MOSFET Without Rs (source resistance) Help

In summary, the individual is encountering a problem in biasing a MOSFET IRF510 with a voltage divider using R1 and R2 both at 100K, Vdd at 9V, and Rd at 600 ohms with an intended Id of 10mA. They do not want to use Rs as a source resistor. However, they are unable to get an output greater than 3V, even though it should be possible. They also mention using a Colpitts oscillator in Common Source configuration, but it fails when Rs is removed. The Kn value is 124 mA/V2 and gfs is 1.3 at 3.4A, with Id being calculated using Kn and Vgs-V
  • #1
vco.asm
14
0
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:
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  • #2
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
yes that's right but in practical in my lab when i build the circuit it's did not success why?

Code: 
9V
|-------------|
|                   Rd=660
R1=100K        |-----------------V0 = should be >5 put in practical about 300mv Why?
|                --
|-----------| IRF510
|                --
R2=100k         |
|                   |
|                   |
---------------
       ---
         -
--
-
 
Last edited by a moderator:

FAQ: MOSFET Without Rs (source resistance) Help

1. What is a MOSFET without Rs and how does it work?

A MOSFET (metal-oxide-semiconductor field-effect transistor) without Rs refers to a MOSFET that does not have a source resistance connected to its source terminal. This type of MOSFET operates by controlling the flow of current between its source and drain terminals using an electric field created by a gate voltage. Without Rs, the source terminal is directly connected to ground, allowing for better control of the transistor's behavior.

2. What are the advantages of using a MOSFET without Rs?

There are several advantages to using a MOSFET without Rs. First, without the source resistance, there is less voltage drop across the transistor, resulting in improved efficiency. Additionally, without Rs, the transistor is less prone to thermal runaway, which can occur when the source resistance causes a positive feedback loop. This can help to improve the stability and reliability of the circuit.

3. Can a MOSFET without Rs be used in all applications?

No, a MOSFET without Rs is not suitable for all applications. It is typically used in low-voltage and low-power applications, as the absence of the source resistance can lead to higher power dissipation and potential damage to the transistor in high-voltage or high-power circuits.

4. How does the absence of Rs affect the performance of the MOSFET?

The absence of Rs can improve the switching speed and reduce the on-resistance of the MOSFET. This is because there is less resistance in the path of the current flow, allowing for faster switching and lower losses. However, it can also lead to higher leakage current and potential instability in certain circuits.

5. Are there any precautions to take when using a MOSFET without Rs?

Yes, there are a few precautions to keep in mind when using a MOSFET without Rs. First, it is important to ensure that the circuit is designed properly and can handle the higher power dissipation. Additionally, thermal management should be carefully considered to prevent overheating. Finally, it may be necessary to use additional components, such as a gate resistor, to ensure stable operation and prevent potential oscillations.

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