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    Working with basic emitter follower

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    Derive the expressions for closed-loop gain Avf, input resistance Rif, and output resistance Rof of the following image.

    © BrainMass Inc. brainmass.com December 24, 2021, 5:01 pm ad1c9bdddf
    https://brainmass.com/engineering/electrical-engineering/working-basic-emitter-follower-22100

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    SOLUTION This solution is FREE courtesy of BrainMass!

    This is a schematic of the basic emitter follower circuit. Just a transistor and a single resistor from the emitter to ground.

    Writing the voltage equation around the input loop,

    Vi - Vbe - Vo = 0 ---------- 1

    Making Vbe very small or the gain large, we have, Vi =(approx.) = Vo

    The output follows the input. thats why this circuit is called an emitter follower.
    The voltage gain Vo/Vi is less than but near unity in magnitude.

    From the load in the emitter circuit,

    Vo = Re (Ib + Ic) = (1+hfe)*Re * Ib -----Since Ic = hfe Ib

    At the input we have Vbe = hie * Ib

    Substituting these back to our equation 1,

    Vi = [hie + (1 + hfe)*Re ]* Ib ------------ 2

    The input resistance of the amplifier follows as,

    Rie = Vi/Ib = hie + (1 + hfe)*Re = approx. = hie + hfe*Re ---- 3

    To find the output resistance,

    (refer to the attached figure) The output resistance at the 2,2 port is found by making the independent source Vi = 0.

    With hie parallel with Re,

    Output resistance, Roc = hie * Re/[hie + Re] --------4

    Voltage gain, Avf = -Ai Re/Ri

    = hfe *Re/hie[1 + gm Re] where gm = hfe/hie

    = hfe*Re/hie*(1+hfe*Re/hie)

    = gm*Re/[1+gm*Re] ---------5

    This content was COPIED from BrainMass.com - View the original, and get the already-completed solution here!

    © BrainMass Inc. brainmass.com December 24, 2021, 5:01 pm ad1c9bdddf>
    https://brainmass.com/engineering/electrical-engineering/working-basic-emitter-follower-22100

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