|
Audio Asylum Thread Printer Get a view of an entire thread on one page |
For Sale Ads |
In Reply to: I'm very confused about mosfet vs bi polar transistors posted by Rudi on April 18, 2002 at 13:40:08:
Mosfets have, even at this late date, too high a gate capacitance to really work well. There is also no such thing as a complementary Mosfet output stage. The p channel device only can act as a constant current sourse for the n channel device above it. So the n channel device is really taking most of the audio load. There is also this famous Mosfet "mist" that everyone with Golden ears hears. A high freq frizzy that surrounds everything like a fog. Until someone is brave enough to bring out the water cooled heatsinks or even stoop to consider a transformers coupled Class A amp then I'm afraid Bipolar transistors operative in complementary emitter followers Class AB is the best of all evils. There is no perfect amplifier. They are all euphonic in their own way. Go listen or borrow amps to hook up with your speakers in your listening space. Most reputable dealers will allow this. Only then can you arrive at what you want. Anyone else recommending an amp is like recommending someone for you to marry. The best! James
Follow Ups:
The problems you mention about MOSFETs have not prevented MOSFET from being used by designers of several very good amplifiers.
Nelson Pass and John Linsley Hood come to mindBipolar designers have have to cope with secondary breakdown, changing and very low gain at high currents, and nonlinear voltage drive response ( Bipolar are current input/output devices )
I must disagree with the Bipolar transistors are current-in current-out devices. The ruling equation for bipolars is:Ic = Is * e^(vbe/vth)
So the output current (Ic) is proportional to the base-emitter voltage. Beta and the current ratio between input and output is just a side effect.
chris m
Yes, so the Ebers-Moll equation tell us. What I was tring to say in a shorthand way, was that it is very nonlinear.( but nearly perfect exp)
Feeding in a sinewave, we get high distortion out.
For audio we want to use the more linear "Sideeffect" Ic=hfeIb.
Right on OLE LUND!Get my email??
This post is made possible by the generous support of people like you and our sponsors: