Source follower and common source output stages

slogan2112 · 8th October 2008, 05:31 PM

I have been trying to wrap my head around the performance of these two output stages. In particular how the Mosfet capacitance effect the frequency response and drive requirements. Looking at text book formulas the source follower has an input capacitance of Cin=Ggd+(1-Av)Cgs and for the common source it is Cin=Ggs+(1-Av)Cgd. From this I would assume the CS is more difficult to drive. But the comment below from Nelson himself leads me to believe otherwise.

http://www.diyaudio.com/forums/showt...972#post319972

Can someone help with an explanation? I am looking at amplifier designs similar to the Aleph, A75 and F5. One idea is a two stage amplifer with complementary diff input stage and complementary CS output stage.

-Scott

Nelson Pass · 8th October 2008, 06:43 PM

No, it's consistent with those formulas. The difference is in the proportion
of drive voltage to output. The same amount of charge current goes into
the gate, but with a follower the voltage is much higher, so the Cgs looks
proportionately lower.

slogan2112 · 9th October 2008, 09:19 PM

It amazes me you can reply to all these messages. I am still having trouble understanding the frequency response and drive requirements. Is there recommended reading you would suggest?

Thanks again for your contribution to this hobby and all the personal time you put in.

-Scott

8th October 2008, 05:31 PM	#1
slogan2112 diyAudio Member Join Date: Dec 2003 Location: austin, tx	Source follower and common source output stages I have been trying to wrap my head around the performance of these two output stages. In particular how the Mosfet capacitance effect the frequency response and drive requirements. Looking at text book formulas the source follower has an input capacitance of Cin=Ggd+(1-Av)Cgs and for the common source it is Cin=Ggs+(1-Av)Cgd. From this I would assume the CS is more difficult to drive. But the comment below from Nelson himself leads me to believe otherwise. http://www.diyaudio.com/forums/showt...972#post319972 Can someone help with an explanation? I am looking at amplifier designs similar to the Aleph, A75 and F5. One idea is a two stage amplifer with complementary diff input stage and complementary CS output stage. -Scott

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8th October 2008, 06:43 PM	#2
Nelson Pass The one and only Join Date: Mar 2001	No, it's consistent with those formulas. The difference is in the proportion of drive voltage to output. The same amount of charge current goes into the gate, but with a follower the voltage is much higher, so the Cgs looks proportionately lower.

9th October 2008, 09:19 PM	#3
slogan2112 diyAudio Member Join Date: Dec 2003 Location: austin, tx	It amazes me you can reply to all these messages. I am still having trouble understanding the frequency response and drive requirements. Is there recommended reading you would suggest? Thanks again for your contribution to this hobby and all the personal time you put in. -Scott

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