similar Common Collector circuits with different answers

Discussion in 'General Electronics Chat' started by donut, Jun 8, 2012.

  1. donut

    Thread Starter Member

    May 23, 2012
    I looked at two different examples of a Common Collector (EF).

    One from horowitz(book) another from hyperphysics(web)

    The problem I am having is that both sources of common collector examples derive different ways/techniques to obtain the resistance for the voltage divider that does the biasing.

    Now i find hyperphysics to be straightforward because they start by explaining:

    find the R1+R2 ratio via I = Vin/R1+R2

    then find R1 via Vout = (R1/R1+R2) * Vin

    then determine R2.

    Ok I follow everything they are telling me in this example.

    Now in the Horowitz example he throws me a curve ball stating:

    Vcc = 15V; quiescent current = 1mA

    Choose R1 and R2. VB is VE + .6V = 8.1V. This determines the ratio of R1 to R2 as 1:1.17. The preceding load criterion requires that the parallel resistance of R1 and R2 be about 75K or less (one-tenth of 7.5k times hFe). Suitable standard values are R1=130K and R2=150K.

    Ok what did he just say? Preceding load criterion?

    Using the technique from hyperphysics in the horowitz example I obtained R1+R2 = 150K, R1 = 81K, R2 = 69K. Obviously not similar.

    So why is it that I cant use the hyperphysics approach in the horowitz design circuit? These circuits are similar the concept to solve seems to be not alike.

    Please advise
  2. #12


    Nov 30, 2010
    You didn't give the starting point information or the answers to the hyperphysics example that are "obviously not similar", but I can say that Horowitz did a better job of it. He included the Vbe loss.
  3. donut

    Thread Starter Member

    May 23, 2012
    Hyperphysics example, step by step process, and results are provided in the link I pasted into the starting thread.
  4. Ron H

    AAC Fanatic!

    Apr 14, 2005
    The loading criterion is just above the example, on the same page. It basically says that the impedance of the voltage divider should be much lower than the DC impedance looking into the base of the emitter follower.
    There are no hard and fast rules for transistor biasing.
    Hyperphysics says make divider current about 10 times the base current.
    Horowitz & Hill says make the Thevenin resistance of the base divider about 10 times the emitter resistance.
    Either method will generally give acceptable results. Both methods will wind up with Ve lower that Vbb/2, because both choose to ignore the effect of base current on the voltage divider. The lower resistance divider (Hyperphysics) will give less error (if you want to call it that), but at the cost of lower input impedance.
    donut likes this.
  5. donut

    Thread Starter Member

    May 23, 2012
    Kind Sir, Ron H. You are one of a few that can provide explanations that I am receptive to! Thanks!