# MOSFET Without Rs (source resistance) Help

1. Jan 10, 2012

### vco.asm

I'm encounter a problem in biasing mosfet irf510 ;
please someone answer me... why i can not get an output over than 3V

i had using voltage divider to bias at 4.5V (R1=R2=100K) ; Vdd=9V; Vout should be = 3V .. Rd=600 OHM where Id=10mA the problem here that i do not like to use Rs as source resistor . the worset of result is not as i calc ...pleeez some one tell me why can not get an out put equal to 3V .. why Id not as i determinted ?

best regard;
MR,10GHz
it should be mentained here that when you put Rs ; you will get what you want; and my colpitts oscillator then work in Common Source; when you remove Rs and recalc the circuit its faild

Kn=124 mA/V2 @ gfs =1.3@3.4A
iD=kn (Vgs-Vgs(th))^2

Last edited by a moderator: Jan 10, 2012
2. Jan 10, 2012

### skeptic2

I'm not sure I understand your question. I think you're saying you have a Vdd or 9V, an Rd of 600 ohms and an Id of 10 mA. The 600 ohms with 10 mA with give you a voltage drop of 6V and 9V - 6V = 3V.

If you want more than 3V at the drain you could either lower your bias voltage or the value of Rd.

3. Jan 10, 2012

### vco.asm

yes that's right but in practical in my lab when i build the circuit it's did not success why?

Code (Text):

9V
|-------------|
|                   Rd=660
R1=100K        |-----------------V0 = should be >5 put in practical about 300mv Why?
|                --
|-----------| IRF510
|                --
R2=100k         |
|                   |
|                   |
---------------
---
-
--
-

Last edited by a moderator: Jan 10, 2012