: Jfet power filtering and running op amp at rails
« Reply #5 on: August 02, 2022, 12:58:58 PM »
Somebody help me (is the heat too much for me?): where does that "FSB"-schemo of a "Pigtronix" derive its envelope from? From a slow ramping-up 9V DC?  :-\

PRR

Re: Jfet power filtering and running op amp at rails
« Reply #6 on: August 02, 2022, 01:42:23 PM »
Even a regulator has voltage drop.

I can't picture an audio JFET stage that needs more than a mA or so. 100r plus 100uFd makes a LOT of passive rail filter at a tenth-Volt drop.
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