To calculate bias, just assume the capacitors are all open circuited (i.e. not conducting). Since the op-amp input terminals can be considered infinite impedance, the voltage at the non-inverting input is determined by two resistors, the 100k resistor to +15V and the 220k resistor with the 1uF capacitor across it. The voltage at the non-inverting input is therefore = 15*(220)/(220+100) which is approximately 15* 2/3.
Since the 4.7 uF cap near the op-amp inverting input is disconnected at DC, the opamp effectively sees only a 100k resistor to the output and therefore acts as a voltage follower at DC. Hence the biasing voltage of the mosfet is controlled by the voltage divider sitting below the input.
Btw - did you mail order all the caps? The only local place for esoteric caps that I know of is Lee's - only WIMAs and Orange drops, small capacitances as well. Where do you find surplus power supplies?
[Edited by Joobu (02-03-2001 at 07:29 PM).]
posted 02-03-2001 03:26 PM CST (US) 
