Linear Operation The region along the load line including all points between saturation
and cutoff is generally known as the linear region of the transistor’s operation. As long as
the transistor is operated in this region, the output voltage is ideally a linear reproduction
of the input.
Figure 5–5 shows an example of the linear operation of a transistor. AC quantities are
indicated by lowercase italic subscripts. Assume a sinusoidal voltage, Vin, is superimposed
on VBB, causing the base current to vary sinusoidally above and below its Q-point
value of This, in turn, causes the collector current to vary 10 mA above and below
its Q-point value of 30 mA. As a result of the variation in collector current, the collectorto-
emitter voltage varies 2.2 V above and below its Q-point value of 3.4 V. Point A on the
load line in Figure 5–5 corresponds to the positive peak of the sinusoidal input voltage.
Point B corresponds to the negative peak, and point Q corresponds to the zero value of the
sine wave, as indicated. VCEQ, ICQ, and IBQ are dc Q-point values with no input sinusoidal
voltage applied.
300 mA.
100 mA
3.4
VCEQ
–
+
10 k
VBB
VCC
10 V
220
–
+
3.7 V
Vin
VBB – 0.7 V
RB
=
Vce
IBQ =
RB
RC
βDC = 100
3.7 V – 0.7 V
10 k
= 300 A
ICQ = βDC IBQ = (100)(300 A) = 30 mA
VCEQ = VCC – ICQRC = 10 V – (30 mA)(220 ) = 3.4 V
Ib
μ
μ
IB = 300 A
IB = 400 μA
10
20
30
40
45.5
1.2 5.6 10 0 VCE
VCC