# E-MOSFET Formula

Discussion in 'General Electronics Chat' started by NeRdHeRd, Feb 11, 2008.

1. ### NeRdHeRd Thread Starter New Member

Feb 5, 2008
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I'm trying to determine what the Vds would be for a given Vgs. Is there a formula I can use for this calculation? I was thinking of using the following formula:

Vds = Vdd - IdRd

I just don't know if Id(on) is the same as Id.

Could someone please clarify this for me. Thanks

2. ### Audioguru Expert

Dec 20, 2007
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A Mosfet has a range of Vgs for it to conduct a certain amount of current.
Most are guaranteed to be fully turned on with a Vgs of 10V. They are barely turned on (called their threshold voltage and their current is only 250uA) with a Vgs from 2V to 4V.

The transconductance is what determines the current with a certain Vgs but it is also a range and it is not linear.

3. ### NeRdHeRd Thread Starter New Member

Feb 5, 2008
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So, if I have a Vgs of say +2.5v is there a way of determining what the vds would be?

Feb 24, 2006
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5. ### beenthere Retired Moderator

Apr 20, 2004
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Or perhaps in the spec sheet for the individual device. As in transistors, all FET's are not alike. Some are "logic-level", meaning that they are fully turned on at 5 Vgs. One might expect that such a device might have a characteristic quite different from a warhorse IRF510.

By the way, Vds is not dependent on Vgs. An N-channel FET will start to enter the conducting region sometime after the gate is taken more than 2 volts positive with respect to the source, but the voltage on the drain is the result of the external circuit. It is a bit more meaningful to inquire as to the degree of conduction resulting from a certain Vgs.

6. ### Audioguru Expert

Dec 20, 2007
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The datasheet shows some Vgs that make a certain drain currents for a typical Mosfet. Your Mosfet might be typical or be below or be above.

7. ### NeRdHeRd Thread Starter New Member

Feb 5, 2008
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Alright to be more specific I have a E-MOSFET in a circuit with the following characteristics:

Vgs(th) = 2v
Id(on) = 5mA
Vgs(on) = 5v
Rds = 500 ohm
VDD = 15v
RD = 1k ohm

With that being said, if I have a Vgs of +2.5v or +3.0v can I determine what Vds would be. Do I need to determine the Id(sat)? I've been searching everywhere for some formulas and I can seem to find one that applies to this.

8. ### beenthere Retired Moderator

Apr 20, 2004
15,808
295
Vgs(th) is the threshold voltage where the device just starts into conduction. If you have the data sheet for the specific device, you could run out the transfer curve and make a guess, but FET's just aren't like BJT's. All the data you have says for certain is that it starts to conduct with 2 volts on the gate, and it's fully on with 5 volts on the gate.

I could be too ignorant to help, too.

9. ### n9352527 AAC Fanatic!

Oct 14, 2005
1,198
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K = Id(on) / (Vgs(on) - Vth)^2

Id = K (Vgs - Vth)^2

Vds = Rds * Id

Assuming that the MOSFET is in active region, where Vds > Vgs - Vth, which you should check after obtaining the Vds value.

10. ### harrypotter New Member

Sep 26, 2007
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How do I find VGS(th) if given: ID(on), k, and VGS(on)?

11. ### n9352527 AAC Fanatic!

Oct 14, 2005
1,198
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Use the first formula above. Vgs(th) is Vth.

12. ### Audioguru Expert

Dec 20, 2007
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The transconductance of a Mosfet also has a wide range of values. The transconductance is not linear at low current.

Simply design the circuit with plenty of negative feedback to lessen the effects of the variations.