Highest voltage, low-noise FET

[MaxDM]

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?

 

[Matador]

What exactly are you trying to do?

 

[Matt Nolan]

How about cascoding a small signal JFET with a MOSFET?

 

[analag]

LND150 depletion mode mosfet

 

[MaxDM]

What exactly are you trying to do?

How about cascoding a small signal JFET with a MOSFET?

I'd like to try and make a high-voltage FET condenser mic, like a tube mic, but with a FET

 

[MaxDM]

LND150 depletion mode mosfet

That looks promising!

I wonder if it's linear enough to use as you would a triode.

 

[analag]

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]

LND150 has been used in guitar amps to buffer the insert sends and returns (https://el34world.com/Forum/index.php?action=dlattach;topic=23555.0;attach=72199).
There are also some attempts to simulate tubes (LND150 tube emulator attempt. etc., see member KMG's posts there, also LND150 Jose style preamp).
Another tube emulation attempt: Tube Emulation & EQ - Page 36 - diyAudio

LND150 has huge gain.

 

[Winston OBoogie]

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]

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]

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.

 

[analag]

I have designed it around three LND150 and a BV-12 opt ( I ran the numbers from my impressive China BV-12). Feedback must be used because the gain is so high, now I will build it. Soon. Currently building an FET and a tube mic.