Mini-boost transistor question

[merlinb]

Thanks, Merlin, that makes sense to me. But in practical, real life terms (like in Jack's variation of the circuit used as a guitar booster), in what region does it spend its time?

Saturation region, so it's resistance is mostly constant. Subject to device variation. Plug in a different device and you'll get different gain, maybe +/- 20% at a guess.

Merlin, one query - is "10mF" on your schematic (set of three schematics) a typo?
I've never seen "mF" before. 10mF would be 10,000uF, right? Quite a big cap for an audio circuit!

I quickly borrowed the circuit from the OP's geofex reference. mF is microfarad, you see it a lot on vintage diagrams (yeah even though logically it should be millifarads).

[Clint Eastwood]

So why then does the circuit with the JFET have much more gain than the same circuit with an ordinary dumb drain resistor? I don't get what you are saying... clearly C2 makes the JFET do more than a dumb resistor can?

Because the upper JET 'looks like' a really big resistance, bigger than you could practically use an actual resistor. But if you did use a resistor of the same size (you would need a much higher supply voltage) you would get the same gain.

I think I understand now what is going on. It is the same trickery as with a bootstrapped current source, with two resistors and a capacitor,  isn't it?
Thanks for the figures explaining the difference between push pull and not pp. I don't want to abuse your patience, but why does a small resistor make it push pull?

[merlinb]

I think I understand now what is going on. It is the same trickery as with a bootstrapped current source, with two resistors and a capacitor,  isn't it?

Precisely.

Thanks for the figures explaining the difference between push pull and not pp. I don't want to abuse your patience, but why does a small resistor make it push pull?

When Q-bottom increases conduction by pulling current from the load it flows through R, which develops a voltage across R which pulls down on the gate of Q-top, so Q-top reduces conduction. When Q-bottom reduces conduction the opposite happens. With a the right-sized resistor you will get equal-but-opposite change in each JFET, which is push pull. But is has to be the right size! R = 1/gm. It's more of a theoretical situation, 99% of the time you will be working with one of the following:
When R is bigger than 1/gm, Q-top will be doing all the work; a source follower that happens to be stacked on top of Q-bottom.
When R is smaller than 1/gm (or zero), Q-bottom does all the work, the top JFET becomes a dumb resistance.

[Clint Eastwood]

Aha.    Thank you for explaining!

[Vivek]

I had done a spice analysis and found that the transfer function is quite like a tube

Asymmetric, with smoother rounded knees

Opamps hit rail hard and have sharp knees

I tried to make a transistor circuit that distorts in similar way, but could not


https://www.diystompboxes.com/smfforum/index.php?topic=127101.msg1217772#msg1217772

[Steben]

I had done a spice analysis and found that the transfer function is quite like a tube

Asymmetric, with smoother rounded knees

Opamps hit rail hard and have sharp knees

I tried to make a transistor circuit that distorts in similar way, but could not


https://www.diystompboxes.com/smfforum/index.php?topic=127101.msg1217772#msg1217772

And have you thought about comparing the transfer functions of minibooster with a single jFET stage?
One of the major advantages (or reasons) still seems gain, not tone.

[ElectricDruid]

Merlin, one query - is "10mF" on your schematic (set of three schematics) a typo?
I've never seen "mF" before. 10mF would be 10,000uF, right? Quite a big cap for an audio circuit!

I quickly borrowed the circuit from the OP's geofex reference. mF is microfarad, you see it a lot on vintage diagrams (yeah even though logically it should be millifarads).

Thanks, understood!

[Steve.mg]

Well,
       I'm glad we got that sorted out...!
So, apart from the Schematic (which we all love!) has anyone actually "made one" using the PCB from AMZ-FX? I didn't
particularly like the idea of buying new JFETs at [AUD]$6.26 each so using some 2N5484's I am still wondering after all
the previous workings out about "top Q" and "bottom Q"... what happens when we introduce "side Q"?  OR, was that only
done when ALL the "Q's" were going to be boring BJT's... IT's still a booster, after all...

Kind Regards,
Steve [AUS]
...

[PRR]

...I've never seen "mF" before. 10mF would be 10,000uF, right? Quite a big cap for an audio circuit!

Better get used to it. At least two meanings:

Older or lazy or ignorant: "m" or "M" is micro

Newer and bigger: Capacitors of many thousand microFarad ARE common in hi-fi power amps. I see 6,800uFd, 16,000uFd, even 4*22,000uFd frequently. "Mine is bigger than yours!!" OK, but now at least one popular schematic package will "automagically" render 16,000uFd as 16mF (16 milliFarad). In fact recent versions of Duncan PSUD do:

[m4268588]

well, then...




"Asymmetric"
"Soft knees"
Other theological words.

[Ben N]

Merlin, one query - is "10mF" on your schematic (set of three schematics) a typo?
I've never seen "mF" before. 10mF would be 10,000uF, right? Quite a big cap for an audio circuit!

I quickly borrowed the circuit from the OP's geofex reference. mF is microfarad, you see it a lot on vintage diagrams (yeah even though logically it should be millifarads).

Yeah, I have seen that, and it can be a real MF to figure it out, IYKWIM.