FET emulation of tubes without trimmer?

[puretube]

ooops - info lost... 

https://tagteamdbserver.mathworks.com/ttserverroot/Download/571_9804v00_JFET_te.pdf
http://w7zoi.net/jfet101.pdf
http://www.prism.gatech.edu/~gtg032i/TechDocs/JFET_VCF.pdf
http://www.faqs.org/docs/electric/Semi/SEMI_5.html
http://socrates.berkeley.edu/~phylabs/bsc/PDFFiles/bsc4.pdf
http://socrates.berkeley.edu/~phylabs/bsc/PDFFiles/bsc5.pdf
http://courses.ece.uiuc.edu/ece443/JFET.pdf
http://www.scribd.com/doc/2634608/Bel-06-field-effect-transistor
http://www.aes.org/e-lib/browse.cfm?elib=1014
http://wps.prenhall.com/chet_boylestad_electronic_9/37/9566/2449036.cw/index.html
http://www.macom.com/Application%20Notes/pdf/an211arev0.pdf
http://www.ibiblio.org/kuphaldt/electricCircuits/Semi/SEMI_5.html
http://www.tpub.com/neets/book7/26i.htm
http://openbookproject.net/books/socratic/output/jfet.pdf

[JDoyle]

and to add a quote:
"However, unlike the resistor in the drain circuit, any current in Rsource causes a voltage across it. The voltage across it raises Vsource and therefore lessens the voltage difference between the gate and source. Rsource causes a voltage that opposes the change in Vbias - it's giving us negative feedback."

from http://www.geofex.com/article_folders/mosboost/mosboost.htm

Good for you! You added a quote! Add that to your elementary math that proved absolutely nothing, and your personally invented definitions for every important term we are dealing with, and you still haven't shown how 'simple' it is that adding a source resistor equates to negative feedback...

I didn't start this nastiness, you did with your 'Simple' comment, implying that I'm an idiot.

As for the comment for R.G., as much as I respect and admire R.G., I believe he either: a - simplified the situation so as not to have to deal with this exact type of 'discussion'; b - forgot to add 'similar to' before 'negative feedback' in the last sentence you quoted of him; or c - has the same misconception of what source degeneration does as you do. R.G. is a human, not a computer, so yes, he too will make mistakes like the rest of us.

Despite the fact that I feel like I'm trying to convince a wealthy audiophile that $10,000 cables aren't really necessary, and that I know whatever I say will be dismissed, I'm going to soldier on here...

So. Let's state a definition:

(1) Negative Feedback is the act of feeding a portion of an opposite phase output signal back to the input of an amplifier.

If you have another definition that you are using, or have invented entirely, please let me know so we are both on the same page here. Hell, I may even agree with you depending on what you've decided how negative feedback is defined. But seeing as I am going to need to 'put the rest of the electronics world right on the subject', I thought I would use the definition that 'the rest of the electronics world' already uses - but again, I'm flexible as to how this discussion can go, if you want to redefine a term, fine, just let me refine my answers in accordance afterwards. Until then, I'm going to use 'the rest of the electronics world'('s) definition of negative feedback.

Now add to that definition of negative feedback two statements of fact:

(2) During operation as an amplifier, the gate of a JFET is separated from the drain to source channel by a reverse p-n diode junction between the gate and the source, giving the JFET's gate an input impedance in the multi-megaohm range.

(3) In a JFET amplifier, when a resistor is added in series with the JFET's source, and the gate is used as the input, the signal on the source is the same phase as the signal at the input/gate.

Again, if you need to redefine or correct me in either of the above statements, please do, but I think we can all agree that points (2) and (3) above are facts.

So if you are right that adding a source resistor to a JFET amplifier is negative feedback, and therefore "feeds a portion of an opposite phase output signal back to the input":

You will have to somehow show that by simply adding a resistor, the signal on the source can traverse a reverse biased p-n diode junction (it's the same input impedance from the source to gate as it is from the gate to source as it is the same reverse biased diode), thereby disproving the entire canon of semiconductor physics; AND, and boy is this one damn big 'and', even considering that you just decimated the entire semiconductor industry if you did manage to somehow prove the former, you will STILL need to somehow also show that the signal at the source REVERSES PHASE as it crosses this reverse biased p-n junction.

