Ever try a JFET power amp?

G

gtt

Member
I was reading about these crazy high voltage and current SiC JFETs (onsemi UJ3N065080K3S) and wondered if they could be used like a power tube in push-pull with a phase inverter and a transformer.

The dream of course is an all-JFET Viceroy as an amplifier not just a pedal. ;)

Chuck did you ever look at anything like this?
 
I was aware of the technology, but have not investigated it. A quick scan of the datasheet tells me that these are intended for power conversion applications where they are used as switches. They can be operated in linear mode and the gain (transconductance) is HUGE. Orders of magnitude higher than a vacuum tube or the typical small-signal JFET. The capacitance is quite high as well, which means a linear power amp circuit will need to have much lower impedances than a tube amp to achieve the same freq response. Intriguing.

This is the pricing at Mouser.
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Seems like it'd be something interesting to play with - especially since you could get quite a lot of power in a single-ended Class A output stage. Feeding it a low impedance is easy enough. The heatsink? Maybe not so easy - or light.
 
I suspect something like this would be easy to adapt, you wouldn't need to parallel the JFETs in the power section of course. Although I wonder, how tube like are power JFETs, a class AB MOSFET section should sound as tube like as JFETs I would think?

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MOSFETs & JFETs have the same transfer function.

Where did that schematic come from?

I'm not seeing the advantage of a power JFET over a power MOSFET in this application.
 
Chuck D. Bones said:
MOSFETs & JFETs have the same transfer function.

Where did that schematic come from?

I'm not seeing the advantage of a power JFET over a power MOSFET in this application.
Click to expand...

Apologies, here's the link, it's one of those Russian circuits, I think the LTP circuit, with an AB Mosfet Power amp if you want real volume, as is this creates a nice little power amp sim that produces ~250mW, you can even load it with an inductive load if you want the speaker impedance response.

 
I'm curious as to the overdrive character of these? Would the overdrive character be different than a mosfet?
 
It could be and probably not in a good way. With a MOSFET you can never overdrive the gate. At audio frequencies, the gate current is essentially zero no matter how hard you drive the gate. with a JFET, it you raise the gate voltage above the source voltage, then the gate is forward-biased and starts conducting current. This changes the harmonic structure. It is analogous to driving a vacuum tube hard enough to cause it to draw grid current.
 
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