Chapter 1 Semiconductor Materials and Diodes 43
calculation. In a final design, we may want to use a computer simulation for better
accuracy. However, it is very important to understand that the diode model or diode
parameters used in the computer simulation must correspond to the actual diode
parameters used in the circuit to ensure that the results are meaningful.
Test Your Understanding
TYU 1.9 Consider the diode and circuit in Exercise EX 1.8. Determine
V
D
and
I
D
,
using the graphical technique. (Ans.
V
D
∼
=
0.54
V,
I
D
∼
=
0.87
mA)
TYU 1.10 Consider the circuit in Figure 1.28. Let
R = 4k
and
V
γ
= 0.7
V. Deter-
mine
I
D
for (a)
V
PS
= 0.5V
, (b)
V
PS
= 2V
, (c)
V
PS
= 5V
, (d)
V
PS
=−1V
, and
(e)
V
PS
=−5V
. (Ans. (a) 0, (b) 0.325 mA, (c) 1.075 mA, (d) 0, (e) 0).
TYU 1.11 The power supply (input) voltage in the circuit of Figure 1.28 is
V
PS
=
10 V and the diode cut-in voltage is
V
γ
= 0.7
V (assume
r
f
= 0
). The power dissi-
pated in the diode is to be no more than 1.05 mW. Determine the maximum diode
current and the minimum value of R to meet the power specification. (Ans.
I
D
= 1.5
mA,
R = 6.2
k
)
1.4 DIODE CIRCUITS: AC EQUIVALENT CIRCUIT
Objective: • Develop an equivalent circuit for a diode that is used
when a small, time-varying signal is applied to a diode circuit.
Up to this point, we have only looked at the dc characteristics of the pn junction
diode. When semiconductor devices with pn junctions are used in linear amplifier
circuits, the time-varying, or ac, characteristics of the pn junction become important,
because sinusoidal signals may be superimposed on the dc currents and voltages. The
following sections examine these ac characteristics.
Sinusoidal Analysis
In the circuit shown in Figure 1.35(a), the voltage source
v
i
is assumed to be a sinu-
soidal, or time-varying, signal. The total input voltage
v
I
is composed of a dc
component
V
PS
and an ac component
v
i
superimposed on the dc value. To investigate
this circuit, we will look at two types of analyses: a dc analysis involving only the dc
voltages and currents, and an ac analysis involving only the ac voltages and currents.
Current–Voltage Relationships
Since the input voltage contains a dc component with an ac signal superimposed,
the diode current will also contain a dc component with an ac signal superimposed, as
shown in Figure 1.35(b). Here,
I
DQ
is the dc quiescent diode current. In addition, the
diode voltage will contain a dc value with an ac signal superimposed, as shown in
Figure 1.35(c). For this analysis, assume that the ac signal is small compared to the dc
component, so that a linear ac model can be developed from the nonlinear diode.
1.4.1
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