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7.2 DIODES 345

Slope = 1/Rr

Slope = 1/Rf

0 von

i

v

i

Ideal

diode

Rf

v Rr

D

von

v

+

−

+

Figure 7.2.6Piecewise-linear model of a diode, including the threshold voltageVon, forward resistanceRf,
and reverse resistanceRr.

Two types of capacitors are associated with apn-diode: thejunction capacitance CJ(also
known asdepletion capacitanceorspace-charge capacitance), which is dominant for a reverse-
bias diode; and thediffusion capacitance CD, which is most significant for the forward-bias
condition and is usually negligible for a reverse-biased diode. For applications where the diode
capacitance is important, the small-signal equivalent circuit under back (reverse)-biased operation
includesRrin parallel withCJ, and the parallel combination ofRf,CJ, andCDfor forward-biased
operation.

Elementary Diode Circuits

Semiconductor diodes are used in a wide variety of applications. Their usage abounds in com-
munication systems (limiters, gates, clippers, mixers), computers (clamps, clippers, logic gates),
television (clamps, limiters, phase detectors), radar (power detectors, phase detectors, gain-control
circuits, parametric amplifiers), and radio (mixers, automatic gain-control circuits, message
detectors). Several simple diode circuits are presented in this section to serve only as examples.
In solving circuit problems with a nonlinear element, such as a diode, a useful technique
employed in many cases is a graphical approach. After plotting a nonlinear characteristic of the
element, such as the volt–ampere characteristic of a diode, one can superimpose a plot of the
circuit response (excluding the nonlinear element), which is the equation of a straight line in the
i–vplane given by the loop equation for the network. Such an equation of the straight line is
known as theload lineequation. The intersection of the load line with the characteristic of the
nonlinear element in thei–vplane determines thequiescent(operating)point, which is the desired
solution. Example 7.2.1 illustrates the procedure.

EXAMPLE 7.2.1

Determine whether the diode (considered to beideal) in the circuit of Figure E7.2.1(a) is
conducting.