Why power JFET’s you ask? Don’t we have enough transistors already? We do have lots of choices in devices, but a power JFET brings some particular advantages to the table that we don’t get elsewhere.
First off, for comparable devices at a given bias current, we see that the power JFET has much less distortion. The original F3 circuit was designed with a MOSFET, and comparing “apples to apples”, the power JFET operated with one-fifth the distortion of the MOSFET.
That’s only the beginning. The JFET has much less distortion operated as if it were a MOSFET, but it’s not a MOSFET, and it has characteristics which allow for even better performance by working the load line of the amplifier, much as found in tube design. To do this, I developed the Modulated Cascode ™ which allows arbitrary ratios of voltage and current (the load line) independent of the actual load value and reactance.
In recent years, advances have been made in vertical JFET design which makes them more competitive in switching applications, and as a result power JFETs have been re-introduced after a thirty-year absence. Earlier this year I published the “Zen Variations #8 – the Power JFET Amplifier” and now First Watt is introducing the model F3. (At introduction, the world’s only commercially available amplifier using power JFETs).
This is what the simplified schematic of this amplifier looks like:
It’s a little hard to believe that such a simple little amplifier and achieve distortion as low as 0.003% distortion at 1 watt into an 8 ohm load.