Biasing AMZ Mosfet Booster - Is this all there is too it?

[DeeBug]

I want to use an 18 volt power supply (please don't hate me  - from my DC Brick)

After searching and reading all I could find, it seems that it comes down to this:

Replace R2 (62K) with a 100K trim and adjust it until point "A" on this diagram, http://www.muzique.com/schem/mosfet.htm , is between 4.5 and 5.5 volts.

So, is that all I need to do for safe 18v usage (besides perhaps increasing the LED resistor too)?  Would a 100K for the trim be sufficient?  (Caps all rated at 25v, by the way.)

Much thanks for your help, y'all!

[petemoore]

  Bias it to around 1/2v, if 18v supply that'd put the drain at around 9v.

[DeeBug]

So, trying to understand this... it's the drain voltage that's key here?  If that's the case, why not just measure the drain voltage instead of the voltage at point "A"?

(Sorry for stupid question - just trying to make sense)

[PerroGrande]

Essentially, you want the drain voltage to be around 1/2 the supply voltage plus the threshold voltage (Vt) of the MOSFET for maximum swing.   Every MOSFET is a little different in its Vt, so that is why the pot is nice to have. So with a BS170 and an 18v supply, I'd want to set the drain voltage somewhere between 9 and 11 volts.  (I usually run mine a little on the higher side, since I don't want to flirt with drifting below Vt). 

The measurement at point A in the circuit *is* the drain voltage -- point A is connected to the drain with a wire (or a trace on the circuit board).  The voltage will read the same at point A as it does on the drain of the MOSFET. 

[DeeBug]

I thought Pete meant between 1 and 2 volts when he said "Bias it to around 1/2v."  D'oh!

OK, so now I understand.  Thanks, Perro and Pete!

[PerroGrande]

Ah -- I see.  It isn't going to be very happy with 1-2 volts on the drain.   

Enjoy your booster!