High voltage transistors for VAS stage

rtarbell · 12th November 2005, 01:57 AM

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!

jcx · 12th November 2005, 04:31 AM

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?

http://www.diyaudio.com/forums/showt...042#post747042

AndrewT · 12th November 2005, 09:31 AM

Hi,
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?

rtarbell · 12th November 2005, 01:30 PM

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?

jcx · 15th November 2005, 01:13 AM

current happily adds in parallel, just connect b-b, c-c and 2 degenerated ce, one for each emitter

12th November 2005, 01:57 AM	#1
rtarbell diyAudio Member Join Date: Sep 2005	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!

12th November 2005, 09:31 AM	#3
AndrewT diyAudio Member Join Date: Jul 2004 Location: Scottish Borders	Hi, 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.

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12th November 2005, 04:31 AM	#2
jcx diyAudio Member Join Date: Feb 2003 Location: ..	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? http://www.diyaudio.com/forums/showt...042#post747042

12th November 2005, 01:30 PM	#4
rtarbell diyAudio Member Join Date: Sep 2005	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?

15th November 2005, 01:13 AM	#5
jcx diyAudio Member Join Date: Feb 2003 Location: ..	current happily adds in parallel, just connect b-b, c-c and 2 degenerated ce, one for each emitter

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