Power JFETs - yes, JFETS - are available again, and offer some intriguing advantages over MOSFETs in audio amp design.
Ever heard of a power JFET? I hadn't (at least by that name) until I ran across The Power JFET Amplifier, a simple low-power DIY power amp project based around a single power JFET device.
As it turns out, the device used is the same or similar to the VFET (or Vertical FET) parts that Sony and Yamaha used in some of their high-end audio amplifiers back in the 70s. The VFETs were eventually replaced with less-expensive alternatives, and apparently stopped being produced not long afterward.
Now they're back - perhaps because of an increased interest in more power-efficient design technologies (although that's certainly not the case for the above DIY amp!). The manufacturer, Qspeed Semiconductor (formerly Lovoltech), claims power JFETs offer several advantages over power MOSFETs, including a lower on-resistance, lower gate turn-on voltage, no inherent body diode and a low-noise linear output.
This is good news for audiophiles. According to high-end audio designer Nelson Pass of Pass Labs, the parts have a "triode character" that is of interest to audio designers, and are more linear than comparable MOSFETs for a given bias current, among other things.
The currently available devices have a VDS of 24 V, an ID of 50 A and power dissipation ratings of 69 or 80 W. Their voltage rating isn't very high for power amplifier purposes, so to get around this the DIY power amplifier mentioned above cascodes a power MOSFET with the power JFET to enable a higher output voltage. (This amplifier is also available as a commercial product - hand built by Nelson Pass himself! - at First Watt.)
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