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Transistors: emitter follower

  1. Oct 5, 2014 #1
    1. The problem statement, all variables and given/known data
    Design a +10V supply with the following specs:
    Iout =0-100 mA
    Vout =10V
    Vin=20-25V (unregulated supply w/ ripple)

    The circuit will have the diagram attached.

    The book I am using is The Art of Electronics, which is very "practical" the author says things like why worry about calculating something to 1 in 100, when you will then reach for a 10% resistor or 20% cap.

    2. Relevant equations
    Ibase * beta = Icollector
    Iemitter = Ibase (1+beta) = Ibase + Icollector
    Iemitter=Ibase +0.6V

    3. The attempt at a solution
    I start at the output, 10V and 0-100mA. The Voltage at the base and also zener must be equal to collector voltage +0.6V. The current in the base is 1/100 the collector current, and output current is 100mA(max) so I base = 1mA and Icollector=100mA. The zener also requires a constant 10mA, which is needed to keep the dynamic resistance under control. My issue is now the resistors.

    So, I know that the resistoprs have 25V on the side facing the power supply, and for R, the voltage on the base/zener side is 10.6V. Does this mean I can say the voltage drop across the res is 14.4 V (using 25V and planning for worst case)? If so, can I just say IBASE + Izener=11mA, asnd the use V/I=R? if so Ic an figure everything else out I belive.

    Thank you
     

    Attached Files:

    Last edited: Oct 5, 2014
  2. jcsd
  3. Oct 6, 2014 #2

    CWatters

    User Avatar
    Science Advisor
    Homework Helper

    You are on the right track but is 25V the worse case? What does the zener current fall to if the input is 20V instead of 25V.
     
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