John Curl's Blowtorch preamplifier part II - Page 13

KSTR · 2nd July 2009, 02:56 PM

Quote:

Originally posted by Justcallmedad
First schematics, those posted by flg where only intended for learning, trying and illustration purposes…

An important note (I already guessed that), thanks!

The more detailed one show buffered followers to unload the refs? BTW, why the JFET drain resistors. practical issues?

- Klaus

KSTR · 2nd July 2009, 03:06 PM

And those are Ciuffoli-type followers, an idea I always liked since I first saw it -- very useful for PSU tasks.

- Klaus

Justcallmedad · 2nd July 2009, 03:09 PM

Quote:

Originally posted by KSTR
An important note (I already guessed that), thanks!

The more detailed one show buffered followers to unload the refs? BTW, why the JFET drain resistors. practical issues?

- Klaus

Not to unload refs but to dont load the RC filter by a bjt, and to get a low impedance for the shunt bjt . Drain resistances are used to reduce the voltage across the fet (Low Vds)

KSTR · 2nd July 2009, 03:16 PM

Thanks, was what I thought, more or less.

syn08 · 2nd July 2009, 03:17 PM

Quote:

Originally posted by Justcallmedad
First schematics, those posted by flg where only intended for learning, trying and illustration purposes…

Last ones in post 93 are fully functional Zout is about 0.5R from 0 to a few tenth of Mhz(I cant measure beyond) noise is -150 db from 100hz to 100 kHz (can't measure beyond neither ) and decreasing to -130 db at 10Hz of course there is no feedback "loop", for me the shunt bjt is 100% “feedback”.

New PSU

Really, that last shunt regulator can't be considered "local feedback", but full blown global NFB. The error amp is now the JFET and the open loop gain of this "opamp" is about Beta*(Gm-jfet/Gm-bipolar). Hence, the output impedance (essentially 1/Gm-bipolar) is divided by the loop gain and is at the end of the day about 1/(Beta*Gm-jfet). The bipolat shunt regulator acts as a Gm-jfet multiplier (by Beta).

I agree this discrete regulator has very good frequency response, but still the LF output impedance is nothing to call home about. I am surprised by the noise performance you measured. Anyway, a wideband, low noise opamp like THS4031 in the same configuration and a bandgap reference instead of the LED will do a much better job.

Magura · 2nd July 2009, 03:18 PM

Quote:

Originally posted by bear

So I am thinking why not a choke or a pi filter in the same spot?

_-_-bear

An air core inductor works rather well in this application.

I made some 5mH ~10DCR coils for heading a shunt reg a while back, as I found it to be the simple, proven solution.

A series reg in place of the inductor was also tested, but the inductor offered at least the same performance, caused less heat, and took up less space.

My scope is not exactly of the newest generation, so in this end of the scale, I can't provide reliable measurements.

Magura

KSTR · 2nd July 2009, 03:40 PM

Syn08, I don't see any other "error amp" than that single Vbe junction. The JFET just buffers the refs to a lower source impedance, driving the base... but that might be semantical issues...

- Klaus

john curl · 2nd July 2009, 03:40 PM

I like common mode chokes, hi H, low R, with external R damping.

syn08 · 2nd July 2009, 03:46 PM

Quote:

Originally posted by KSTR
Syn08, I don't see any other "error amp" than that single Vbe junction. The JFET just buffers the refs to a lower source impedance, driving the base... but that might be semantical issues...

- Klaus

It might be semantical

The inverting input is the gate, the non inverting input is the source and the output is the drain. The output is loaded by the resistor and the bipolar rb'e, that is about Rload=rb'e. The non-inverting input at the ground is nothing else but "unity loop gain".

The voltage gain of the "opamp" is then Vo/Vi=Gm-jfet*Rload=Gm-jfet*(Beta/Gm-bipolar)

KSTR · 2nd July 2009, 03:56 PM

But then again (we are refering to this, neg side, are we?) the JFET drain is not involved in the circuit at all as could be hooked to any potential to make the FET work with no change in circuit behavior (EDIT: and same thing for the collector, except for the return current path we want in the first place)? But we can leave it at that, it's not worth any hair-splitting IMHO.

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2nd July 2009, 03:06 PM	#122
KSTR diyAudio Member Join Date: Jul 2007 Location: Central Berlin, Germany	And those are Ciuffoli-type followers, an idea I always liked since I first saw it -- very useful for PSU tasks. - Klaus

2nd July 2009, 03:16 PM	#124
KSTR diyAudio Member Join Date: Jul 2007 Location: Central Berlin, Germany	Thanks, was what I thought, more or less.

2nd July 2009, 03:40 PM	#127
KSTR diyAudio Member Join Date: Jul 2007 Location: Central Berlin, Germany	Syn08, I don't see any other "error amp" than that single Vbe junction. The JFET just buffers the refs to a lower source impedance, driving the base... but that might be semantical issues... - Klaus

2nd July 2009, 03:40 PM	#128
john curl diyAudio Member Join Date: Jul 2003 Location: berkeley ca	I like common mode chokes, hi H, low R, with external R damping.

2nd July 2009, 03:56 PM	#130
KSTR diyAudio Member Join Date: Jul 2007 Location: Central Berlin, Germany	But then again (we are refering to this, neg side, are we?) the JFET drain is not involved in the circuit at all as could be hooked to any potential to make the FET work with no change in circuit behavior (EDIT: and same thing for the collector, except for the return current path we want in the first place)? But we can leave it at that, it's not worth any hair-splitting IMHO.