What is this transistor (BJT+FET) arrangement called?

[midwayfair]

Sorry for the ascii art ... for clarification, it's like a BJT buffer hooked up to the drain of a FET, but without a coupling cap between them. Q1's collector is connected directly to the +9V. R1 and R2 form a voltage divider between the 9v and the base of Q1, and in series they form a bias resistor to the collector of Q2. The BJT's source and FET's drain bias similarly, and adjustments to R3 change both. I can't remember seeing this. The gain is super high like this, much higher than it is if a coupling cap is placed between R1 and the base, and also much higher than Q2 by itself. So it seems to be a lot more than just a buffer for sure.

9V --_-------_
       |         |C (Q1)
       R1--->]B
       |         |E >--C1
       R2
       |D (Q2)
in--> ]G
       |S
       R3>Ground

Q1: NPN BJT. Q2: N-channel FET

Again, sorry for the ascii art; it's not my circuit and I can't show someone else's work (but it IS a DIY project). But I would like to know so I don't say something dumb in a demo I'm doing if I comment on the circuitry.

[psychedelicfish]

Is this what you mean:


circuits.io didn't have a JFET symbol, so I used a MOSFET

I'd say it's mostly just a DC coupled emitter follower. I've been thinking about this sort of thing lately, in that you quite often see DC coupled source followers in various FET based amp simulators. In a valve amp the DC coupled cathode followers "steal" current from the previous amplifying stage (thanks Merlin),  but with a JFET as the source follower the gate wouldn't pull much, if any, current from the previous stage, meaning you wouldn't get the same sonic benefits from a JFET source follower as from a valve cathode follower. A solution, I thought, would be to use a BJT instead of a FET, because a BJT will draw a current from the previous JFET stage. I never got around to testing this theory, but it looks like they've done something similar here, perhaps experimenting with extra resistors and finding that they provide far more gain. Why has every idea of mine been done before? 

[midwayfair]

Dude, thank you so much! That's exactly what I was talking about. Thanks for the quick rundown.

[Keppy]

It seems strange to me that you're getting high gain. R0 & R1 actually form a voltage divider between Q2 drain and +9v, so in theory they're bringing the overall gain DOWN. As far as I can tell, you'd actually get more gain by moving Q1 base to Q2 drain.

You're not confusing "gain" (relative signal amplitude) with "distortion" are you? Depending on the resistor values you could get a lot of distortion, but like I said this doesn't seem to be configured for maximum gain.

Oh, and sorry for interjecting my curiosity. I have no name for this circuit, and therefore am not helpful at all.

[psychedelicfish]

^I think they're doing something like in ROG's Omega, i.e. high value drain resistor, meaning lots of voltage swing for a small amount of current change through the FET.

[B Tremblay]

^I think they're doing something like in ROG's Omega, i.e. high value drain resistor, meaning lots of voltage swing for a small amount of current change through the FET.

Well spotted! That stage is the same as the Omega with a BJT in lieu of the JFET buffer, probably for lower noise.