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Power amp output - BJT or MosFET?
Power amp output - BJT or MosFET?
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Old 25th April 2004, 03:58 AM   #21
slowhands is offline slowhands  United States
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Join Date: Feb 2004
Location: Silicon Valley
Default Re: About vertical and lateral mosfet

Originally posted by lumanauw
I have built amp with K1058 pairs, it is lateral mosfet. In the manufacturer's datasheet it is definitely for audio use. But why I feel that with vertical mosfet (IRF) the sound just seems tobe have more character?
There is also controvercy about a certain device is good or bad for audio, like 4558 opamp.
Is it if a device is said for audio it is not automaticly that it will sound good?
The laterals are inferior for switching, compared to vertical MOSFETs. Their on resistance is too high, dissipation capability and currents too low. But they have the wonderful ability to share current in parallel, and avoid thermal runaway, for the simple reason that their transconductance decreases with temperature. This makes them ideal for audio use.

The main reason they have survived as a product is continued demand in the audio market. It is questionable whether there is enough demand to ensure long term availability from the manufacturers. The major player, Hitachi, has transferred its discrete semiconductor manufacturing to Renesas, a joint venture company (with Mitsubishi). Toshiba supplies a few parts, but is rumored to be ending production. There are several boutique semiconductor makers who produce them, but I personally would not bet my company on them.

The vertical MOSFETs have higher transconductance, lower on resistance, and higher power capability, and with care can perform as well or better. They usually need to be matched for parallel operation, and need temperature compensating bias generators for reliable operation in audio amps.
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Old 9th February 2015, 06:40 PM   #22
DanielNielsen is offline DanielNielsen
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Mosfets have a poor linearity in transconductance compared to BJTs. This puts a non linear load on the voltage amplifier stage, and thereby creates more cross over distortion. On the other hand, a BJT output often has trouble with switching off quickly enough and therefore makes more cross over distortion.
These are the down sides. But it just so happens to be the case that it's not all that hard to make the BJT output stage switch proberly in the cross over point and thereby making the switch off distortion almost go away. It will be at least an order of magnitude less than the cross over distortion of the mosfet stage. And there might be ways to make the transconductance of a mosfet stage more linear but I havent seen this work, nor managed to come up with a way of dealing effectively with it.
So in my opinion a well designed BJT output stage have higher potential than a well designed mosfet outout stage. And BJT is also cheaper.
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