0195136047.pdf

7.2 DIODES 353

(g)

Load line

Motion of

load line

ωt

Q 2

Q

Q 1

VQ VB

IQ

ωt

i

(VB + √ 2 VS)

v

Combination of

direct and

time-varying levels

Distortions

(VB − √ 2 VS)

Figure E7.2.5Continued

Zener Diodes

Most diodes are not intended to be operated in the reverse breakdown region [see Figure 7.2.4(a)].
Diodes designed expressly to operate in the breakdown region are calledzener diodes. A nearly
constant voltage in the breakdown region is obtained for a large range of reverse current through
the control of semiconductor processes. The principal operating region for a zener diode is the
negative of that for a regular diode in terms of both voltage and current. Zener diodes are employed
in circuits for establishingreference voltagesand for maintaining a constant voltage for a load in
regulator circuits. Figure 7.2.7 shows the device symbol along with the linearizedi–vcurve and
the circuit model.
As seen from thei–vcharacteristic, a zener diode approximates an ideal diode in the forward
region. However, when the reverse bias exceeds the zener voltageVZ, the diode starts to conduct
in the reverse direction and acts like a small reverse resistanceRZin series with a batteryVZ. Zener
diodes are available with values ofVZin the range of 2 to 200 V. The circuit model of Figure
7.2.7(c) incorporates two ideal diodes,DfandDr, to reflect the forward and reverse characteristics
of the zener diode. Thei–vcurve thus has twobreakpoints, one for each ideal diode, and three
straight-line segments.