Fabriquer un U47FET DIY

poctop
Posts: 2185


Re: D-U47 FET Microphone Project PCB Build thread

« Reply #118 on: October 12, 2012, 12:10:09 AM »

To clear some confusion before it happens the Bias on the U47FET is not actually a Bias , it permits a certain adjustment to the source voltage to operates half the Rail voltage, it also permitted me to set the FET in a consistant VDS ( about 15V in this case) condition , as i went further into this circuit design with the help of any extraordinary electronician guy who fixes stuff by wetting is finger and touch the PCB ;D, to understand the that FET source current is actually regulated by the transistor below and it actually feedbacking to the Drain Voltage for the change in Voltage implying the constant current source , so this design is actually AutoBiasing and regulates a predetermined idle current, i had noticed no change in the THd Level as well at the oprating point set ,
So basically brings us over a section of the curve where there is not much change in behavior,


So for the Folks who will build, the pot is not really necessary unless you want to learn about the circuit.
if you dont install it just short the 2 used pad with a fine wire and then use R6 = 6.8K there is some extra in the bom anyway as you need a pair to match the front end , the value is not critical,

Best,
Dan,