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Homework Help: Discrete Voltage Regulator w/ BJTs

  1. Nov 8, 2008 #1
    My problem is very simple, I have a project in which I have to design a voltage regulator out of discrete BJTs, Zeners and resistors etc. the only limitation is that I cannot use an IC. I would prefer not using a zener because i would like to make the output variable through a voltage divider bias if possible.
    the main problem is that i have looked around a lot, tried to attempt a lot, but i have no idea about any basic design that would allow me to make a low-dropout voltage regulator with mentioned elements. i would be very grateful if someone could nudge me in the right direction, i do not ask for a specific answer, just a general explanation so that i can start working on it and tweak the design as much as i can to my specifications.
    let's suppose i have a 15 V V(in) and need a 10 V V(out), with as good load and line regulation. can anybody point me to a basic configuration which could allow me to accomplish the task? or any simple configuration that lets the BJT to act as a voltage regulator?
  2. jcsd
  3. Nov 8, 2008 #2


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    Welcome to PhysicsForums!

    Here's a hint: What is the purpose of a BJT Voltage Follower?

    NOTE: a resistor divider does not give good line regulation (variations in input voltage).

    Good luck!
  4. Nov 10, 2008 #3
    Yeah i did make a circuit in which i used CE configuration for two NPN transistors, one after the other, where the voltage at the base of Q1 was set by a rheostat and the base of Q2 was attached to the emitter of Q1.
    Pathetic line regulation, as you pointed out, yet it may be because i have no idea what a voltage follower is :(
    I have tried many sites, but all of them show followers with op-amps :(
    any help there?
    and yeah, thanks for the help :)
  5. Nov 10, 2008 #4
    You can consider an op-amp as pretty much a BJT circuit that's designed to have a specific gain. But ask, and ye shall receive. http://www.national.com/appinfo/power/files/f4.pdf" [Broken], and see the datasheets and application notes corresponding to the voltage regulators you see there. Good luck!
    Last edited by a moderator: May 3, 2017
  6. Nov 10, 2008 #5
    I did look at the paper you referred me to, but still there is one little problem that i'm having. All these designs use an error-correcting op-amp. I understand it's simply a negative feedback comparator kinda thing, no? or is there something else to it? in any case, I can't use an IC, so how do i go about doing this thing?
  7. Nov 10, 2008 #6
    http://www.satcure-focus.com/tutor/page5.htm" [Broken]
    I'm amazed that you can Google just about anything these days...
    Last edited by a moderator: May 3, 2017
  8. Nov 11, 2008 #7


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    You're over-complicating things. A BJT *IS* a voltage follower (in the right configuration). For small variations in current, the base-emitter voltage is pretty constant (and in any case, roughly 0.7V). The common collector configuration is typically known as a voltage (or emitter) follower:

    Now, how would you go about generating a reference voltage to follow? Under what circumstances would it work? Are these good assumptions when using a common collector configuration?
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