Variable current source supply

Question

I am designing a power supply with a variable current source capability. I am using this circuit for my design(just a sample). Here I have a simple 15V transformer based power supply to give the voltage at the collector. I am getting abit problems including the voltage drops due to which I decided to use mosfet instead. But as far as I have calculated mosfet is not able to give me the desired results. My load is variable means I have change the load(5ohms shown) and change the currents accordingly. But with a mosfet i am getting biasing issues. Is there any way to go for mosfets or should I work on with transistors. I have usually seen transistors working in power supplies so is the biasing of mosfet a reason for that?, mosfets being used in switching mostly. Kindly suggest me some answers.

Here the 5Vs are from a regulator. The question is just wheather mosfets should be used or not in this case...

Answer 1

Regarding to use mosfet as a current source:

Yes, it its possible. You do not need a buffer transistor then (the another one) because mosfets need a very little current to control.
You need to implement an huge offset on potentiometer for about 3V Vgs threshold otherwise most the pot range will not be usable (just one resistor is series with pot).
See two left charts of IRF44Z below, after mosfet is heated the current will be shifted. Example, at 4.5V Vgs and temp rise from 25 to 175 degC the current is shifted from 10 to 10.5A.
You need a mosfet with most Vds vs. Id flatness (again see chart at 4.5V). There is a small current rise from 10.1A to 10.3 after Vds has to be changed duet load variability.

Schematic:

^{simulate this circuit – Schematic created using CircuitLab}