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

 

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.

 

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

 

The answer might be here
http://ece-classweb.ucsd.edu:16080/spring05/ece53b/Chapter11.pdf

 

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.

 

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.

 

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.

 

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.

 

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.

 

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

 

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

Use the first formula above. Vgs(th) is Vth.

 

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.