Depletion mode mosfet cascoded constant current source. - diyAudio
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Old 10th April 2003, 10:28 AM   #1
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Default Depletion mode mosfet cascoded constant current source.

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
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Old 10th April 2003, 11:44 AM   #2
316a is offline 316a  England
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Default Should work

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

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Old 10th April 2003, 11:46 AM   #3
316a is offline 316a  England
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Default battery

battery is the wrong way round !

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Old 10th April 2003, 11:50 AM   #4
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Bas

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

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Old 10th April 2003, 11:54 AM   #5
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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
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Old 10th April 2003, 11:56 AM   #6
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Quote:
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
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Old 10th April 2003, 12:54 PM   #7
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316a wrote:
Quote:
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.

Quote:
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
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Old 10th April 2003, 01:04 PM   #8
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Thanks for your input Gary!
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Old 10th April 2003, 02:19 PM   #9
mlloyd1 is offline mlloyd1  United States
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Default Re: Depletion mode mosfet cascoded constant current source.

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

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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
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Old 10th April 2003, 02:26 PM   #10
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Default MOSFET.

Hi,

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

Cheers,
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