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Old 26th December 2012, 03:18 PM   #1
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Default Discrete MOSFET 12db High Pass Filter

I drew up this High Pass filter that I want to use in my BOSOZ preamp, and use the same PSU as the BOSOZ (why i'm using a 77v rail), the FET's I will use in the finished circuit are the FQPF2N60C's, are there any improvements that I can do to the circuit to improve performance? (it simulates well).
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Old 26th December 2012, 03:41 PM   #2
DUG is offline DUG  Canada
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M3 gate protection?
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Old 26th December 2012, 04:37 PM   #3
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Quote:
Originally Posted by DUG View Post
M3 gate protection?
I am going to add 27V zeners to each FET gate, in the final version.
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Old 27th December 2012, 01:01 PM   #4
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Does anyone know the minimum current that the FET's can run at with low distortion I am running the FET's in this circuit at 5ma for the first stage and 9ma for the second stage.
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Old 27th December 2012, 02:56 PM   #5
DUG is offline DUG  Canada
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FQPF2N60C seems a bit of an overkill if you are running 5mA @ 30V...150mW

General "Rule of thumb": The higher the max V, max I, and Pd that a mosfet has, the greater the capacitance you have to deal with.

Just sayin.

Distortion comes from the non linear relationship between Vgs and Id.

Active filter design usually requires high bandwidth active components to keep unwanted phase shifts from messing up the designed characteristics.

It would be interesting to see the results of this design.

Keep us posted.
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Last edited by DUG; 27th December 2012 at 02:59 PM.
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Old 27th December 2012, 07:43 PM   #6
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My rail voltage is 77v I calculated that each fet will dissipate 200mw at 5ma and 300mw at 9ma, does distortion lessen when the voltage on the drain of the fet is increased or is it dependent only on the current through the fet.
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Old 30th December 2012, 07:43 AM   #7
Calvin is offline Calvin  Germany
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Hi,

whats Your reason to use vertical MOSFETs for low level signals here?
JFETs or depletion MOSFETs would make things easier and probabely better too.

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Old 30th December 2012, 12:34 PM   #8
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Im using vertical fets because i have a bunch of them, the rail voltage that I have to work with is very high 77v, to high for jfets. It matches the BOSOZ that is before this stage.
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