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Transistor Design. Emitter Follower with Stiff Source

  1. Jul 30, 2011 #1
    Use a follower with base driven from a voltage divider to provide a stiff source of +5 volts from an available regulated +15 volt supply. Load current (max) - 25mA. Choose your resistor values so that the output voltage doesn't drop below more than 5% under full load.


    So I have to choose the base to be at 5 volts using R1 and R2. I'm fine with that. R2/(R1+R2) = 1/3.

    Then the Emitter voltage will sit at about 4.4V. So I have to choose R3 to give 25mA when 4.4 volts are across it. That gives me 176 Ohms.

    What I'm confused about is how to choose resistor values to get a less than 5% voltage swing. Someone mentioned to me that a rule of thumb is to make the current through the voltage divider 10 times the output current. But then I have R1 R2 and R3, and I don't know how to check if the voltage swing parameter is met.

    Any help is greatly appreciated
  2. jcsd
  3. Jul 31, 2011 #2
    I'm pretty sure you are supposed to choose the resistors so that Vout (the emitter) is around 5 V, not Vin.

    25 mA is the max load current, not the no-load R3 current.

    You need to use a large signal model for a BJT; the simple thing to do is just treat the base-emitter voltage as constant (around 0.7 V) when the BJT is operating forward active.

    Using the beta value, you should be able to solve the circuit under no-load conditions so that the output is 5 V, and use this to constrain the resistor values.

    If you divide the worst-case (lowest) output voltage by the worst-case (highest) load current you should get a worst-case (lowest) load resistance. You can solve the circuit again using this resistance in parallel with R3, and make sure the voltage is within range.

    Since in you probably should choose resistor values so that the voltage drop is less than 5% under these conditions, the current through that "worst case" resistor may exceed 25 mA, but this is okay.
    Last edited: Jul 31, 2011
  4. Jul 31, 2011 #3
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