variable current source

[FvM]

please give some suggestion to keep the NMOS operating in Active region

You're really great in raising nonexisting problems.

 

[keith1200rs]

The circuit will work as long as you have enough gate voltage to drive the transistor. Saturation or ohmic doesn't really matter.

Keith.

 

[saeed_pk]

Check LM317 Datasheet there is an application circuit for 1A Current source using a simple resistor so you change your resistor for your requirement ( better use an POT) you can have a variable current source

 

[ssingh]

hi
. My suggestions to operate the NMOS in active region:
determine the drain current(Id) and Vds. look at the I/V characteristic curves for the MOSFET. they can be found in the datasheet. You are going to use the MOSFET in the active (saturation) region .find the current level you need to drive the load, and look at the curve for the Vgs you intend to use....if the current level falls in the ohmic region (ie- the part before the line goes semi-horizontal) then you must select another device in which it does, or decreasee the voltage with which you drive the gate but it must be greater than the threshold voltage,Vt(Enhancement mode)..........but remember operating the mosfet in saturation region will dissipiate a lot of power,.. (P=Vds*Id)..

 

[jerkymotion]

thank u...
for your suggestion

 

[keith1200rs]

Why do you think the circuit will not work when the transistor operates in the ohmic region?

Keith

 

[jerkymotion]

i maybe wrong here...
But a Mosfet can behave as an constant current source in active region only. Basically it's ohmic region is used for switching purpose only...

 

[keith1200rs]

The circuit doesn't rely on the ability of the MOSFET to be a current source. The closed loop with the opamp is what makes the circuit a constant current source.

Keith