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PAPER CODE ………………..ELEC271……………PAGE….1……..OF …………..10……………………..CONTINUED
PAPER CODE NO. EXAMINER: Dr. Ian Sandall
ELEC 271 DEPARTMENT: EE&E TEL. NO: 49796
SECOND SEMESTER EXAMINATIONS 2022/23
ELECTRONIC CIRCUITS AND SYSTEMS
TIME ALLOWED: THREE HOURS
INSTRUCTIONS TO CANDIDATES
The numbers in the right hand margin represent an approximate guide to the marks
available for that question (or part of a question). Total marks available are 100.
Answer ALL Questions.
The use of a calculator IS allowed.
Additional Information:
Amplifier properties attached
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1. a) Sketch a typical frequency response curve (Bode plot of voltage gain versus
frequency) of an ac-coupled common-emitter voltage amplifier. Explain the form
of the curve, indicating clearly the reasons for the low and high frequency roll-off.
5
b) You are required to design a 2-stage voltage amplifier (find values for RE, RC1, RC2)
to meet the following criteria: an input resistance of 500 k and an overall voltage
gain equal to or greater than 300, with a resistor output load, RL.
Use a common-emitter with emitter degradation (RE) stage for the input, followed
by a common-emitter amplifier with bias current equal to 0.75 mA.
(VCC = 15 V, βo = 250 and the DC levels of the first and second stage outputs need
to be set at half the supply voltage. Allow 10% of VCC across RE). Proceed as
follows:
i) Draw a schematic circuit of your amplifier, omitting the bias resistors
and coupling capacitors.
ii) Work out the bias current and emitter resistor value to meet the input
resistance specification. Hence find the value of the collector resistor
of the first stage.
iii) Work out the gain of the first stage and hence find the required gain of
the 2nd stage to meet the specification.
iv) Estimate the minimum value of load resistance, RL to satisfy the
specification of voltage gain.
2
6
3
5
c) From the amplifier designed in 1b), comment on the ‘quality’ of your voltage
amplifier and suggest how it could be improved.
4
Total
25
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2. a) Draw small-signal ac equivalent circuits of bipolar and MOSFET transistors at
mid-frequencies, as defined between the two -3 dB points. Compare and contrast the
ac parameters for transconductance and output resistance for application in analogue
circuits.
4
b) Figure Q2b shows a differential amplifier with current source biasing.
Explain the good features of this design.
Figure Q2b
3
Question continues overleaf
Ib
RB1
RB2 RE
VEE
Vi1
RC
v
i2
0V
X
VCC
VOUT
IO
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Question continued
c) Design the circuit in Figure Q2b (estimate values for the resistors) to meet the
following specification: IO = 1.5 mA and the DC level at VOUT = 0 V.
Hints: Allow 2 V across RE, the current in the bias circuit, Ib = 0.1× IO and the
forward-bias diode drop is 0.6 V. You may assume ideal transistors with high DC
current gain, with VCC = 15 V, and VEE = 15 V.
10
d) Give a physical description of the terms ‘differential mode gain’ and ‘common mode
gain’ with reference to the Figure Q2b.
4
e) Draw an ac equivalent circuit for the differential amplifier. Use this circuit to estimate
the differential gain.
4
Total
25
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3. a) An ideal transconductance amplifier is connected as shown in Figure Q3a. The overall
voltage gain of the circuit, vo/vg is 500 when RL is 10 k . Calculate the gain of the
ideal transconductance amplifier.
Figure Q3a
5
b) State the advantages and disadvantages of negative feedback. 5
Question continues overleaf.
RL
Rg
+
vg
vo
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Question continued
c) Identify the feedback topology of the amplifier shown in Figure Q3c and hence the
amplifier type. Represent the circuit as a negative feedback system and hence estimate
the appropriate gain.
State any approximations used in your analysis.
v
i
io
v
o
-VEE
+VCC
11kΩ
1kΩ
Ω
15kΩ
1kΩ
220Ω
2k2Ω
Q1
Q2
9kΩ
Ω
Figure 3c
15
Total
25
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4. a) Design the operational amplifier circuit of Figure Q4a to have an input resistance of
20 k and a time constant (R × C) of 15 μs. What function does the circuit
perform
Figure Q4a
5
b) Draw an appropriate equivalent circuit for the amplifier shown in Figure Q4b, and
hence derive the following expression for the input impedance.
=
(1 + ) + 0
+
–
+
vg
vo
Figure Q4b
8
Question continues overleaf.
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Question continued.
c) Derive the following expression for the loop gain of the amplifier in Figure Q4b .
d o
d
ol
r r
r
T A
+
=
Estimate values for T, β, Rin and comment on the values obtained.
Assume Parameter values are rd = 10 kΩ, ro = 50 Ω and Aol = 105
.
7
d) Explain the meaning of the terms, ‘voltage offset’ and ‘offset current’, in relation to
operational amplifiers. How would you alleviate these problems for the case of the
inverting operational amplifier circuit
5
Total
25
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ELEC271 / Amplifier properties
Common Emitter
Common emitter with emitter degradation
Common collector (Emitter follower)
Ri = rbe
Ro = RC
Ro = RC
vi
Ri
RC
Ro
io
RL
vo
RE
R
r R
R o
be S
o
E =
+
1+
/ /

vi
Ri
Ro
i
o
RL
vo
RE
RS
vs
A
v
v
g R R V
o
i
m C L = = / /
( ) R r R i be o E = + 1+
A
g R R
g R
V
m C L
m E
=
+
/ /
1
vi
Ri
RC
Ro
io
RL
vo
( ) R r R R i be o E L = + 1+ / /
A
g R R
g R R
V
m E L
m E L
=
+
/ /
/ / 1
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Common base
Ro = RC
AV = gmRC
/ /RL
R
r
g r i
be
o
m e
=
+
=
1
1

/
vi
Ri
RC
Ro
i
o
RL
vo