0195136047.pdf

7.3 BIPOLAR JUNCTION TRANSISTORS 359

vS(t) = VS sin ωt

(a)

Primary

Ideal

transformer

1:2 Secondary Ideal diode

D 1 D 1

Ideal diode

D 2 D 2

−

+

RL

RL

vS vL(t) vL

vS

+ +

+

−

−

−

vS

−

+

vS

−

+

− +

vS > 0 (positive half-cycle)

(b)

RL

vL

vS

−

+

vS

−

+

− +

vS < 0 (negative half-cycle)

vL(t)

(c)

vS(t)

π 2 π 3 π

ωt

VS

RL

vL

vS

−

+

vS

−

+

− +

Figure 7.2.10Full-wave rectifier.(a)Circuit.(b)Circuit configurations for positive and negative half-cycles.
(c)Rectified output voltage.

currentsiB,iC,andiEare all negative quantities such thatiE=iB+iC, that is to say, the bias
voltages as well as current directions are reversed compared to those of annpnBJT.
In annpnBJT, current flow is due to majority carriers at the forward-biased BEJ. While the
electrons diffuse into the base from the emitter and holes flow from the base to the emitter, the
electron flow is by far the more dominant part of the emitter current since the emitter is more
heavily doped than the base. Electrons become minority carriers in the base region, and these are
quickly accelerated into the collector by action of the reverse bias on the CBJ because the base is
very thin. While the electrons are going through the base region, however, some are removed by
recombination with majority-carrier holes. The number lost through recombination is only 5% of
the total or less. Due to the usual minority-carrier drift current at a reverse-biasedpnjunction, a
small current flow, on the order of a few microamperes, denoted byICBO(collector current when