Swapping an NPN for an FET

[Noplasticrobots]

I've Googled and searched the forums, but can't find any information so I'm gonna be lazy and just ask: how do you substitute an FET for an NPN tranny? As in, what pin gets grounded, what pin gets the input, and what pin gets the ouput? Thanks for any help.

[R.G.]

The Devil makes me ask - why would you want to?

But I'll content myself with the following.
- an N-channel FET has similar voltage needs to an NPN bipolar. The input goes into the gate; a noninverted signal appears on the source, analogous to the NPN emitter. An inverted signal appears on the drain, analogous to the collector. The gate is the control electrode, and it controls current flowing from the drain to source, analogus to the base controlling current flowing from the collector to emitter.

The biasing voltages are different among the devices. The NPN base must be about 0.5V to 0.7V higher than the emitter for linear amplification. In MOSFETs, the gate must be pulled up by Vthreshold (which can vary from 0.2 to 3.0V) before anything happens at all. In JFETs, the maximum current the device can supply flows when Vgate-source (Vgs) is zero; the gate must be pulled negative with respect to the source to turn JFETs off.

Some circuits have enough lattitude in them that you could sub in a low-Vth MOSFET and have the circuit kind of work, albeit at a lower gain. I don't know of any circuit where a JFET can be directly subbed for an NPN and have it work semi-properly, because the JFET gate has to be biased in the other direction from the bipolar base.

[Noplasticrobots]

I was just gonna experiment with an FET in a Bazz Fuss, but after reading your post I don't think it's worth it. I thought I read that FET's sound more like tubes, so I was gonna throw together a Bazz Fuss on the breadboard with an FET. Thanks for the help!

[R.G.]

The old "FETS sound more like tubes" myth keeps getting bandied about.

JFETs *bias* in the same method as tubes. FETs in general saturate less suddenly and abruptly than bipolars, but not as softly as tubes can.

FETs in general are somewhat between bipolars and tubes in the audio goodness spectrum, but they are closer to bipolars than tubes. Simply subbing in a FET won't make a circuit sound like tubes.

[Steben]

Sometimes you CAN subtstitue a MOSfet, depending on how well the original NPN circuit was biased towards them. Jfet's are far more difficult (negative biasing). However I swapped the NPN pregain stage of a Boss DS-1 with a J201 Jfet. Sounded quite high-gain tubey. You have to swap a lot of "collateral" components too, though...

[Noplasticrobots]

I'm glad I asked because I've learned quite a bit about FETs just in the last few posts. Thanks again all!