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Why do JFET/FET amps experience the miller effect? There isn't any feedback

Could someone explain why a self biasing JFET amp (for example:

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would display the miller effect? There isn't any feedback, so there shouldn't be any miller effect.. right?

From what I've read "The miller effect is the effective multiplication

of a impedance across a negative gain device."

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In Erno Boberly's great article "JFETS: The New Frontier Part I"

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he mentions how the JFET amp can have this huge input capacitance with a large gain. But.. these amps don't have feedback... so why?

Thanks Stephen

Reply to
Stephen
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There is a capacitance between the FET drain and gate. This arises due to the fact that the gate and channel are separated by only a PN junction. That capacitance acts as a feedback element. So your premise, "don't have feedback", does not apply. (Your conclusions would be sensible, otherwise.)

--
--Larry Brasfield
email: donotspam_larry_brasfield@hotmail.com
Above views may belong only to me.
Reply to
Larry Brasfield

It also arises from the capacitance of the silly plastic breadboard thingie.

John

Reply to
John Larkin

good point! Thanks to all.

Reply to
Stephen

I read in sci.electronics.design that Stephen wrote (in ) about 'Why do JFET/FET amps experience the miller effect? There isn't any feedback', on Tue, 12 Apr 2005:

There IS feedback, via the drain-gate capacitance. This is analogous to the very original Miller effect, due to the anode/plate-control grid capacitance in triode valves/tubes.

Note and admire the bilingual exegesis. (;-)

If the gate voltage goes down by a small amount, the drain voltage goes up by that amount multiplied by the stage gain. So the capacitor demands charge as if it had a capacitance larger by a factor equal to the stage gain.

--
Regards, John Woodgate, OOO - Own Opinions Only.
There are two sides to every question, except
\'What is a Moebius strip?\'
http://www.jmwa.demon.co.uk Also see http://www.isce.org.uk
Reply to
John Woodgate

What? ?8-\

Reply to
Rich The Newsgropup Wacko

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