On Wed, 1 Sep 2010 16:17:08 +0100, "ian field"
wrote:


"John Larkin" wrote in message
.. .
On Wed, 1 Sep 2010 15:00:35 +0100, "ian field"
wrote:

Does a source follower present lower apparent input capacitance?

I'm thinking in terms of 2N7000/BS170 sized devices.

Thanks.


Yes. If the follower voltage gain is close to 1, the gate-source
capacitance is bootstrapped and goes away. All that's left is
drain-gate capacitance. You can bootstrap that, too, with another
follower.

John



Thanks.

Is there a cheat sheet anywhere on the web to estimate the % of Cin that
disappears with various amounts of source degeneration?


The small signal gain (gate-to-source) of an (R-loaded) MOS follower
is ...

(gm*R)/(1+gm*R)

Take it from there ;-)

...Jim Thompson
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