mosfets or JFETS with low Ciss

[RichardJones]

This is the problem.

I want to make a pre that uses a tube input i.e. at 15-25mA operating current.

This is driving a MOSFET or JFET follower running at 100mA Class A. running off +-20vdc.

the tube is obviously cap coupled to the gate of the MOSFET.

Any recommendations on MOSFETS (N channel) that would work with low Ciss to minimize HF roll-off.... oh and high S.

Thanks!!!

 

[Nelson Pass]

Assuming you are driving the Mosfet off the plate,
what is the plate resistance and the voltage across
that resistor?

 

[RichardJones]

input tube

it depends on what tube I use.

I may just use a CCS to load the tube... that way I have more flexibility on tube types.

Or even a choke load....

For explanation sake...

ECC99 at 150v, 25mA, plate resistance is 2.2K.
gain =22

Thanks!

 

[Nelson Pass]

Something like a IRF610 will get you out to 50+ KHz
without feedback and a 2.2K source.

 

[RichardJones]

how to couple the 2

So cap off the plate of the tube connected directly to gate of MOSFET, or do I need a resistor to ground at the gate?

whats the smallest value cap I can get away with?

Thanks!!!

 

[tiroth]

There are a bunch of JFET discussions floating around, one of which strongly encouraged referencing all gates to ground with 1M.

 

[Nelson Pass]

No reason you can't DC couple to the Gate of the
MOSFET follower. You do of course want to use a
higher voltage type MOSFET and protect its Gate
with a Zener from Gate to Source, and you want some
resistance on the Gate of the MOSFET.

 

[RichardJones]

??

I am confused then.

Say my plate voltage is 150v.

Surely then the source of the MOSFET is around 146v..?

I was going to run my MOSFETS from a +-60vdc supply at 100mA.

How can an input stage be direct coupled to this output?

......

 

[Nelson Pass]

154 volts.

Maybe you want to capacitively couple the Gate to the
Plate.

 

[RichardJones]

gate vs source

I thought the gate is positive wrt source in a source follower configuration?

So if The gate is at 150vdc the source is at 146vdc or approx.

So plate resistor on tube to B+ At the plate a cap (value?) to a resistor to ground. gate connected at point of resistor and cap.

Like a 100K to ground?

Maybe a 10uf?

Thanks!!!

 

[Nelson Pass]

If you capacitively couple the output of the plate, then
the Gate of the Mosfet follower should be biased at the
midpoint of its swing +4 volts or so. In your case it
sounds like +4 V referenced to ground. Any resistance
that works is OK for this, as the DC input impedance of
the Mosfet is near infinite. 22K would probably be the
kind of value you want to load the plate after the cap,
and 10 uF or more film cap is what you probably want
for coupling.