MOSFET Quasi Complimentary ???

N-channels rule!

Look in any catalogue and you will find seriously big N-channel Hexfets and IGBT's. Big enough to need bolts to attach the conductors. In contrast, P-channel devices wimp out comparitively early on the buahahaha scale. Super duper sized amplifiers (not necessarily audio) may well need to use a pair of N-channel devices for this reason.


Nelson Pass

The one and only
Paid Member
2001-03-29 12:38 am
I like to be agnostic about it, and I've made plenty of
money with both.

Bipolar are great if you are trying to get decent distortion
with a low bias AB amp. Home theater stuff is a good example.

Mosfets are the choice if you have high voltages and power,
or if you want simple Class A circuits. In general, they tend
to be easier to design with if you can afford lots of bias.
Here is a quasi-comp Mosfet power amp


I just browsed my MJ mag articles and found a kaneda Power amp from 1999, which has both N-type Mosfets in the output stage. Though he drives them rather symmetrically, in my opinion.



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Nelson Pass said:
Properly designed, the Mosfets are a lot more rugged
since they don't have second breakdown at high
voltages. Your typical 250 watt Bipolar becomes a
wimpy 50 watt device at 150 volts. Not so with the

MOSFET amps lose about 7 volts across the bias stage so unless you use a slightly higher voltage rail for the driver stage you lose a bit of power.

I designed a 450WRMS MOSFET power amp it has been very reliable for disco and guitar work.
I do tend to overdesign my amps with huge heatsinks and a pair of fans to keep it cool. It just never even gets warm.
I did find care has to be taken setting up the bias level and output DC offset voltage.