Highest voltage, low-noise FET

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MaxDM

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I’ve been wanting to experiment with a FET with a 150v to 200v B+ In a condenser mic

I read something a long time ago on Facebook regarding one, which I looked up, and it seemed to be worth trying.

I wrote the part number down but seem to have misplaced it.

Can anyone tell me what currently manufactured FET can be used in such an application?
 

analag

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That looks promising!

I wonder if it's linear enough to use as you would a triode.
If you care to experiment, you could do a grounded source in a source follower into say a 4:1 output transformer. That would leave you with enough gain to apply a small amount of feedback to linearize things a bit.
 

mhelin

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Last edited:

Winston OBoogie

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I know that the LND150's work well as low current, high voltage current sources, but no experience with them other than that.
Might be worth a shot.
My gut is that I'd probably opt for a cascoded 2SK/LSK170 but, lots of ways we can skin cats.
 

Matt Nolan

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I'm out of practice with this stuff. What's the Miller capacitance of the LND150 going to be in a typical triode-alike circuit, compared with the capsule capacitance?
 

MisterCMRR

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Miller C occurs only when the drain has an inverted version of the signal at the gate - so it doesn't apply to followers, for example. Miller effect will multiply the Crss (drain-to-gate) C by the actual in-circuit voltage gain. So, if you use it "grounded source" (it's what the last s in Crss signifies, BTW), and choose a drain load resistor that gives the stage a gain of 10, then the input capacitance seen at the gate will appear to be 10 times the Crss figure. This is in addition to the normal Ciss (gate-to-drain) capacitance. So, in the gain of 10 circuit, the input capacitance would be (10 x 0.5 pF) + 7.5 pF = 17.5 pF ... using the "typical" numbers from the data sheet.

Of course, stage gain is transconductance (Gfs) x R load - in this case 2.0 milli-mhos x 5 kΩ = 10 ... and 50 kΩ would give a gain of 100, etc. ... again using the typical Gfs figure from the data sheet.
 
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