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. enter image description here

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

  • \$\begingroup\$ As the current flowing through power transistor (due to load variability) changes the Vbe is also changed (0.5-0.75V) what causes Ic shift. Another words, it will not to hold the constant current precisely. Also the Vbe changes significantly with temperature so you have another source of inaccuracy. \$\endgroup\$ Jul 4, 2023 at 19:42
  • \$\begingroup\$ yes I just wanted to ask if the mosfet could be used for that or not? \$\endgroup\$
    – user340506
    Jul 4, 2023 at 19:50
  • \$\begingroup\$ Linear operation is mainly done with BJTs. MOSFETs have a current instability that means their full power is only available when switching, linear requires huge derating, not many MOSFETs have a DC line on their SOA. \$\endgroup\$
    – Neil_UK
    Jul 5, 2023 at 5:10

1 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.

enter image description here



simulate this circuit – Schematic created using CircuitLab

  • \$\begingroup\$ But I am working on variating the base voltage. I want that for each of the load I should be able to control its current. But I am facing a challenge in helding it constanlty in the saturation region, because it gets it into the linear region. \$\endgroup\$
    – user340506
    Jul 4, 2023 at 20:39
  • \$\begingroup\$ A current source works with transistor in linear region. \$\endgroup\$ Jul 4, 2023 at 20:48
  • \$\begingroup\$ @EEK21 just to be clear here, if you wish to regulate current, then the transistor (either MOSFET or BJT) should be in the linear region. When saturated or cut-off, it is no longer regulating current. \$\endgroup\$ Jul 5, 2023 at 6:00
  • \$\begingroup\$ @SimonFitch I mean the saturated in case mosfet. Because the saturated in mosfet is equal to active in transistor. How linear??? \$\endgroup\$
    – user340506
    Jul 5, 2023 at 7:18
  • \$\begingroup\$ @MichalPodmanický this circuit is not giving me a high current variation ability as 3A-5A. As i would have If I had a transistor and I change the base voltage then the emitter being 0.7 less would give the 4.3/1=4.3A. \$\endgroup\$
    – user340506
    Jul 5, 2023 at 7:21

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