Minibooster/mu-amp with one BJT and one JFET?

[Buran1997]

After reading a couple of Geofex articles, I understand that the principle of the mini-booster/mu-amp is that the lower JFET is set up as a common source amplifier which does the work of amplifying (and nicely colouring) the signal, while the upper JFET acts as a source follower contributing to the voltage swing of the amplifier. I'm interested in building this, but there's a problem: JFETs are getting expensive, often one or two dollars per unit. BJTs, on the other hand, cost pennies. My question is: can the JFET source follower be replaced with a BJT emitter follower (as suggested in the first article)? Can I do a one-to-one substitution, or does some other part of the circuit have to change as well? Finally, is it possible to do this with the SRPP amplifier described in the second article:



TL;DR: What are the advantages (other than reduced cost), disadvantages, and implementation details of replacing the upper JFET with a BJT in a mu-amp?

[amptramp]

If you want to get equal swing positive and negative, the transconductance of the upper transistor should be the reciprocal of the source resistor.  In this circuit, the upper FET transconductance should be 1000 µmhos.  You can define a transconductance for a bipolar transistor but it is usually large, meaning the emitter resistor would be much lower in value.

The value of the load is important - consider the two transistors to be current sources operated in series.  With no load, one completely overpowers the other and the signals are driven to the rails.  (This could be the basis for a fuzz/distortion box.)  You need a load resistance that takes the difference in current between the upper and lower current sources and serves as a sink for the difference current.

[PRR]

> JFETs are getting expensive, often one or two dollars

How much do they charge for beer these days? (I don't buy beer anymore.) Discounting mare-pee beers, looks like $1-$2 (to $6 at the ball park). And you don't own beer, you rent it. And you feel like crap the next day.

I think JFETs are a much better buy than beer. (If you don't do beer, you probably have some other indulgence: shirts, strings, guitars, llama-wrapped guitar cords...)

Making a BJT do what a JFET does naturally is not simple (as Ron interjected). There may be a better way to skin this banana, but more features, not a lot simpler.

[Buran1997]

> JFETs are getting expensive, often one or two dollars

How much do they charge for beer these days? (I don't buy beer anymore.) Discounting mare-pee beers, looks like $1-$2 (to $6 at the ball park). And you don't own beer, you rent it. And you feel like crap the next day.

I think JFETs are a much better buy than beer. (If you don't do beer, you probably have some other indulgence: shirts, strings, guitars, llama-wrapped guitar cords...)

Making a BJT do what a JFET does naturally is not simple (as Ron interjected). There may be a better way to skin this banana, but more features, not a lot simpler.

Seems fair enough. What about MOSFETs? My local electronics store stocks the 2N7000 for 20 cents a piece, which is still considerably cheaper than a JFET.

[Rob Strand]

Seems fair enough. What about MOSFETs? My local electronics store stocks the 2N7000 for 20 cents a piece, which is still considerably cheaper than a JFET.

There's a thread about 3 weeks ago on a MOSFET muamp; thread started by bushidov.

https://www.diystompboxes.com/smfforum/index.php?topic=124961

[Steben]

Seems fair enough. What about MOSFETs? My local electronics store stocks the 2N7000 for 20 cents a piece, which is still considerably cheaper than a JFET.

There's a thread about 3 weeks ago on a MOSFET muamp; thread started by bushidov.

https://www.diystompboxes.com/smfforum/index.php?topic=124961

I like the note on using a single MOSFET.


Using two MOSFETs is AFAIK succesfully done on using CMOS buffers as opamp stage. You could make a discrete one.