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

S

Stephen

Jan 1, 1970
0
Could someone explain why a self biasing JFET amp (for example:
http://www.mission-technology.com/ELECLABS/images\21lb10_1.gif) 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."
http://www.odyseus.nildram.co.uk/RFIC_Theory_Files/Miller_Effect.pdf

In Erno Boberly's great article "JFETS: The New Frontier Part I"
(http://www.tkhifi.com/div/Erno_Borbely_fet_articel_1.pdf).. 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
 
L

Larry Brasfield

Jan 1, 1970
0
Stephen said:
Could someone explain why a self biasing JFET amp (for example:
http://www.mission-technology.com/ELECLABS/images\21lb10_1.gif) would
display the miller effect? There isn't any feedback, so there
shouldn't be any miller effect.. right?

of a impedance across a negative gain device."
http://www.odyseus.nildram.co.uk/RFIC_Theory_Files/Miller_Effect.pdf

In Erno Boberly's great article "JFETS: The New Frontier Part I"
(http://www.tkhifi.com/div/Erno_Borbely_fet_articel_1.pdf).. 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?


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.)
 
J

John Woodgate

Jan 1, 1970
0
I read in sci.electronics.design that Stephen <[email protected]>
wrote (in <[email protected]>) about
'Why do JFET/FET amps experience the miller effect? There isn't any
feedback', on Tue, 12 Apr 2005:
Could someone explain why a self biasing JFET amp (for example:
http://www.mission-technology.com/ELECLABS/images\21lb10_1.gif) would
display the miller effect? There isn't any feedback, so there
shouldn't be any miller effect.. right?

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.
 
J

John Larkin

Jan 1, 1970
0
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.

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

John
 
R

Rich The Newsgropup Wacko

Jan 1, 1970
0
I read in sci.electronics.design that Stephen <[email protected]>
wrote (in <[email protected]>) 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. (;-)

What? ?8-\
 
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