Parallel JFETs vs. Parallel BJTs

[midwayfair]

Just taking a basic single gain stage from something like this (this is a clone of the Wampler 57):
http://home.comcast.net/~gprause/BuildDocs/Tweedy_V2.0_Build_Doc.pdf

I've tested this before on the breadboard and was reminded of it today. A matched pair of paralllel JFETs "seems to work" -- you seem to get a little more gain -- while parallel BJTs seems not to -- one just seems to take over even with matched hfe.

Is there a good reason, or is my imagination making me wrong about one or the other here? Others have reported similar results on the BJTs in the past, though I can't remember where I saw it. (I think it was in a thread about one of Jimi Photon's fuzzes.)

[Paul Marossy]

You might get something out of this thread: http://www.diystompboxes.com/smfforum/index.php?topic=101274.0

Reply #16: "You don't have the same problem with paralleling JFET's as you do with bipolar transistors.  With a bipolar transistor, one slightly stronger transistor will hog the current because the higher temperature from more current flow will reduce Vbe and therefore, devices that start with nearly equal current will shift entirely to the transistor with lower Vbe.  This can only be corrected by separate equal emitter resistors.  There is no such effect with a JFET or MOSFET - the channel is modelled as a resistance with a positive tempco of resistance and if one starts to heat up, its resistance increases and it tends to share the current without the need for source resistors.  This of course implies the Vgs on voltages are equal to start with."

[midwayfair]

You might get something out of this thread: http://www.diystompboxes.com/smfforum/index.php?topic=101274.0

Reply #16: "You don't have the same problem with paralleling JFET's as you do with bipolar transistors.  With a bipolar transistor, one slightly stronger transistor will hog the current because the higher temperature from more current flow will reduce Vbe and therefore, devices that start with nearly equal current will shift entirely to the transistor with lower Vbe.  This can only be corrected by separate equal emitter resistors.  There is no such effect with a JFET or MOSFET - the channel is modelled as a resistance with a positive tempco of resistance and if one starts to heat up, its resistance increases and it tends to share the current without the need for source resistors.  This of course implies the Vgs on voltages are equal to start with."

Thanks, Paul! I remember seeing that original post up to Gus's "Don't guess do the math" post, but didn't keep following it. Some good info there. Now to ponder whether to stick a matched pair into my Azabache's final gain stage ...

[Paul Marossy]

Glad to be of service. 

Paralleled JFETs sound to my ears very similar to paralleled triodes.

[PRR]

Transconductance (Gm) of BJT varies directly as current.

2 devices, each takes half current, gives half Gm, two halves is a whole, gain is unchanged. (Further: it is difficult to get two BJTs to split equally.)

Field-effect devices (tubes, JFETs, MOSFETs) in the saturation range (normal current): the Gm varies slower than the current. Assume square-root. Half current is 0.7 of Gm. Two 0.7s is 1.4. Gain goes up to 1.4.