# Small signal model BJT

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

1. ### aamirali Thread Starter Member

Feb 2, 2012
415
2
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.

Feb 2, 2012
415
2

3. ### Papabravo Expert

Feb 24, 2006
11,779
2,495
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
473
145
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
7,049
675
If Re is AC bypassed with a capacitor whose reactance << re, then AC emitter current≈vb/re, where re=.026/Ie.

Definitions:
Re=external emitter resistance
vb=AC base voltage
re=dynamic emitter resistance (internal)
Ie=DC emitter current

6. ### ecesoul New Member

Sep 8, 2012
8
0
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.