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Swinburne Higher Education Exam Paper
Examination Cover Sheet
SEMESTER 1, 2010
EXAMINATION DETAILS
Faculty: FAC ENG&IND SCI
Subject Code: HET386
Subject Title: ANALOGUE ELECTRONICS 2
Duration:
min
180
Reading time:
15 min
60
% of overall assessment covered by this exam
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SWINBURNE UNIVERSITY OF TECHNOLOGY
FACULTY OF ENGINEERING & INDUSTRIAL SCIENCES
HET386 ANALOGUE ELECTRONICS 2
FIRST SEMESTER EXAMINATION 2010
Note : State any assumptions made in your answers. Unless specified
otherwise, ⏐VBE(on)⏐= 0.7 V, ⏐VCE(sat)⏐= 0.1 V, VT = 25 mV, rb = 0,
and ro = ∞ .
Q1. An operational amplifier with fT = 2 MHz, slew rate of SR = 1 V/μs, and
Vo(max) = 10 V, is used in the design of a non-inverting amplifier with a
nominal gain of 10. Assume the input is a sine-wave with peak
amplitude of Vp, and at a frequency f.
(a) If Vp = 0.5 V, what is the maximum frequency before the output
distorts?
(b) If f = 20 kHz, what is the maximum value of Vp before the output
distorts?
(c) If Vp = 50 mV, what is the useful frequency range of operation?
(d) If f = 5 kHz, what is the useful input voltage range?
( 8 marks)
Q2. For the circuit of Figure 1, find the approximate values of IO and IZ. The
Zener diode has a breakdown voltage VZ of 2.7 V. Assume that β for the
transistor is very large in value.
IO
IZ
Figure 1
( 8 marks)
Page 1 of 9
-2HET386 Analogue Electronics 2
Semester One Examination 2010(Cont'd)
Q3. A power MOSFET, for which θjC = 4 oC/W, is mounted on a heat sink
with θjA = 5 oC/W. The mounting method uses a 0.2 mm thick mica
spacer that introduces an additional thermal resistance of 1 oC/W
between the transistor case and the heat sink. During its operation, the
transistor carries an average drain current of 1 A at an average drainsource voltage of 10 V. If the ambient temperature is 25 oC, find the
operating temperatures of the transistor case and the heat sink.
(8 marks)
Q4. For the circuit of Figure 2, the transistor parameters are Kn = 500 μA/V2,
VTN = 1.2 V, and λ = 0. The resistor values are R1 = 383 kΩ, R2 = 135
kΩ, RS = 3.9 kΩ, and RD = 16.1 kΩ. Find the drain current ID.
(8 marks)
10μF
Figure 2
Q5. For the ideal class-B output stage shown in Figure 3, V+ = 25 V, V- = -25
V, and RL = 4 Ω. If the peak voltage at vO is 17 V, find the power
dissipated by each transistor.
(8 marks)
Figure 3
Page 2 of 9
-3HET386 Analogue Electronics 2
Semester One Examination 2010(Cont'd)
Q5. Sketch and clearly label the transfer characteristics (Vout versus Vin) for
the circuit shown in Figure 4. The operational amplifier saturates at ±12
V.
(8 marks)
Figure 4
Q6. The circuit of Figure 5 represents one form of a phase-shift oscillator.
3
Show that oscillations will occur at ω o =
as long as the product
RC
A1A2A3 is equal to -8, regardless of the values of the individual stage
gains.
(10 marks)
A1
A2
A3
Figure 5
Page 3 of 9
-4HET386 Analogue Electronics 2
Semester One Examination 2010(Cont'd)
Q7.
Clearly explain the operation of each of the circuits shown in Figure 6.
(15 marks)
Figure 6a
Figure 6b
Page 4 of 9
-5HET386 Analogue Electronics 2
Semester One Examination 2010(Cont'd)
Q8. EITHER
OR
Explain (clearly) the operation of a 3-bit flash analogue-todigital converter. Details of the encoder circuit are NOT
required.
Explain the operation of the digital-to-analogue converter of
Figure 7. Each of the N-channel enhancement FETs have a
threshold voltage VTN of 1 V and negligible ON resistances.
Treat each FET as a switch: closed if the gate input (for
example, b0) is greater than the threshold voltage VTN,
otherwise the switch is open. Also assume that Rd << 2R.
b0, b1, b2, and b3 are digital inputs, with values of either 0 or 5
V.
(15 marks)
Figure 7
Page 5 of 9
-6HET386 Analogue Electronics 2
Semester One Examination 2010(Cont'd)
Q9. For the circuit of Figure 8, the transistors have parameters: β = 100, VA =
∞, Cμ = 2 pF, and fT = 400 MHz. From the DC viewpoint, base currents
may be neglected.
