Depletion mode mosfets for 2mA CCS ?

[mach1]

Does anyone know of a depletion mode mosfet suitable for use in a 2mA ccs capable of handling 350V ?

I have use the LND150 very successfully for currents < 1mA, and the DN2540 / IXCP10M45s for currents > 7mA.

1mA appears to be the upper limit for the LND150.

The DN2540 datasheet indicates that gm is miniscule at 2mA, implying low effective resistance.

Althought the IXCP10M45 datasheet implies an operational current bandwith commencing at 2mA, I suspect that effective gm will also be very low at this current.

ideas / comments ?

[David Davenport]

Have you considered a cascode with a DN2540 or 10M45 on the top and LND150 on the bottom? The top will take the voltage and the bottom will set the current.

Dave

[mach1]

Thanks for the reply Dave.

Unfortunately Vgs for the LND150 transitions from -ve to to +ve at approx 1.7mA, which effectively prevents its use as a 2mA ccs.

[David Davenport]

Do you think that you could use two of the LND150 in parallel?

Dave

[Bandersnatch]

Originally posted by David Davenport
Do you think that you could use two of the LND150 in parallel?

Dave

Or try the DN3545N3. At 2 mA, you'll be able to get a significant value for the set resistor which will improve the performance.
cheers,
Douglas

[mach1]

Thanks guys,

I have used the DN3545 before as the lower element in a cascode with the DN2540. Surprisingly, it has a very similar current v transconductance characteristics to the DN2540 (ie it is also miniscule at 2mA), which pretty well rules it out in terms of the resultant ccs effective resistance.

The paralleling of LND150 sounds like a great idea (why didn't I think of it?) which I will definitely trial.

[jackinnj]

From AN-D16 -- when I simulate it, I get no distortion -- wassup?