Small signal model BJT

Discussion in 'General Electronics Chat' started by aamirali, Sep 4, 2012.

  1. aamirali

    Thread Starter Member

    Feb 2, 2012
    Hi we consider small signal model, a signal means using small voltage signal....... but how small, of what value, any formula to calculate. How does this small signal voltage with input dc voltages and what are other factors which afects

    I had read a book & its written that transistor is in small model if
    ie(pp) < 0.1I(EQ)
    where ie(pp) is peak to peak ac emitter current & I(EQ) is dc emitter current.
    I know how to calclulate dc emitter current but I don't know how to calculate ac current. Any help about this.
  2. aamirali

    Thread Starter Member

    Feb 2, 2012
    help about this.
  3. Papabravo


    Feb 24, 2006
    From the biasing circuit you can find the Q-point on the DC load line. The peak-to-peak current on the base will be multiplied by the AC beta to give the peak-to-peak collector current. The slope of the AC load line is often different than the slope of the DC load line, but they intersect at the Q-point.
  4. ramancini8

    Active Member

    Jul 18, 2012
    You calculate the DC emitter current by Ie=Ve/Re. A tenth of this yields Ve= 0.1ReIe.
  5. Ron H

    AAC Fanatic!

    Apr 14, 2005
    If Re is AC bypassed with a capacitor whose reactance << re, then AC emitter current≈vb/re, where re=.026/Ie.

    Re=external emitter resistance
    vb=AC base voltage
    re=dynamic emitter resistance (internal)
    Ie=DC emitter current
  6. ecesoul

    New Member

    Sep 8, 2012
    AC base current, iB = IBQ exp(vbe/VT)
    When vbe<<VT, we can expand the exponential term in a Taylor series, keeping only the linear term. This approximation is what is meant by small signal.

    iB = IBQ (vbe/VT)
    iB = (IBQ / VT) vb ,this is a linear equation
    Normally we take vbe < 10 mV, so that above equation can be valid
    If vbe is high, we have to include other terms in Taylor series expansion in first equation, causing harmonic distortion.