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Transistor problem/question

  1. Jun 12, 2007 #1
    I'm trying to create a simple power supply for a project, but it's not working correctly. I have a reference voltage (4.1V) and a supply voltage (4.8-5.0V). My goal is to connect an opamp functioning as a voltage follower with a transistor used to supply the adequate current.

    Circuit1.jpg

    That is a drawing of my circuit, sorry for the quality.

    Edit: Forgot to mention that it isn't working. My output is stuck at Vcc, when it should be pulled to the same value as my input. Any suggestions would be helpful. Thanks.
     
  2. jcsd
  3. Jun 12, 2007 #2
    The circuit you have drawn can't work. You have a transistor configured as an inverting amplifier and you have negative feedback to the opamp. In the end, you have positive feedback.
     
  4. Jun 12, 2007 #3

    berkeman

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    Staff: Mentor

    If you want the opamp circuit to be a current-amplifying voltage follower, use an NPN transistor with the collector at Vcc, the base driven by the opamp, and the emitter connected to the load. Connect the emitter to the inverting input of the opamp directly, so that Ve = V+ (the positive input of the opamp).

    Be sure that the opamp power supplies are wide enough that V+, V- and Vout of the opamp are within the voltage specifications of the opamp.
     
  5. Jun 12, 2007 #4
    The problem is that my supply voltage is pretty much capped at 4.8 volts, and I am using a single supply op amp, so Vcc = 4.8V (insead of +-4.8V).

    That is why I was leaning toward using a PNP instead of an NPN. I switched my input to go into the negative and I now have positive feedback. The circuit works like a charm... with no load. When I throw any load on it, the voltage drops drastically on the output.
     
  6. Jun 12, 2007 #5

    berkeman

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    Staff: Mentor

    Maybe try putting an NPN stage between the opamp output and the PNP input. You're going to have to start being careful about high-frequency oscillations, though with that extra delay....
     
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