U niversity of S outhern C alifornia
School Of Engineering
Department Of Electrical Engineering
EE 348:
Homework Assignment #05 & #06
(Due 03/27/2001)
Spring, 2001
Choma
Problem #21:
In the circuit shown in Fig. (P21), Rl = 1,030 , R1 = 1,520 , R2 = Ree =
170 , Rs = 300 , C1 = 3.5 μF, C2 = 100 pF, and VCC = 3.3 volts. The two transistors are
matched and are each identical to the TN2219AM NPN transistor studied in Problem #19.
+VCC
R1
Rl Rout
Rin
C1
Vo
C2
Q1
Rs
Q2

Vs

R2
Ree
Fig. (P21)
(a). Derive expressions for, and numerically evaluate, the quiescent collector current, say ICQ,
and the quiescent collector-emitter voltage, say VCEQ, pertinent to transistor Q1.
(b). Simulate the circuit on SPICE. Verify your Q–point calculations and note the Q–point values of the small signal parameters for each transistor.
(c). Neglecting the forward Early resistance, ro, of each transistor, perform a small signal
analysis to ascertain general expressions for, and numerical values of, the voltage gain, Avo
= Vo/Vs, the input resistance, Rin, and the output resistance, Rout. Assume that the signal
frequency indigenous to the source voltage, Vs, is sufficiently large to enable C1 to be represented by a short circuit and sufficiently small to allow C2 to be supplanted by an open
circuit; that is, the circuit is operating in the so-called mid–frequency band.
(d). Derive expressions for, and determine numerically, the time constants associated with
capacitors C1 and C2.
(e). Use SPICE to simulate the subject circuit under small signal (“.AC”) analytical conditions.
Compare the mid–frequency values of Avo, Rin, and Rout with the calculations executed in
Part (c). How do the lower and upper 3–dB frequencies respectively compare with the
EE 348
University of Southern California
J. Choma, Jr.
inverse time constants associated with capacitors C1 and C2?
Problem #22:
At signal frequencies that are large enough to approximate capacitor C1 as a
short circuit, the small signal Thévenin equivalent model of the output port of the amplifier in
Fig. (P21) can be diagramed as shown in Fig. (P22a).
(a). Determine general expressions and numerical results for the indicated Thévenin parameters, Kth and Rth.
(b). The amplifier in Fig. (P21) is modified by incorporating an emitter follower output buffer
as depicted in Fig. (P22b). Choose the resistance, Rk, so that transistor Q3 conducts a quiescent collector current that is identical to the static current conducted by transistor Q1.
All three transistors are identical to the device utilized in the preceding problem.
(c). Use the results of Part (a) to determine the small signal voltage gain, Avo = Vo/Vs, of the
buffered amplifier. For computational purposes, assume that the entire amplifier operates
in its mid-frequency range. Confirm your results through SPICE simulations and compare
the computed and simulated gain with that deduced in the preceding problem.
(d). What is the output resistance, Rout, of the buffered structure at mid-frequencies? Validate
your result through appropriate SPICE simulations.
+VCC
R1
Rl
Rin
C1
Q3
C2
Q1
Rth

KthVs
Q2
Rs
C2
Vo

Vs
R2


Rout
(a).
Ree
Rk
(b).
Fig. (P22)
Problem #23:
Problem #4.37, Page 236 of assigned text. Do only parts (c) and (d), and
assume that the transistor is characterized by the small signal parameters, rb = 100 , re = 2 ,
r = 2 K,  = 50, ro = , and rc = 70 . The capacitors can be assumed to emulate short circuits over the signal frequency range of interest.
Homework #05 & #06
47
Spring Semester, 2001
EE 348
University of Southern California
J. Choma, Jr.
Problem #24:
Problem #4.39, Pages 236-237 of assigned text. Assume that the transistor
is characterized by the small signal parameters, rb = 100 , re = 2 , r = 2 K, ro = , and rc
= 70 . The capacitors can be assumed to emulate short circuits over the signal frequency range
of interest.
Problem #25:
Problem #4.50c, Page 239 of assigned text. Assume that the transistor is
characterized by the small signal parameters, rb = 100 , re = 2 , r = 2 K,  = 100, ro = ,
and rc = 70 . The capacitors can be assumed to emulate short circuits over the signal frequency range of interest.
Problem #26:
Problem #4.51, Page 239 of assigned text. Do only Parts (c) and (d).
Assume that the transistor is characterized by the small signal parameters, rb = 100 , re = 2 ,
r = 2 K,  = 100, ro = , and rc = 70 . The capacitors can be assumed to emulate short circuits over the signal frequency range of interest.
Problem #27:
Problem #4.52, Page 240 of assigned text. Assume that the transistor is
characterized by the small signal parameters, rb = 100 , re = 2 , r = 2 K,  = 100, ro = ,
and rc = 70 .
Problem #28:
Problem #4.53, Page 240 of assigned text. Assume that the transistor is
characterized by the small signal parameters, rb = 100 , re = 2 , r = 2 K,  = 100, ro = ,
and rc = 70 . The capacitors can be assumed to emulate short circuits over the signal frequency range of interest.
Problem #29:
Problem #10.28, Page 630 of assigned text.
Problem #30:
The transistors in the circuit of Fig. (P30) are identical and conduct identical current densities.
(a). Derive an expression for the quiescent collector current conducted by transistor Q1.
Assume that the static beta, hFE, is large enough to allow for the tacit neglect of all transistor base currents.
(b). What design strategy would you invoke to minimize thermal drift of the Q1 static collector
current?
Homework #05 & #06
48
Spring Semester, 2001
EE 348
University of Southern California
J. Choma, Jr.
(c). What design requirements would you invoke to ensure that all static transistor base currents are indeed negligible?
(d). Derive an expression for the small signal time constant attributed to the capacitor, C. For
both transistors, ignore internal emitter resistances and assume that the forward Early
resistance is infinitely large. Assume further that capacitor C1 behaves as a short circuit
for the signal frequencies of interest.
+VCC
R1
Rl
Vo
C1
Q1
Rs
Rx

Vs

Q2
Ry
Re1
R2
Re2
C
Fig. (P30)
Homework #05 & #06
49
Spring Semester, 2001
EE 348
University of Southern California
J. Choma, Jr.
U niversity of S outhern C alifornia
School Of Engineering
Department Of Electrical Engineering
EE 348:
Homework Assignment #05 & #06
(SOLUTIONS: Due 03/27/2001)
Spring, 2001
Choma
Problem #21:
Homework #05 & #06
50
Spring Semester, 2001