So either you are right and need to think seriously about writing one hell of an earth shattering dissertation...


Or you are wrong.


Regards,

Jay Doyle

[puretube]

(3) In a JFET amplifier, when a resistor is added in series with the JFET's source, and the gate is used as the input, the signal on the source is the same phase as the signal at the input/gate.

so, will it subtract from an oppositely phased Drain-signal?
(in the Vdd to Vss signalpath...)

[JDoyle]

I don't see what the big deal is- it's just a question of semantics. Either way, negative or degenerative, the overall effect is to reduce gain.

It isn't just semantics Doug, though I can definitely see your point.

While both do reduce gain, negative feedback does it by sending part of the amplifer's output signal back to the amplifer's input signal, thereby subtracting from the input - the amplifier stage itself is unchanged; while degenerative feedback alters the operational Q point, load line, and limits of the amplifier stage itself.

With negative feedback, a portion of any amplifier induced distortion is fed back to the input inverted, to then be amplified by the amplifier, which, because it was fed back out of phase, then cancels out an amount of the amplifier induced distortion proportional to the amount fed back. Basically, by using negative feedback, you are 'pre-subtracting' a portion of the distortion that will be induced by the amplifier, so when it IS induced, it doesn't have as great an effect. Not the most elequent or accurate of ways to put it, but one that I think makes sense.

Degenerative feedback reduces distortion by bringing the transistor stage into a more linear and usable portion of the transistor's transfer curve. It reduces distortion simply because the stage is then biased to be more linear, not because any induced distortion is fed back to the input to then cancel out the distortion in the amplifying process itself, but simply because there is less of it.

I hope that made sense.

Regards,

Jay Doyle

[JDoyle]

(3) In a JFET amplifier, when a resistor is added in series with the JFET's source, and the gate is used as the input, the signal on the source is the same phase as the signal at the input/gate.

so, will it subtract from an oppositely phased Drain-signal?
(in the Vdd to Vss signalpath...)

I hope so - otherwise we're going to need to redefine the whole Av=Rd/Re equation... 

[alanlan]

Take the bog standard source follower:

The input to the circuit is vin which is tied to the gate so vg = vin.

The input to the device is the voltage across the gate and source i.e. vg - vs or vgs.

vout = vs i.e. voltage on source wrt gnd.

so the device input = vin - vout

vout = gm.(vin - vout) for small signals - ignoring non-linearities

So, negative feedback has been applied to the device.  More specifically, it is series voltage negative feedback.

Have a look at this:
http://en.wikipedia.org/wiki/Image:Feedback_amplifier_input_resistance.PNG

[earthtonesaudio]

Says you can minimize device-to-device gain variations (page 4-6) using the graphs and some calculations, but I tried and came up with weird numbers.  Anyone else tried successfully?

I read that years ago, and decided I could not make it work reliably.

There's one of those engineering limits things going on here. If there was a simple way to make JFETs have much less unit-to-unit variation by how you biased them, just by using a couple of resistors, they would be used in more circuits as something other than switches or input followers.

Cool, thanks R.G. for the reply.  I got the impression that the devices used as examples in the appnote were chosen because they fit the method well... not many other devices fit the profile of the one they used.  So maybe it's a "trick" that can be used, but only with certain JFETs, and maybe not the ones the DIY stompbox people like to use.  I guess I'll have to bang my head against it for a while before I decide it can't be done.  Not to discount your advice; I just gotta see it for myself.

[DougH]

It isn't just semantics Doug, though I can definitely see your point.

While both do reduce gain, negative feedback does it by sending part of the amplifer's output signal back to the amplifer's input signal, thereby subtracting from the input - the amplifier stage itself is unchanged; while degenerative feedback alters the operational Q point, load line, and limits of the amplifier stage itself.

