#1: Mosfet Power Amplifiers

Mosfet: Metal Oxide Silicon Field Effect Transistor A discussion of the benefits of the lateral Mosfet output devices used in today’s High Power audio amplifiers. Some points for consideration are: 1/ Thermal Characteristics Mosfet devices are inherently self regulating. Since they exhibit a positive thermal co-efficient, they don’t suffer from the dreaded BiPolar thermal runaway. As the device's junction increases in temperature, it’s on resistance increases also, limiting the amount of current transfer. 2/ Frequency response Mosfet devices are very fast and switch several Amperes in nano seconds, which is 30 to 100 times faster than equivalent BiPolar devices. Mosfets can also deliver full rated output into their loads, at up to 20KHz+. 3/ Overvoltage and overcurrent durability Unlike BiPolar devices, Mosfets will handle short overvoltage and overcurrent operating conditions without terminal failure.

4/ Output Device reliability In practice the reliability of Mosfets in surviving real world abuse, usually overheating and driving output shorts, is vastly higher than that of comparably rated Bi Polar output stages, even with their usual extensive protection. Should a Mosfet expire, it will usually fail "soft" without affecting associated devices or vaporising the drive circuitry. This is in marked contrast to BiPolar amplifiers, where one faulty device can destroy the complete output and drive stage. 5/ Simple drive circuit Mosfet devices require very small amounts of current to operate correctly, allowing a much simpler drive circuit to be used, maximising reliability and performance. Compare the minimal amount of drive circuitry in a high power Mosfet amp with the layer upon layer of boards in a similar power BiPolar amplifier. 6/ Clipping characteristics Mosfets exhibit a "soft clip" characteristic when driven into clipping levels. This is predominantly made up of even order harmonics rather like that of Valve Amplifiers, not the harsh odd order harmonic clipping produced by BiPolar Amplifiers. 7/ Damping factor This is more an issue of Loudspeaker Cabling and (where used) Passive Crossovers rather than the type of output devices, whether they are BiPolar or Mosfet. The misconception that Mosfet Amplifiers have by definition low damping factors was primarily due to early low output Hitachi device based amplifiers with marginal Power Supplies. This is not a factor with today’s Third generation high output Mosfet devices that are used throughout the ARX range. When using multiple output devices in conjunction with global negative feedback (featured in all ARX Power Amplifiers), then Damping factors equivalent to BiPolar Amplifiers are realised, especially with the all important Low Frequencies < 500Hz. All ARX amplifiers use multiple output devices. Some amplifier models that use the new "double die" devices have the electrical equivalent of 20 Mosfet devices in the output stage.

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