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Unformatted text preview: Homework 3 Solutions (
P3.1) In the circuit shown,
(a) Find the fraction of the ac source cycle during which the diode conducts?
(b) Find the peak charging current?
(c) Find the maximum reverse bias across the diode. Assume that the diode is ideal. (a) ()
() ; NOTE: only measures from 12V to 18V; conduction occurs from
12V to 18V, AND then back down to 12V so… % conduction =
(b) ( ( ) ( ) ; Forward Bias Ideal Diode = short circuit
) (c) Reverse Bias:
( ) P3.2) Assuming ideal diodes, find the value of the currents, I1 and I3, in the two circuits below (a) First, open-circuit the diodes and find the voltage across the diodes. KVL through D1 gives: KVL through D2 gives:
Since VD2 > VD1 > 0, replace D2 with the ideal diode equivalent circuit for forward bias, ie.
a short. Find current through D2 short : Find new D1 voltage : With both diodes shorted: ( ) (b) Same process as before: KVL through D3 gives: KVL through D4 gives:
VD4>VD3>0, Short D4 Find current through D3 short : Find new D3 voltage : For parts (a) and (b), one can also assume that both diodes are forward biased (short circuit)
and perform KVL. Part (a) will give you the same result of ID1 = 2mA. Part (b) will result in
ID2 < 0A, which means the diode is reversed bias (open circuit), and ID3 = 0mA.
P3.3) At what temperature is the thermal voltage equal to 25mV? Use the values for the Boltzmann
coefficient and electronic charge given in lecture to compute T. = 289.9K
() P3.4) A particular n=1 diode D1A is found to exhibit a forward current of 3 mA at an applied diode
junction voltage of 0.7V and at an operating temperature where Vt=25mV.
(a) Determine the saturation current, Is?
(b) Another diode D1B is manufactured using the same fabrication process as D1A. The only
difference is the cross-sectional area of D1B is 10x larger. Find the reverse leakage current of diode
D1B at a diode voltage of -1V.
GIVEN: n = 1,
(b) When area is 10x larger,
Given P3.5) Sedra and Smith 4.23
(a) ( ) (b) P3.6) Sedra and Smith 4.41 ( ) Similar analysis to P3.2
(a) Assume both diodes are forward bias, so replace them with DC 0.7V sources. ( )( ) = 0.16mA
( ) (b) Same analysis as A ( )( ) Voltage
Since I = 0A, D1 node is omitted, leaving only the two resistors and D2 replaced by DC 0.7V
Find total current;
) ( ) You can check by computing voltage across 10k resistor and doing KVL ...
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- Spring '11