0195136047.pdf

350 SEMICONDUCTOR DEVICES

(IQ,VQ) by graphical analysis. Extend the graphical analysis for different values of (i) load

resistance, and (ii) supply voltage.

(b) If, in addition to the constant potentialVB, an alternating or time-varying potential

vS(t) =

√

2 VS sin ωtis impressed across the circuit, discuss the dynamic (ac)

characteristics of the diode in terms of (i) small-signal current and voltage waveforms,

and (ii) large-signal current and voltage waveforms.

Solution

(a) The circuit of a forward-biased diode with a load resistanceRLis shown in Figure

E7.2.5(a). The KVL equation yields

VB=IRL+V or I=

VB−V

RL

which is the load-line equation. The device equation (Boltzmann diode equation) and

the load-line equation involve two variables,IandV, whose values must satisfy both

equations simultaneously. As seen from Figure E7.2.5(b),Qis the only condition

satisfying the restrictions imposed by both the diode and the external circuit. The

intersectionQof the two curves is called the quiescent or operating point, indicated

by the diode currentIQand the diode voltageVQ.

Extension of the graphical analysis for different values of load resistance and

different values of supply voltage is shown in Figures E7.2.5(c) and (d).

(b) The diode circuit with dc and ac sources is shown in Figure E7.2.5(e). The total

instantaneous voltage impressed across the circuit is given by

vt=VB+

√

2 VSsinωt and vt=v+iRL

The maximum and minimum values ofvtare(VB+

√

2 VS)and(VB−

√

2 VS),

corresponding to the values of sinωtequal to+1 and−1, respectively. The small-

signal current and voltage waveforms, for values ofVSmuch less thanVB, are shown

in Figure E7.2.5(f). The large-signal current and voltage waveforms, for values ofVS

comparable to those ofVB, are shown in Figure E7.2.5(g).

The motion of the load line traces the shaded area of the characteristics. The line segment

Q 1 Q 2 is the locus of the position of the operating pointQ. It is clear from the figures that the

waveforms of the diode voltage and current are functions of time. A point-by-point method

should be used for plotting the waveforms. For the small-signal case, the diode can be considered

to behave linearly and the segmentQ 1 Q 2 is approximated by a straight line. For the large-signal

case, on the other hand, the behavior is nonlinear. The time-varying portion of the response is not

directly proportional tovS(t), and a simple superposition of the direct and alternating responses

does not apply.