0195136047.pdf

358 SEMICONDUCTOR DEVICES

iL(t)

vS(t) = VS sin ωt

vS(t) vS(t)

C RL

iL

VS

t 1 t 2

RL

CRL

Capacitor charges

Time

constant

Capacitor

discharges

t

Load

resistance

Filter

capacitor

Ideal diode

(a)

(b)

−

+

C RL

Diode

0 ≤ t ≤ t 1

(c)

−

+

C RL

Diode

t 1 ≤ t ≤ t 2 + t 1

−

+

Figure 7.2.9Rectifier with filter capacitor.(a)Circuit.(b)Output current of rectifier with filter capacitor.

(c)Circuit configurations while capacitor gets charged and discharged.

Thefull-wave rectifierusing ideal diodes is shown in Figure 7.2.10(a). Figure 7.2.10(b)

shows circuit configurations for positive and negative half-cycles of the input source voltage

vS(=VSsinωt), and Figure 7.2.10(c) shows the rectified output voltage across the load resistance

RL. The full-wave rectification can be accomplished by using either a center-tapped transformer

with two diodes or a bridge rectifier circuit with four diodes.

7.3 Bipolar Junction Transistors

The family of bipolar junction transistors has two members: thenpnBJT and thepnpBJT. Both

types contain semiconductor junctions which operate with bipolar internal currents consisting of

holes and electrons. These are illustrated in Figure 7.3.1 along with their circuit symbols. The

emitter in the circuit symbol is identified by the lead having the arrowhead. The arrow points

in the direction of conventional emitter current flow when the base–emitter junction is forward

biased. A transistor can operate in three modes:cutoff, saturation,andactive. In the active mode,

for annpnBJT, the base–emitter junction (BEJ) is forward-biased by a voltagevBE, while the

collector–base junction (CBJ) is reverse-biased by a voltagevCB. Thus for annpnBJT, as shown

in Figure 7.3.1(a),iBandiCare positive quantities such thatiB+iC=iE. For apnpBJT on the

other hand, in the active region, the base–emitter and collector–base voltages are negative, and