Bias Generator for Source Follower OPS - diyAudio
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Old 7th September 2005, 01:50 PM   #1
Pwoida is offline Pwoida  Australia
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Default 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?

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Old 7th September 2005, 07:20 PM   #2
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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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Old 8th September 2005, 01:22 AM   #3
Pwoida is offline Pwoida  Australia
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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?

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