piggybacking JFETs

[squidsquad]

Didn't find anything in a search.  I'm aware of it's use w/transistors...lowering the HFE.  Anyway...I've noticed a couple of times...to be specific...in R G's SRPP (bottom portion)...and in ROG's  Fetzer Valve...that adding a JFET piggyback style adds a DB or two in gain.  It appears 2 do a better job than one.  Anyone else experimented w/this?

[brett]

Hi
JFETs are voltage-based current controllers that consume almost no ampage at the gate.
So lots (by which I mean thousands) can work together, if needed.  Or, you could just buy one big one.

So I'm guessing that by putting two in parallel, you can double the transconductance and current output.  Three would triple it, and four would quadruple it.

For our small signals, you can achieve a similar result by replacing a low transconductance JFET (e.g. 2N5486 or MPF102) with a high transconductance one (e.g. the J201.  That's why folks love 'em.).

cheers

[mac]

JFETs are voltage-based current controllers that consume almost no ampage at the gate.
So lots (by which I mean thousands) can work together, if needed.  Or, you could just buy one big one.

So I'm guessing that by putting two in parallel, you can double the transconductance and current output.  Three would triple it, and four would quadruple it.

Cool. I´m going to try it.

For our small signals, you can achieve a similar result by replacing a low transconductance JFET (e.g. 2N5486 or MPF102) with a high transconductance one (e.g. the J201.  That's why folks love 'em.).

I´m using 2sk117, very high transconductance.

mac

[JDoyle]

This could be really interesting! With the gates both conducting so little current, neither input loads the other down so if the parameters of the JFETs are matched you have a larger JFET; OR you could have the two working in parallel for a portion of the signal, and then if you bias them right have each take over on it's own at either extreme! OR, with the J201/MPF102 combo, after the J201 cuts off, the MPF102 should still have some room to move... It's all about the bias...

As one JFET drops out leaving the other along, the transfer curve will change, which means distortion... I know you will get it, I just don't know what it will sound like!!!

Let us know how it turns out!

Jay Doyle

[aron]

It's sounds more exciting than it is. We've have done this and it's a useful tool. To me, it "focuses" the JFET stage a bit more, but single JFETs sound more "alive" to me.

I guess don't expect a major amazing change in tone.

[brett]

Hi
interesting!

To me, it "focuses" the JFET stage a bit more

I'd guess that using different devices and spreading out the bias points will tend to linearise the output overall. (ie "soften" the distortion)

Here's an idea: Different numbers of different devices might work best together.  For example, one J201 (biased to a low -Vgs) in parallel with three MPF102s (biased to a high -Vgs).  This *might* be helpful because the J201 has lots higher transconductance than the MPF102s, and "overshadow" the effect of a single MPF102.

Note that the JFETs don't need to be matched.  IIRC, Rd of 6.8k for a J201 and 15k for an MPF102, both with with Rs = 2.4k put them into a good zone of bias.  Hey...I wonder whether they could all share one Rs? 
cheers

[JDoyle]

It's sounds more exciting than it is. We've have done this and it's a useful tool. To me, it "focuses" the JFET stage a bit more, but single JFETs sound more "alive" to me.

I guess don't expect a major amazing change in tone.

I'm not so much talking about simply piggybacking another same-type JFET with the first, but using another type altogether and biasing it seperately so that the only shared part is the drain resistor; each JFET has it's own bias circuitry.

If I'm thinking right, you can mess with the bias and set each so that one of the JFETs is still in linear mode as the lower Vgs(off) part cuts off, changing the gain.

The other swing towards saturation would be hard to seperate the same way, esp. without my morning coffee (I'm guessing you need to have a shared source resistor to do this), but even so, if you couldn't, or can't, get that swing to act in the same manner, at the least you have built in harmonic distortion with the asymetrical gain for each polarity.

Of course I haven't done anything like this, it hit me yesterday reading this post, so you are most likely right that it isn't very exciting at all in practice...

Jay Doyle