Depletion mode mosfet cascoded constant current source.

[Bas Horneman]

Can anyone tell me if this would work.

Copied if from Gary Pimm and drew the schematic as described...but want some extra verification


Cheers,
Bas

[316a]

No problems here but do you really need such a high impedence current source ? A single mosfet should do the trick in most cases . Check the Supertex datasheet to calculate the value of 'R' , just like reading tube curves , simple as that . Advise fitting a multiturn pot , will make adjustment far easier

316a

[316a]

battery is the wrong way round !

316a

[Brett]

Bas

where are you getting them from? The few places I've contacted in the US are out of stock.

Cheers

[Bas Horneman]

Thanks...

Don't need that high an impedance really I suppose..

But since I bought 50 of those mosfet's might as well make it a cascode jobby. And have the lithium batteries as well in my spares box.

See attached performance graph by Gary Pimm

[Bas Horneman]

where are you getting them from? The few places I've contacted in the US are out of stock

Hi Brett,

Got them from Mouser!

If you don't want 50 of them..I'll sell you a few for $2 a piece. That way I can recover my shipping costs

Kevin from K&K audio will also sell them for $3 a piece. And Paul Barker in the UK has also got a few to sell.

Cheers,
Bas

[Gary P]

316a wrote:

No problems here but do you really need such a high impedence current source ? A single mosfet should do the trick in most cases .

From my point of view it is not the low frequency impedance number that should be looked at. It is the shunt capacitance that is most audable and un-desireable.

You can use the formula 1/2pi*F*R using the impedance number at 10K for "R" to calculate the shunt capacitance.

battery is the wrong way round !

Nope. The idea is to increase the voltage across the current set resistor. The higher the voltage across the current set resistor, the less that thermal drift and noise from the mosfet effect the performance of the circuit. Also improves the local feedback. You can see this in the graph by compairing the DN2540 cascode (purple) to the DN2540 cascode 1 +6 (yellow).

Gary

[Bas Horneman]

Thanks for your input Gary!

[mlloyd1]

I don't see how this will work properly unless the top MOSFET is a depletion mode device, BUT I don't have data for that device. For a MOSFET, you need some VGS for drain current to flow.

mlloyd1

Originally posted by Bas Horneman
Can anyone tell me if this would work.

Copied if from Gary Pimm and drew the schematic as described...but want some extra verification


Cheers,
Bas

[fdegrove]

Hi,

It is indeed a N-channel depletion mode vertical D-MOS FET.

Cheers,