Advantage of active bias circuit
The active bias circuit has a tempco .The idea is that the tempco tries to cancel the tempco of the Fets that you are biasing .The Gate source threshold voltage of most power mosfets is -3.3 mV/K so in linear mode there is the potential for thermal runaway .I always mount the active bias devices on the main heatsink .When I set up the bias for my 5W class A source follower car audio amplifiers I deliberately over compensated so the Idle current fell slightly with increasing heatsink temp.Another advantage of this active biasing scheme is that Idle current varies less with supply volts .This is inportant when considering classic cars that still have the origional generator and mechanical regulator charging system.The active circuit is also valid and common with BJTs .