Digital Electronics Homework #1
Question #1:
Given τp = 1µs, R = 1kΩ, Vf = 2V, Vr = -20V and VD = 0.65V at t = 0, find the
storage time in the diode in the figure below:
Question #2: Note the change from the original assignment!
Given If = 10mA, Ir = -20mA, and R = 500Ω, determine Tr & Ts of the diode in
the above figure. The diode is characterized by Is = 2 x 10-15 A and Cj(av) = 5pF
when it is reverse-biased, and NA = 100ND = 1019cm-3. τp = 167ns.
Question #3:
Consider two diodes, one with Is1 = 10-14A and one with Is2 = 100Is1, connected
in series. Calculate the diode current, I, and the voltage across each diode if
the applied voltage is 1V.
Question #4:
The following are provide for a pn junction: NA = 1015cm-3, ND = 1017cm-3, and
the area is A = (40x40)µm2. Calculate the following:
A. VD, Cjo, and Cj(av) as V changes from –10V to 0.5V.
B. W and Cj for V = -10V and V = 0.5V
Digital Electronics Homework #1 page 1
Question #5: Switching Diode:
Vd = 0.7V, VD(ON) = 0.7V, CT(0) = 2pF, τS = 10ns. Prior to t = t1, s is closed. At t
= t1, s is opened and left opened.
A. Determine ID at t < t1.
B. Determine ID at the instant t = t1.
C. Plot ID versus time showing TS, the storage time, and TR, the recovery
time, until ID = 0.
D. Plot VD showing TS and TR.
E. Determine the storage time, TS.
F. What is the recovery time constant value = R*CT(av)?
G. What is the final stored charge in the diode after the switch is opened for
about 1 second.
Question #6: Switching Diode
R = 1kΩ, Vo = 0.7V, VD(on) = 0.7V, Cj(0.7V) = 2Cj(0) = 4pF, τT = τs = 10ns,
Vi = -10V to +10V 1kHz square wave.
A. Plot Vi versus time and VD versus time showing TD, TS, and TR.
B. Determine Cj(-10V) and CD(Vi=10V) .
C. Determine Iav(R), the average current in R during the diode voltage fall
time, TR.
D. Determine the change in charge in the depletion capacitance, Cj, during
the diode recovery time, TR.
E. Determine If and Ir during the storage time, TS.
F. Determine TS.
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