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MOSFETS are NOT high impedance devices -

216.158.42.31

Posted by Victor Khomenko on May 11, 2000 at 14:17:35

In Reply to: Re: IGBT Update and Question posted by Thorsten on May 11, 2000 at 11:44:49:

- they only are when used in DC mode.

***IMHO you cannot "just replace Mosfet's with Bipolars"..... Bipolar Transistors are very diffcult to drive really well in high current applications, tend to be a lot less rugged and overall need a very different Circuit to operate best than FET's.

In terms of thermal run-away, the MOSFET's used in most power amps behave not better than typical BJT's. The legend about their "negative thermal coefficient" is mostly a legend, because they are never used in their "negative tempco" areas. Doing so would usually require bias currents in several amps per device range. Most MOSFET's are biased in the 100mA - 200mA range, where their temco is decidedly positive, and they do happily ran away, if allowed.

***Yes, but now the Bipolar Driver has it's own Base Current (and a good deal of it) and a Predriver Stage optimised for Voltage Amplification into very high Impedance load now operates into one that is of comparably low and variable (with signal) Impedance.

I would take *some* issue with that. I don't think anyone today designs the driver stage for MOSFET output as "voltage amplifier". If has been understood for quite some time that MOSFETS need about as much current as the BJT's to work properly - that is a good rule for a robust design. Not on an average power dissipation basis, of course, but in terms of what they can deliver as maximum current, for sure. So yes, I could easily envision a driver stage that is suitable for either. Well, perhaps with some minor tweaking.

Again, for all practical applications - MOSFETS are NOT voltage driven devices, they are pretty much current-driven ones.

>2) For my application, would an IGBT offer any advantages over
>either a bipolar output driver or bipolar output devices.

***Yes. You can basically make a "discrete" IGBT from a FET and Bipolar Transistor of opposite polarity. So you have the very low "on" resistance of the BJT combined with the very high input Impedance of the FET.

Again, in audio band the MOSFETS DON'T HAVE high input impedance. A typical power MOSFET has 3000 to 5000pF gate capacitance, times the number of devices in parallel - that is anything BUT high impedance.

There have been some amps that drove the MOSFET with voltage amplifiers and these didn't last long before redesign.

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