Depletion MOSFET as gain device (not CCS) ? - diyAudio
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Old 26th February 2012, 04:09 PM   #1
pixpop is offline pixpop  United States
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Default Depletion MOSFET as gain device (not CCS) ?

I have 9 of these Supertex DN2540 devices that I bought for making constant current laser diode drivers. They work great for that purpose, but now I have these left over.

I see on the data sheet that they cite 'Linear amplifiers' as a recommended application. Doing some initial searching, I only find references to them being used as a CCS load element.

Is there an amplifier topology that makes use of these as the gain element? Or would all of the relevant topologies be better covered by JFETs ?
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Old 26th February 2012, 04:14 PM   #2
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They are just like any other MOSFET except that their threshold voltage is negative rather than positive. Therefore, you can use them as a common source amplifier or a source follower or in any other circuit where you would use a MOSFET.
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Old 26th February 2012, 08:17 PM   #3
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common gate amplifier...
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Old 26th February 2012, 08:21 PM   #4
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You can check The Arch Nemesis by Nelson Pass for inspiration. There, he tests depletion mode MOSFETs and JFETs in the same simple circuit.
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Old 27th February 2012, 01:10 AM   #5
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Quote:
Originally Posted by sawreyrw View Post
They are just like any other MOSFET except that their threshold voltage is negative rather than positive. Therefore, you can use them as a common source amplifier or a source follower or in any other circuit where you would use a MOSFET.
Huh? No. You are thinking of N-channel versus P-channel enhancement-mode MOSFETs, only.

Depletion-mode MOSFETs are fully "ON" when the gate-source voltage is zero. Applying a negative Vgs (for an N-channel device) tends to turn the device off.

They are like a "normally-closed (ON) switch", always conducting unless a non-zero Vgs is applied.

The much-more-common enhancement-mode MOSFETs are like a "normally-open (OFF) switch", and don't conduct unless a proper Vgs is applied (depends on threshold and whether device is n-type or p-type).

http://en.wikipedia.org/wiki/MOSFET#...n-mode_MOSFETs

Last edited by gootee; 27th February 2012 at 01:24 AM.
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Old 27th February 2012, 02:10 AM   #6
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I think I understand that depletion mode MOSFET's can have their gates driven negative and positive without any problems. There just aren't that many to choose from and someone mentioned that they are noisy.
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Old 27th February 2012, 05:17 PM   #7
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Physically they are different than N-ch J-fets, but bias in a similar way, -Vgs turns it off. I do not believe there exists yet a P-ch depletion MOSfet.
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