With negative feedback, a portion of any amplifier induced distortion is fed back to the input inverted, to then be amplified by the amplifier, which, because it was fed back out of phase, then cancels out an amount of the amplifier induced distortion proportional to the amount fed back. Basically, by using negative feedback, you are 'pre-subtracting' a portion of the distortion that will be induced by the amplifier, so when it IS induced, it doesn't have as great an effect. Not the most elequent or accurate of ways to put it, but one that I think makes sense.

Degenerative feedback reduces distortion by bringing the transistor stage into a more linear and usable portion of the transistor's transfer curve. It reduces distortion simply because the stage is then biased to be more linear, not because any induced distortion is fed back to the input to then cancel out the distortion in the amplifying process itself, but simply because there is less of it.

I hope that made sense.

Regards,

Jay Doyle

I agree it's not global negative feedback, Jay. But the way it's being tossed around in this thread, I believe it means feedback that reduces gain. I'm not going to argue terms, I have a life. But I agree that degnerative and global-negative feedback are two completely different things.

The input to the circuit is vin which is tied to the gate so vg = vin.

The input to the device is the voltage across the gate and source i.e. vg - vs or vgs.

[...]

So, negative feedback has been applied to the device.

Like I said, semantics. See my global comment above. (The italicized emphasis in the quote is mine.)

[Dragonfly]

Sometimes I think people get to arguing so much that they forget what the thread was about in the first place. 

[StephenGiles]

jesus. all over a @#$%ing fuzz box........how pathetic

[DougH]

Agreed. (With Andy & Stephen.)

[Dragonfly]

Not that anyone cares, as they're too busy arguing...but in case you do, you might find a few clues in these docs....


http://www.aronnelson.com/gallery/main.php/v/DRAGONFLY-LAYOUTS_0/JFET+INFO/
http://www.pacificrecone.com/JackDarrBook.html

[gez]

jesus. all over a @#$%ing fuzz box........how pathetic

Not just any fuzz box Stephen, but one with degenerative feedback...or possibly negative feedback.

Makes a hell of a difference! 

[StephenGiles]

Perhaps, but to me fuzz is fuzz and not worth arguing over, when there are far more interesting areas to explore, inevitably ignored by most! For unmodulated clock krackers it's OK I suppose.

[gez]

Perhaps, but to me fuzz is fuzz and not worth arguing over

Indeed! (my post was in jest)

[puretube]

Sometimes I think people get to arguing so much that they forget what the thread was about in the first place. 

Yes, the initial purpose/question of this thread (like mentioned earlier in reply #54) has been answered/solved (by me).

The results don`t have anything in common anymore with "tube-clone-circuits",
but went way beyond that...

We went to pedal/CV-modulated variable envelopecontrolled frequency-multiplication through zero, back, and FeeDonWard...

Lawyers were mentioned (in an other thread, during the time),
who were simply patent-guys investigating the novelty
but have been falsely misinterpreted by selfnominated foreign "jurors" and their Madmen
who out of their nonabilty of using their searchyourownrationality even decided
it were neccessary to freeze tompboxes and forumice them into their
builderbook kindergarden... rant,rant,rant



and now I`m sitting here figuring out how in heaven to replicate those effects/sounds, using your average dualtriode t**bz...

[Dragonfly]

and now I`m sitting here figuring out how in heaven to replicate those effects/sounds, using your average dualtriode t**bz...

[puretube]

Hehe; DONE!   : just replace those JFETs with E-H 12AX7s, lift "V_DD" to ~250V,
and don`t forget to adapt the caps voltage, and keep the cat out of the lab...

[wampcat1]

...

nevermind...

[DougH]

Hehe; DONE!   : just replace those JFETs with E-H 12AX7s, lift "V_DD" to ~250V,
and don`t forget to adapt the caps voltage, and keep the cat out of the lab...

Amazing how that works, isn't it?

Except... I'd use JJ's...

Bwahahaha!

Speaking of cats my favorite Monty Python quote is from the surgeon: "Get that cat outta here!"