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Old 20th December 2003, 10:34 PM   #1
azira is offline azira  United States
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Default MOSFETS vs BJTs

What's so great about MOSFETS over BJTs and why are they always so advertised? The layman consumer has been convinced that MOSFETS raise the bar to a whole new quality level.

I used to think MOSFETs were gods gift to amplifiers. Huge input impedance, no gate current -> easy to design. Then I found out about parasitic gate capacitance and random oscilations... And a Vt of 4 volts actually kind of sucks.

Also, the early voltage of a MOSFET is typically less than the VA of the BJT, so the BJTs are more constant in their F.Active region than the MOSFETS in saturation which leads to less distortion.

I've now been convinced that, at best, FETS are good for the input stage but BJTs are far better for VAS and especially the output stage. Lower distortion, and less feedback needed to linearize, so more gain.
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Old 20th December 2003, 10:40 PM   #2
sam9 is offline sam9  United States
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It's just Ford vs. Chevy, Nissan vs. Toyota, Acura vs. Lexus, Mecedes vs. BMW. They are different but with either one you can get where you need to go.

The debate is just so entertaining, though.
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Old 20th December 2003, 10:54 PM   #3
Dave is offline Dave  New Zealand
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How about BJT output devices with MOSFET drivers?
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Old 20th December 2003, 11:23 PM   #4
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Quote:
Originally posted by Dave
How about BJT output devices with MOSFET drivers?

isn't that called igbt?



I think it is pretty much moot to claim superiority based on the type of output devices. MOSFETs have their +s and -s, just as the BJTs.

I happen to like MOSFETs more because I have used them for a long time and have lots of them in my drawer. I would not based on purchases on MOSFETs or BJTs, because doing so makes zero sense.
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Old 21st December 2003, 12:05 AM   #5
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It's a bit more complicated. At first, power mosfets looked like audio nirvana. They had high input impedance, a more linear G(m), and were free from the dreaded 2'nd breakdown that made transistors fail. When we actually tried them, however, we found big problems. First, the Hitachi 'lateral' power mosfets, while rugged and high voltage, had LOW G(m) and low peak output current.
The American 'vertical' fets had lots of G(m), fairly high voltage and current, BUT they were prone to breakdown. You can't use them at anywhere near their rated voltages, except as switches. And over the years, bipolar transistors got faster and more rugged.
In the last 5 years or so, power mosfets have actually gotten worse. Many parts that are really superior, have been discontinued. Still, I will use selected mosfets, before I use bipolars, where I can, because they tend to give lower amounts of higher order distortion in my designs.
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Old 21st December 2003, 04:27 AM   #6
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Default One big reason that they are worse:

They don't have to be good to make an H-bridge. All N-types, full on, or full off......how good do they need to be?

As long as the gate capacitance and Rs is predictable, what else do they need?

Jocko
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Old 21st December 2003, 06:27 AM   #7
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Sounds like these breakdowns may have occured when the MOSFETs were operated out of their SOAs. Wrt SOA range, the Toshiba 2SK1530 and 2SJ201 seem to have much more robust pulse SOAs in the 1-100msec range than, say the IRFP240 & IRFP9240 even if their absolute peak current ratings aren't quite as high. On top of that, their Vgs thresholds are significantly lower than average for devices of their type and their g(m)'s appear competitive.
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Old 21st December 2003, 12:52 PM   #8
lucpes is offline lucpes  Romania
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Quote from a mail I received from Randy Slone related to bipolar vs mosfets (2SK1058/2SJ162); I had a 'peak current capability' question relative to his OPTI-MOS kit.

* On an "overall" basis, L-MOSFETs can provide much greater peak output currents than bipolar devices. You see, a typical bipolar power transistor that may be rated for an Ic (max) of 15-amps will probably have a secondary-breakdown point (depending on where the SOA falls for the operating conditions) of about 2-amps, or less.

Thus, the overload protection circuitry must restrict output currents very tightly within certain operational regions, and such restrictions can result in distortion
from the protection circuitry under extreme impedance variations. This is the reason that many "esoteric" audiophiles have abandoned inherent protection circuitry, which is "always" a bad idea (if you desire any type of reliability from the amplifier). In contrast, MOSFETs do not have a secondary-breakdown mechanism, and this is the primary reason for their much
greater reliability. Hitachi's tests on these devices show that they can conduct over 700-amps for 8-milliseconds with no damage.
Unlike bipolar devices, which can be destroyed in microseconds by exceending maximum current ratings, the 7-amp current rating on the specified L-MOSFET devices is an "average" parameter (such as in a simple resistive device). Thus, MOSFET devices are capable of much, much higher peak current outputs than comparable bipolar devices.

However, the Rd(on) parameter of L-MOSFETs is a little higher than the "on" resistance of a saturated bipolar power transistor, so the overall efficiency of L-MOSFET output stages is a little
less than comparable bipolar devices. This equates to the need for slightly higher rail voltages than needed for bipolar devices (for equivalent power outputs), depending on the number of parallel legs in the output stage design. I hope this helps. :-)
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Old 21st December 2003, 06:28 PM   #9
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I would agree with sloan on this. I have used V-mosfet since the 1980s and have always loved their ruggedness (?). I have not managed to blow a single one of them. each time, the fuse is always the one to first go away on mosfet-based amps.

From that point of view, I think DIYing mosfets has a big advantage.
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Old 21st December 2003, 09:26 PM   #10
djk is offline djk
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"Hitachi's tests on these devices show that they can conduct over 700-amps for 8-milliseconds with no damage. "

Sloan is blowing smoke again.

He quotes numbers for V mos when he uses L mos in his designs.

At room temperature the 2SK1058/2SJ162 he uses have an Rds of about 1 ohm, about 1.7 ohm over temperature.

They put out about 1A per volt of gate drive, and the gate will be permanently damaged above 14V.

It is not possible to get 700A out of one of these things under any real operating condition.
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