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(ELEC101)T11Example.pdf
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Example 1: (From P11-7, Circuits, Devices & Systems, by R.J. Smith and R.C. Dorf)

It is desired to increase the conductivity of silicon by a factor of 1000 by doping with arsenic (Group V) element. Specify the doping density and identify the important charge carrier.

Given
Intrinsic carrier density of silicon atoms per m3

Electron mobility
m2 / V-s
Hole mobility
m2 / V-s
Electron charge C



Answers to Example 1:





As Arsenic is a Group V element, when Si doped with As, the important charge carrier is electron, so for the doped Si, we can assume
and






Well, we can calculate the concentration of the hole in the doped Si




We can see that
so, concentration of hole can be ignored.




Intrinsic, n = p = ni

Example 2: (From 11-28, Circuits, Devices & Systems, by R.J. Smith and R.C. Dorf)

The switching circuit of Fig. 3 contains two silicon diodes. Assume the diodes are PL diode with VF = 0.7 and rs = 0

(a)
For switch S closed, estimate IA and IB for VA = .1, .0.1, +1, and +2V


(b)
For switch S open, estimate IA and IB for VA = +1V





Answers to Example 2:
(a)
Redraw the circuit, we have














VA(V)
VP(V)
D1
D2
IA(mA)
IB(mA)

.1
.0.3
ON
OFF
5
0

.0.1
+0.6
ON
OFF
4.1
0

1
+0.7
OFF
ON
0
4

2
+0.7
OFF
ON
0
4




(b)
When S is open, redraw the circuit, we have














We can see that VP = VA + 0.7, if VA = 1V, then VP = 1.7V
Current flowing at the top 1k resistor will be
, By KCL,





Example 3: (From 8.30, Introduction to Electrical Engineering by Irwin)

An electronic calculator operates on a dc supply 6V. This supply is obtained from a half-wave rectifier with an RC filter. If the calculator can be modeled as a 10k resistance and it can tolerate a maximum peak-to-peak variation of 0.1V around the dc value. Calculate the minimum value of capacitor that can be used in the filter. Use a piecewise linear diode model with rs = 50, VF = 0.7V. Assume ac input is 60Hz.

Answers:

The circuit can be modeled as








The question stated that the calculator operates on dc 6V. That means Vout must be 6V. In that case, the model of the diode is not important.

In addition, the calculator can only tolerate a maximum peak-to-peak of 0.1V.
i.e. 5.9V < Vout < 6V

















Using equation








6V

5.9V




Example 4: (From Page 349, Circuits, Devices & Systems, by R.J. Smith and R.C. Dorf)

A load draws a current varying from 10 to 100mA at a nominal voltage of 100V that can vary a maximum of 1%. Specify V1 for a regulator consisting of RS = 200 and a Zener diode represented by VZK = 100V and rZK = 20. For IL = 50mA, determine the increase in dc i