ECEG 351
Electronics II
Spring 2016
Homework Assignment #5 – due in class Friday, Apr. 15, 2016
Instructions, notes, and hints:
Provide the details of all solutions, including important intermediate steps. You will not receive
credit if you do not show your work. Some problems might be solvable (or must be solved) using
good engineering approximations or assumptions. In those cases, your answer might differ from
the posted answer by a fairly large margin. Given typical device variations and component
tolerances, a little discrepancy is often reasonable. If you justify any approximations you make,
you will be given full credit for such answers.
Assignment:
1. The quiescent point in the common-source amplifier shown below has been set at
approximately VD = 8 V and ID = 10 mA. If the MOSFET has Vt = 2.1 V, find the mid-band
gain vo/vin and the approximate values of the break frequencies fPi, fPo, and fPS associated with
capacitors Ci, Co, and CS, respectively, that determine the low-frequency response. Also
estimate the lower frequency limit fL of the midband range. Hint: gm = 2ID/VOV. You may
ignore the Early effect (i.e., assume ro → ∞).
VDD = 12 V
Rsig = 50  v
in
vsig
+
−
Ci
0.1 F
RA
1 M
RB
1.1 M
CDD = 10 F
RD
390 
Co = 1 F
+
vo
−
RS
390 
(continued on next page)
CS
100 F
RL
10 k
2. Estimate the upper and lower limits of the midband range for the common-gate amplifier
circuit shown below. Before finding the numerical values of the pole frequencies, first find
symbolic expressions for the equivalent resistances seen by each capacitance in the circuit.
The quiescent drain current is 2 mA, and the MOSFET has parameter values kn = 4 mA/V2,
Vt = 1 V, Cgs = 30 pF, Cgd = 2.0 pF; Cds is negligibly small, and ro is large enough to ignore.
Identify which single capacitors have the most dominant effect in determining the upper and
lower limits of the midband region.
Rsig
50 
Ci
10 F
Co
1 F
vo
vsig
+
−
RS
2 k
CG
3300 pF
RL
10 k
RD
2 k
RG1
1 M
VDD = +12 V
RG2
1 M
Cby
10 F
3. Find the required values for the capacitors Ci, Co, and CE in the common-emitter amplifier
shown below so that the low end of the midband region is at 200 Hz with the dominant
capacitor contributing 80% toward the lower cut-off frequency and the other two each
contributing 10% (i.e., the dominant pole contributes 80% of the value of fL, and the other
two poles each contribute 10%). Minimize the total required capacitance. Note that you will
first need to find the quiescent point of the amplifier. Also find the mid-band gain Av = vo/vin.
You may assume that n = 1, T = 290 K, VBE = 0.7 V, VCE|sat = 0.3 V, and  ≈ 200. You may
also assume that ro → ∞, and you may use some of the results presented in class.
CDD = 10 F
VDD = 12 V
Rsig = 75  vin
vsig
+
−
R1
68 k
Ci
RC
7.5 k
Co
+
vo
−
R2
43 k
RE
7.5 k
(continued on next page)
CE
RL
50 k
4. Using the capacitor values from the previous problem, find the midband gain and lower cutoff frequency for the circuit shown below. Note that it is the same as the circuit considered in
the previous problem except that the resistor RD has been added (the subscript D is for
degeneration), and RE has been correspondingly reduced. By what factors have the gain and
cut-off frequency been reduced? Comment on the performance trade-off this circuit change
represents.
VDD = 12 V
Rsig = 75  v
in
vsig
+
−
R1
68 k
Ci
R2
43 k
CDD = 10 F
RC
7.5 k
Co
+
vo
−
RD
200 
RE
7.3 k
CE
RL
50 k