Electronics > Beginners

JFET Audio preamp, diode set constant drain-source voltage, why not with BJT's?

(1/4) > >>

ELS122:
Many JFET buffer stages I see have a diode or some other circuit that keeps the JFET source-drain voltage constant, I presume this is done for increased linearity right?
But why isn't it done for BJT buffers? I've never seen an emitter follower set up like that.

ELS122:
Here's a couple circuits like that:

ELS122:
The closest thing I've found is the Radford preamp. But it's very much NOT the same, as you can see.

Kleinstein:
JFETs are not that ideal as BJTs. The drain voltage has more effect and often also the thermal offset drift is larger. A variable voltage causes a change in temperature and this way some extra error (though to a large part linear and mainly low frequency).

As part of chips a constant collector - emitter voltage is still used with super beta transistors, as these usually have too low a maximum voltage. There are few discrete ones too and these would in many cases need a similar bootstrapping of the collector side too.

The Circuit shown as the Radford preamp is the classic Sizlaky stage - not directly related.

Anyway most modern circuits now use OP-amps and feedback - the open loop buffers are a bit outdated in the audio range.

CaptDon:
One thing for sure, that m.buffer circuit is a current limiter safety circuit, not a voltage stabilizer circuit.

Navigation

[0] Message Index

[#] Next page