simple gain stage using power mosfet like 5n50c?

[Ryanhardy]

Hi, salvaging power mosfets is quite easy.  5n50c pops up quite a bit.  As usual it has three terminals.  I am quite adept at using BJTs to build overdrives etc but havnt managed to produce a simple working gain stage using a power mosfet running off 7-9 volt power supply.  Any diagrams to point me to that will result in a working gain stage.  From there i can progress onwards to making distortions / tone stacks.

[antonis]

MMMMMMM...

Fresh meat for Sir Mike...


http://www.muzique.com/schem/mosfet.htm

[Transmogrifox]

Power MOSFETs with threshold voltages of 2V to 4V make it tricky to operate the in a high gain linear (ish) region without relatively high currents for a 9V circuit (like 50mA or more).

The first thing to consider is that the variability in linear region is not very well controlled since these are optimized for performance in the saturation region.

The first thing to consider is that you may need to bias the gate at up to 4V.  This is probably ok for distortion since you can bias the drain at around 6V or 7V.  You may need to formulate the distortion stage using a resistor and bypass capacitor in the source and then squeezing the last 1V out of your 9V rail for signal swing at the output.

It seems it could be reasonably done, but the power MOSFET makes it more difficult due to the poorly defined behavior at low currents.

[Rob Strand]

If you use Feedback biasing you will probably get less grief.   It finds the right bias point much more reliably.
See this example. [Do not take this as a working circuit.]





The input capacitance of power MOSFETs is quite high.   The input capacitance goes up with the amount of gain.   You pretty much need a preceding buffer stage to prevent high frequency loss.

Not that I believe the simulation much, but to give you an idea how bad it is:   A 1k ohm source impedance driving the above circuit had a roll-off of 2.4kHz!  The gain was about 40dB (probably higher than reality).

You can knock the gain back with a source resistor and it will increase the hf cut-off.

[EDIT: It occured to me a slightly better connection is to ground the lower side of R2 then connect the lower side of R3 to C2/R2/R4.  The value of R2 will then need to be increased to say 4M7.  Now R3 isn't that critical, choose  say 1M.  Probably not enough to worry about while you are playing around.]

[antonis]

I was ready to ask you if R3 is realy necessary, Rob, untill I realized we deal with MosFet (and NOT BJT).. 

(another verification of your signature..)

[Rob Strand]

I was ready to ask you if R3 is realy necessary, Rob, untill I realized we deal with MosFet (and NOT BJT)

It would work probably work without R3 but the bias won't be optimal.

[Gus]

A link to something that has been in the Schematics link at the top of the page for years
https://www.diystompboxes.com/pedals/mosfet.gif