Designing emitter follower transistor circuit

Discussion in 'Homework Help' started by umichfan1, Aug 7, 2012.

  1. umichfan1

    Thread Starter Member

    Jun 16, 2012
    32
    0
    I have a few questions about pp. 90-93 of the "Student Manual for the Art of Electronics" (attached), which describes how to choose the specific component values for an emitter follower circuit. On p. 92, it talks about making the bias "stiff enough." What do they mean by "stiff"? And stiff enough...for what?

    Also on p.92, they calculate the impedance looking into the entire circuit. To do this, they use the Thevenin equivalent of the two bias resistors. I don't see how this makes sense, however...shouldn't you just use the lower bias resistor (the one directly connected to ground), since current passing through the capacitor cannot possibly pass through the upper bias resistor (since to do so it would be going to higher potential)? It seems to me that to calculate the impedance of the entire circuit, looking in from the capacitor, you should calculate R_2 parallel (R_E parallel R_load). Why is this wrong?

    Thanks in advance for the help.
     
  2. mlog

    Member

    Feb 11, 2012
    276
    36
    "Stiff" means to choose the voltage divider resistors on the base so that the current through the resistors is large compared to the base current. In other words, R1 || R2 << HFE RE, assuming that R1 and R2 are the base voltage divider. Otherwise a change in base current will affect the bias.
     
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  3. vk6zgo

    Active Member

    Jul 21, 2012
    677
    85
    In theoretical circuit analysis,the assumption is made that for an AC signal,the DC supply source has zero internal impedance,so that the DC supply & ground are at the same AC potential,&,(again for AC),may be shown as shorted together,hence the two bias resistors are connected in parallel.

    In the real world,power supply sources don't quite meet this ideal,but by the use of "decoupling" capacitors,the supply connection to each circuit can be made to approach the ideal so closely,that the "ideal" assumption may be used,with any error being so small that it may be safely ignored.

    There is a "sticky" about decoupling capacitors in the General area,& several discussions on these throughout the forum.
     
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