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Old 12th November 2005, 12:57 AM   #1
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Join Date: Sep 2005
Default High voltage transistors for VAS stage

Okay, I've looked EVERYWHERE but have had no luck in finding the following:

I need to find a transistor model (BJT preferred, but MOSFET would do as well) with a breakdown voltage of at least 300V, power dissipation of 5w or more, current handling of at least .5 amps, and either NPN or PNP.

They will be used as both a common emitter and a cascode common base pair. I know most power transistors have these specs, but they are usually not good for common emitter or common base configuration. Currently, I'm using ZETEX FZT857 and FZT957 for my stage (3.5W per transistor), but they're getting over-powered in simulation for worst case.

Thank you! Your help is much appreciated!
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Old 12th November 2005, 03:31 AM   #2
jcx is offline jcx  United States
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you could consider paralleling, if your'e using emitter degeneration it is easy

another option is cascaded/totem pole like the super leach

maybe somewhere in earlier discussions you could find 1 or 2 transistor solutions?

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Old 12th November 2005, 08:31 AM   #3
AndrewT is offline AndrewT  Scotland
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Join Date: Jul 2004
Location: Scottish Borders
are you designing an audio power amp?
Is 500mA necessary for a VAS stage?
Often a 50mA to 140mA device will do here.
A low current device will usually have a lower Cob which is often a prerequisite for this location.
Can an EF be used to help the loading on the VAS?
If you are really in trouble then I second the staggered cascode as a solution. But what are the drawbacks?
regards Andrew T.
Sent from my desktop computer using a keyboard
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Old 12th November 2005, 12:30 PM   #4
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I was considering paralleling the devices for double the power hanlding, and I could do it for the common emitter stage (with emitter degenerator resistors), but how would I do that with the common base stage afterwards? Ideas?
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Old 15th November 2005, 12:13 AM   #5
jcx is offline jcx  United States
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current happily adds in parallel, just connect b-b, c-c and 2 degenerated ce, one for each emitter
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