Question for Jack (`s) --AMZ Mosfetboost

[Eddie]

I built the mosfet booster.I ge a light boost and distortion at the end of turning down the 5 k pot (from 200 to 0 ohms).
Ithink that there is a biasing problem. The gate voltage is too low.
I do not know much about mosfet biasing and I couldn`t find a good article dealing with that. Maybe I ´m too stupid for that.The orientation of the mosfet is ok, I checked the datasheet for this Mosfet(and tried it reversed too, I tried everything....??).
Could you or others help me?

I measured these voltages:

vr :5,8
D:5,4 Testpoint A
G:2,9
S:3,46

I want this unit working! I m really frustrated.I can´t find the mistake in  my layout.


Which resistors influence the gate voltage? where could i Use a trimpot?


Thanks for any help

Eddie

[bwanasonic]

I get a strong sense of Deja Vu from this post. Someone recently had the same issues with their mosfet boost. Do a search to see if they came up with a solution. If it's at all helpful, here is the layout I used for mine:

http://home.earthlink.net/~bwanacentral/html/mosfetboost.htm

I used a BS170 for mine, and managed to zap the first one I tried. The second one worked like a charm though.  BTW- the voltages I read on mine are:

vr=D or 4.5
D=4.5
G=0.5
S=3.47

Good luck

Kerry M

[Eddie]

Thanks bwansonic!

If i turn the voltage on the drain or Testpoint A to 4, 5 Volts i get the your Source Voltage (around 3,6 Volts) but I get a gate voltage of 2,9 volts.

So my problem is that the gate voltage is too high.

What is your Vr? Vr is the Voltage after R2 (a trimppot maybe or 62 k)?
My Vr is arond 6,6 Volts.

Can anybody help me from that measurements?How do I get the gate voltage down?

Thanks Eddie

(the earlier mosfet posting was mine)

[Thomas P.]

There's a good article over at GEO!
If I see it right the resistors R1 and R2 are creating a Vr of 5.5V