FET Phase inverter to drive valve PA

bassetrox · 5th November 2011, 08:19 PM

Hello,

I haven't been able to find anything by searching the archives or googling, I'm very much a beginner - just after an opinion on a PI circuit I threw together... have mercy...

I'm attempting to put a solid state phase inverter to feed a low power valve output stage.
I'm using low voltage rails, 110V before rectification (130 after?), as I have a trafo that will do that.
Ive used IRF840 mosfets because I have some, I'm not sure if MOSFETs are the right choice...?
I'm using LTP, because its a circuit I think i know.

I havent just swapped the fet in as the Vak isnt same as Vgs(on), I tried to bais properly. But I'm not sure, have I done it right?

Thanks in advance.

DF96 · 5th November 2011, 08:37 PM

IRF840 is an enhancement mode FET, so it needs positive gate bias to turn it on. You have given it negative bias, like a valve or depletion mode FET.

As it is an 8A power FET, I'm not sure how well it will work with only 1mA drain current.

bassetrox · 5th November 2011, 09:31 PM

Thanks for your reply DF96, could you recommend a better device that will be happy with the voltages/currents involved?

sorry for my stupidity, thinking about it, I was treating it as a JFET so I was bound to **** up. I havent ever designed with mosfets before.

Is this the correct form for an enhancement mosfet (ignore the values)?

bassetrox · 5th November 2011, 10:11 PM

Sorry for the double post... I can't edit.

I guess a straight forward voltage divider will do the trick...

Thanks for your help...

DF96 · 5th November 2011, 10:25 PM

Ignoring the component values, yes this is nearer the mark. Not something I have tried, so maybe others can help?

hemgjord · 5th November 2011, 10:31 PM

What's R14 and 15 about? To me the common "tail" should be as "long" as possible. A CCS would be preferable and since you are into ss already, easily implemented.

/Olof

bassetrox · 6th November 2011, 12:59 PM

I've attempted to re-bias this...

Does anybody have any recommendations for a more suitable device?

Alastair E · 6th November 2011, 03:14 PM

Quote:

Originally Posted by bassetrox

I've attempted to re-bias this...

Does anybody have any recommendations for a more suitable device?

Yup......

A Valve!

artosalo · 6th November 2011, 03:18 PM

Quote:

Does anybody have any recommendations for a more suitable device?

Simply a double triode such as ECC83.
As far as I remember IRF840 has very high input capacitance.

bassetrox · 6th November 2011, 03:59 PM

Thanks for your comment. I was looking for a solid state alternative...

Thanks,

Quote:

Originally Posted by artosalo

Simply a double triode such as ECC83.
As far as I remember IRF840 has very high input capacitance.

5th November 2011, 08:19 PM	#1
bassetrox diyAudio Member Join Date: Apr 2011	FET Phase inverter to drive valve PA Hello, I haven't been able to find anything by searching the archives or googling, I'm very much a beginner - just after an opinion on a PI circuit I threw together... have mercy... I'm attempting to put a solid state phase inverter to feed a low power valve output stage. I'm using low voltage rails, 110V before rectification (130 after?), as I have a trafo that will do that. Ive used IRF840 mosfets because I have some, I'm not sure if MOSFETs are the right choice...? I'm using LTP, because its a circuit I think i know. I havent just swapped the fet in as the Vak isnt same as Vgs(on), I tried to bais properly. But I'm not sure, have I done it right? Thanks in advance.

5th November 2011, 10:31 PM	#6
hemgjord diyAudio Member Join Date: Apr 2007	What's R14 and 15 about? To me the common "tail" should be as "long" as possible. A CCS would be preferable and since you are into ss already, easily implemented. /Olof __________________ I'm not allowed to do magic, union rules...

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5th November 2011, 08:37 PM	#2
DF96 diyAudio Member Join Date: May 2007	IRF840 is an enhancement mode FET, so it needs positive gate bias to turn it on. You have given it negative bias, like a valve or depletion mode FET. As it is an 8A power FET, I'm not sure how well it will work with only 1mA drain current.

5th November 2011, 09:31 PM	#3
bassetrox diyAudio Member Join Date: Apr 2011	Thanks for your reply DF96, could you recommend a better device that will be happy with the voltages/currents involved? sorry for my stupidity, thinking about it, I was treating it as a JFET so I was bound to **** up. I havent ever designed with mosfets before. Is this the correct form for an enhancement mosfet (ignore the values)?

5th November 2011, 10:11 PM	#4
bassetrox diyAudio Member Join Date: Apr 2011	Sorry for the double post... I can't edit. I guess a straight forward voltage divider will do the trick... Thanks for your help...

5th November 2011, 10:25 PM	#5
DF96 diyAudio Member Join Date: May 2007	Ignoring the component values, yes this is nearer the mark. Not something I have tried, so maybe others can help?

6th November 2011, 12:59 PM	#7
bassetrox diyAudio Member Join Date: Apr 2011	I've attempted to re-bias this... Does anybody have any recommendations for a more suitable device?

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