Bias Generator for Source Follower OPS

Pwoida · 7th September 2005, 01:50 PM

I have read the Slone book on power amplifiers and have decided to build a class B MOSFET source follower output stage. This means there will be no pre-driver BJTs and I am not interested in any protection circuits(it doesn't have to last very long). The output stage must be biased with about 1v to turn on the two output FETs. The bias will be placed in the output of the VA stage. If I was using BJTs I know the bias must track the Vbe characteristic of the output(or pre-driver) transistors. I understand that the bias does not need to be as accurate as if I was using BJTs but what do I use to create the bias? Will a potentiometer work or does it need to thermally track some property of thr FETs?

Thanks
Pwoida

chalky · 7th September 2005, 07:20 PM

If you only need 1V of bias then the mosfets must be lateral types. In this case no temperature compensation is required, and you can use a simple potentiometer. This only works for lateral mosfets, you need thermal compensation for vertical mosfets.

Pwoida · 8th September 2005, 01:22 AM

Yes they are lateral MOSFETs: The 2SK1058 and 2SJ162 that are used in Slone's book.

I understand that FETs will not undergo thermal runaway but to minimise crossover distortion is some kind of thermal tracking still needed?

Thanks
Pwoida

7th September 2005, 01:50 PM	#1
Pwoida diyAudio Member Join Date: Aug 2005	Bias Generator for Source Follower OPS I have read the Slone book on power amplifiers and have decided to build a class B MOSFET source follower output stage. This means there will be no pre-driver BJTs and I am not interested in any protection circuits(it doesn't have to last very long). The output stage must be biased with about 1v to turn on the two output FETs. The bias will be placed in the output of the VA stage. If I was using BJTs I know the bias must track the Vbe characteristic of the output(or pre-driver) transistors. I understand that the bias does not need to be as accurate as if I was using BJTs but what do I use to create the bias? Will a potentiometer work or does it need to thermally track some property of thr FETs? Thanks Pwoida

7th September 2005, 07:20 PM	#2
chalky diyAudio Member Join Date: Dec 2003 Location: North of the border	If you only need 1V of bias then the mosfets must be lateral types. In this case no temperature compensation is required, and you can use a simple potentiometer. This only works for lateral mosfets, you need thermal compensation for vertical mosfets. __________________ If it ain't broke don't fix it

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8th September 2005, 01:22 AM	#3
Pwoida diyAudio Member Join Date: Aug 2005	Yes they are lateral MOSFETs: The 2SK1058 and 2SJ162 that are used in Slone's book. I understand that FETs will not undergo thermal runaway but to minimise crossover distortion is some kind of thermal tracking still needed? Thanks Pwoida