Fabriquer un U47FET DIY

De fait, il n’y a plus de réglage du bias, tout est figé et le trimpot dégage

The trimmer in this microphone PCB is no longer necessary , make sure you jump the potentiometer pad contact and use for R6 = 6.8K instead of 3.6K, the trimmer has been removed from the BOM As well to avoid confusion.

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,

Mais on peut pas le mesurer directement sur le multi (la prise PNP/NPN) ?

L'Idss est mesurée par un multimètre inséré entre D et + de la pile (9V) dans ce montage :



(Source : http://stompville.co.uk/?p=112 )

La meilleure page d'explication sur ce qu'est l'Idss est celle-ci :
http://www.learningaboutelectronics.com/Articles/What-is-IDSS-of-a-FET-transistor

Je relie le G et le S au négatif de la pile et le D au + c'est bien ça ?
Et je dois prendre le plus élevé comme idss ?