Hi all I need to build a Jfet pushpul low power amp like those in transistor radios of the 70's with audio transformer output .
The problem is how to bias the output Jfets when I use a single 9VDC supply .
Any Ideas ?
Thanks BarNash
Decoupled source resistors. Degrades the efficiency. Watch for imbalance which might bias the transformer.
Graham H
That's very weird. Why do you need to do that?
John
Probably class B or AB.
Have Fun! Rich
And an output filter to deaden the high harmonic blips while it's slewing inbetween?
Ewww...
Tim
-- Deep Friar: a very philosophical monk. Website: http://webpages.charter.net/dawill/tmoranwms
I measure noise and distortion in PPM... single digits of same.
Class AB bipolar amps are hardly linear open-loop.
John
So you've got ~80dB NFB? Yea, at what frequency?
Then you didn't design them correctly. Hell, lots of emitter resistors and a few more gain stages and you don't even need a loop.
Tim
-- Deep Friar: a very philosophical monk. Website: http://webpages.charter.net/dawill/tmoranwms
50 KHz net bandwidth is good for NMR and MRI. The real trick is to get any hum way below 1 PPM and to have steps settle to PPMs in 100 usec or so.
It's not a single overall fb loop... there's some local stuff around the output stage.
The driver, usually dueling current sources, slamming into the output transistor bases is a major source of nonlinearity. All sorts of stuff happens near the zero crossing, or elsewhere for reactive loads.
I prefer to drive the loads out of the drains of the fets, not the sources. One of the many benefits is that you can bolt all the fets to a common heat sink with no insulators.
ftp://66.117.156.8/Amp.jpg
John
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