MOSFET

09-01-09: Kijanki
There is no perfect device. Mosfets output stages are:

- more robust than transistors
- have higher bandwidth
- are more tolerant of difficult loads

but are:

- less efficient since have lower voltage swing from the same supply voltage
- are more nonlinear than transistors around cutoff region
- require more gain in preceding stages to eliminate crossover distortions
- limited ability to apply local feedback (low gain)

Kijanki, some of the material you have written doesn't make sense to me & also appears to be contradictory: in the advantages you wrote that MOSFET have higher bandwidth than BJTs. In the disadv you wrote that they have lower voltage swing from the same supply voltage. This is contradictory to me.
Lower voltage swing from the same voltage supply compared to the BJT implies lower gain. Bandwidth & gain (Gm) are directly related. Lower gain (for the same amount of bias current) compared to the BJT implies lower bandwidth. This seems to be what I observe too.

I also don't know if there is any substantial evidence of BJTs being less robust than MOSFETs. Ditto re. the ability of MOSFETs to handle difficult loads any better than BJTs.

MOSFETs are a square law device hence the distortions are more along the tube-like behaviour. That's why one often reads of MOSFETs being tubey sounding. In comparison the BJT is an exponential device & the distortions are odd-order harmonic based. If the implementation is not correct/good enough this can lead to very discordant sound very quickly.

I agree w/ most of the other members that MOSFETs should sound pretty good if implemented "correctly" (another audio fuzzy term!). I believe that even better than the MOSFET is its cousin the JFET in the output stage - MOSFET electrical characteristics but BJT-like sound. Preferred by the discriminating amp builders. The sad part is that no one is making large quantities of power JFETs like they are power MOSFETs & BJTs. Thus amps seldom use JFETs.
Now, First Watt *seems* to be using JFETs since they output only a few watts.

Another device, not yet mentioned is the IGBT, Isolated Gate Bipolar Transistor. I have NO idea if there are audio applications here.

Just FYI, the older Thresholds (T200, T100, T400, etc from that era) & several of the down-market Forte amps also built at the same time used IGBTs in their output stages. The sound of these class-A amps & the Forte class-A, class-AB amps is quite good. I find it a bit confined compared to my class-A BJT amp but this is just 1 amp compared against my own.