Yes, the biasing of the op-amp controls the bias of the gate voltage of the mosfet. By the schematic there is no separate bias of the gate voltage of the mosfet. The 4.7k resistor at the op-amp output is for driving the op-amp into class A - it does not affect the gate voltage. R1 and Rg are seemingly used to stabilize the Mosfet.Of course, you can just change the value of R4 to control the quiescent current as well as altering the op-amp bias.
I am somewhat puzzled why R1 needs to be in the DC-coupled version. I would think Rg is sufficient.
"ok now I just need to find some musical opamps that will take more V supply then... I want a 10V version."
- What do you mean? Most op-amps I've looked at (other than low-power op-amps) can take up to ±15V, which means they're suitable for a 0 to +30V supply.
[Edited by Joobu (02-03-2001 at 09:35 PM).]
posted 02-03-2001 05:29 PM CST (US) 