(a) Draw a clearly labelled small-signal equivalent circuit at mid-band
frequencies.
(b) From your diagram of part (a), calculate the mid-band voltage gain,
Av = Vout/VS.
(c) Estimate the upper -3 dB frequency, fH.
(5+5+10 = 20 marks)
Figure 8
(Total for the paper is 8+8+8+8+8+10+15+15+20=100 marks)
END OF QUESTIONS
Page 6 of 9
-7HET386 Analogue Electronics 2
Semester One Examination 2010(Cont'd)
Appendix – some basic information and formulae.
1.
A Bipolar Junction Transistor (BJT) is in the active mode when its baseemitter junction is forward biased and its base-collector junction is
reverse biased.
2.
When a BJT is in the active mode of operation,
I C = β I B = αI E
3.
β
β +1
The small-signal parameters of a BJT are :
gm =
re =
I CQ
VT
rπ
β +1
ωT =
3.
where α =
rπ =
ro =
β
gm
VA
I CQ
gm
where ωT is the unity − gain frequency.
Cπ + C μ
The current-voltage characteristics of an N-channel MOSFET are as
follows :
for vGS > VTN ,
k n' W
2
2
[2(vGS − VTN ) v DS − v DS
] = K n [2(vGS − VTN ) v DS − v DS
] for v DS < v DS ( sat )
2 L
k n' W
=
(vGS − VTN ) 2 = K n (vGS − VTN ) 2
for v DS ≥ v DS ( sat )
2 L
where v DS ( sat ) = vGS − VTN ,
iD =
k n' W
Kn =
,
2 L
and k n' = μ n C ox .
For an N-channel enhancement-mode MOSFET, VTN > 0, and for an Nchannel depletion-mode MOSFET, VTN < 0.
Page 7 of 9
-8HET386 Analogue Electronics 2
Semester One Examination 2010(Cont'd)
4.
The small-signal voltage gain of a common-emitter amplifier is given by:
AV (base → collector ) ≈
− RC'
R E + re
where RC' is the effective resis tan ce in the collector , and
R E is the external resis tan ce in the emitter.
5.
The small-signal voltage gain of a common-base amplifier is given by:
AV (emitter → collector ) = g m RC'
6.
The small-signal voltage gain of a common-collector amplifier is given
by:
RE'
AV (base → emitter ) = '
RE + re
7.
where RC' is the effective resis tan ce in the collector .
where RE' is the effective external resis tan ce in the emitter.
The time constant methods are :
ωL ≈ ∑
i
1
Ris C i
and ω H ≈
1
∑ Rio Ci
i
where Ris is the resis tan ce seen by C i , with all other capaci tan ces short − circuited ,
and Rio is the resis tan ce seen by C i , with all other capaci tan ces open − circuited .
External sources are removed when evaluating Ris or Rio .
8.
For the basic differential amplifier, the magnitude of the differential-mode
gain is gmRC if the output is between the two collectors, and is 0.5gmRC if
the output is single-ended. RC is the effective resistance at the collector.
9.
The average power dissipation, PD(average), in a BJT, is given by:
PD(average) = IC(average) X VCE(average)
Page 8 of 9
-9HET386 Analogue Electronics 2
Semester One Examination 2010(Cont'd)
10.
Table below gives the relationship between the filter type and the
biquadratic functions
Filter type
Low-pass
Transfer function
Kω 02
H LP = 2
s + (ω 0 / Q) s + ω 02
High-pass
H HP =
Band-pass
H BP
Ks 2
s 2 + (ω 0 / Q) s + ω 02
K (ω 0 / Q) s
= 2
s + (ω 0 / Q) s + ω 02
K ( s 2 + ω 02 )
s 2 + (ω 0 / Q) s + ω 02
Band-reject
H BR =
All-pass
H AP = K
s 2 − (ω 0 / Q) s + ω 02
s 2 + (ω 0 / Q) s + ω 02
12. The step response of a first-order RC circuit is given by :
vC (t ) = vC (∞) + [vC (0 + ) − vC (∞)]e − t / τ
where vC (t ) is the voltage across the capacitor ,
vC (∞) is the final or steady state value,
vC (0 + ) is the initial voltage at t = 0 + , and
τ (= RC ) is the time cons tan t.
13. The small-signal parameters for a FET are:
g m = 2 K n I DQ and
ro =
VA
1
where V A =
λ
I DQ
END OF PAPER
Page 9 of 9
Swinburne Higher Education Exam Paper
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