Physics 160
Spring 2006
Name:
Final Exam
Closed book. You may use one page, two sides (8.5 by 11)
of your own notes. Calculators are okay, but no computers
are allowed.
In all of the following problems, assume that a bipolar
transistor has β = h fe = 100 and VBE = 0.6 V .
Problems
1–3
4−6
7,8
9−11
12−14
15
16,17
18,19
Total B:
Score
6
8
20
16
12
8
7
23
100
Numerical results only need to be good to about 5% (no
more than 2 significant figures). To be eligible for partial
credit, you must show your work leading up to all
numerical results. Round resistor and capacitor values to two significant figures. You do
not need to worry about whether a component value is commercially available.
When doing sketches where numbers are available, indicate clearly the voltage, current,
or frequency scale on your sketch.
Other than perhaps the last two problems, in which you provide the design, each section
of each problem should be a quick calculation. If you get bogged down on a section,
move on and come back to it later if there is time.
The exam includes 8 pages and 19 problems and questions.
1. (2 pnts) The output impedance of a good current source should be
a) as high as possible
b) as low as possible
2. (2 pnts) The output impedance of a good voltage source should be
a) as high as possible
b) as low as possible
3. (2 pnts) Two voltage amplifier/filter stages are connected in series, as indicated
below. In order that the gain and filter characteristics of A are not significantly
affected by the presence of B and vice versa,
a) The output impedance of A should be high and the input impedance of B should
be low.
b) The output impedance of A should be low and the input impedance of B should
be low.
c) The output impedance of A should be low and the input impedance of B should
be high.
d) The output impedance of A should be high and the input impedance of B should
be high.
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Physics 160
Spring 2006
4. (2 pnts) What is the small-signal output impedance of this emitter follower?
15V
10k
144k
-15V
a) 100 Ω
b) 125 Ω
c) 250 Ω
d) 350 Ω
e) 10 kΩ
f) 144 kΩ
5. (2 pnts) What is the small-signal input impedance of this emitter-follower circuit?
15V
40k
In
Out
40k
a) 10 Ω
b) 17 kΩ
1k
c) 20 kΩ
d) 40 kΩ
e) 80 kΩ
f) 120 kΩ
(
)
6. (4 pnts) The voltage at point A in the circuit below goes like V (t ) = V0 1 − e − t τ after
the switch is closed. What are the values of V0 and τ ? Note that the solution can be
obtained very easily if you make use of circuit theorems that you know.
1
A
2
6kohm
5V
3uF
3kohm
6uF
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Physics 160
Spring 2006
7. (14 pnts) Consider the following amplifier circuit.
7.5k
100k
In
Out
0.1uF
15Vdc
5k
10uF
10k
750
a)
b)
c)
d)
What is the bias voltage at the transistor base?
What is the value of the transistor collector current?
What is the approximate voltage gain of the amplifier?
What is the DC impedance of the bias network, as seen from the node at the
transistor base?
e) At operating frequencies (e.g. 10 kHz) what is the effective impedance of the
5 kΩ resistor, as seen from the node at the transistor base?
f) What is the small-signal input impedance of the amplifier at operating
frequencies?
g) What is the small-signal output impedance of the amplifier?
8. (6 pnts) Consider the following filter circuit. Assume an input sinusoidal voltage of
amplitude unity.
In
Out
C
R
a) Is the output voltage leading or lagging in phase relative to the input voltage?
b) At what frequency (in terms of R and C) is the magnitude of the phase change
from input to output equal to 45°?
c) Sketch the output phase as a function of frequency.
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Physics 160
Spring 2006
9. (6 pnts) Consider the following filter.
In
Out
R
L
C
a) Sketch the gain (Vout/Vin) as a function of frequency.
b) If you change the product LC, what aspect of your graph will change, and in what
direction or sense?
c) If you change the value of R, what aspect of your graph will change, and in what
direction or sense?
10. (4 pnts) Explain why this emitter-follower circuit will not operate reliably.
15V
750k
In
C
Out
7.5k
11. (6 pnts) Explain what is wrong with each of these bad circuits and what single
component should be added, and where, in each case to make the circuit at least
functional. You do not need to give values for the added components.
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Physics 160
Spring 2006
12. (3 pnts) Explain why this high-gain common-emitter A/C amplifier will not work
reliably, even though it appears to simulate fine in PSpice, with a collector current of
about 1 mA and a gain of about 270.
In
RC
6.8k
R1
56k
C1
3.3uF
15Vdc
Out
Q1
Q2N3904
R2
2.7k
13. (3 pnts) Sketch a modification to the amplifier in the previous problem that will make
it reliable while still maintaining the same high gain for A/C signals. You do not
need to specify the values of the new or changed components.
14. (6 pnts) Answer the following questions about the schematic below, of an A/C
amplifier with a push-pull output stage.
3
0.68uF
+
B2
OUT
2
V4
-
B1
5
6
A
Out
1
4
FREQ = 200
V-
VAMPL = 0.1
U1
V+
In
7
V2
15V
RLoad
10k
100
9k
V1
9k
15V
1k
a) What is the overall voltage gain? (For this you can assume that RLoad=1 Mohm.)
b) Suppose a 200 Hz sine wave of 100 mV amplitude is input. Sketch the waveform
(voltage versus time) at the output and at point A in the circuit.
c) What property of the op-amp will limit its ability to remove crossover distortion
at higher frequency (e.g. 10 kHz)?
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Physics 160
Spring 2006
15. (8 pnts) The following 3 plots are from the 2N3819 JFET data sheet. Make use of
them for the following four questions.
LOAD
10V
2N3819
180
a) What is the approximate current delivered by the current source illustrated above
at room temperature?
b) What is the approximate compliance range (in volts) for this current source?
c) What is the approximate output impedance of this current source?
d) What is the approximate output impedance of the following source follower at
room temperature?
In
10V
2N3819
10k 0.01uF
1Meg
6
180
Out
Physics 160
Spring 2006
16. (4 pnts) Consider the following Schmitt trigger, in which the output swings between 0
and 5V.
5V
8
20k
+
1k
V+
3
-
4
2
Out
1
V-
OUT
In
100k
10k
a) What is the threshold for a rising signal at the input?
b) What is the threshold for a falling signal at the input?
17. (3 pnts) In the following common-emitter amplifier (for which the bias-circuit details
are not shown), the capacitance from collector to base of the transistor is 2 pF.
Suppose that the voltage gain of the transistor is 500. What will be the frequency of
the 3dB point caused by the Miller effect?
Out
RS
In
80
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Physics 160
Spring 2006
18. (10 pnts) Draw a schematic showing how to connect up a 723 regulator (shown
below) to deliver a 3V regulated output from a car battery (12V input). The 723
reference voltage is 7.15V. Make sure that your supply is protected against loads that
would draw more than 150 mA of current. Remember that the regulator needs a
100 pF frequency-compensation capacitor.
VCC+
VC
12
+
5
3
-
4
2
V+
8
11
+
OUT
V-
-
1
4
10
Vref
OUT
2
CL
3
CS
6
7
13
COMP
VCC-
19. (13 pnts) Design a DC-input bipolar-transistor differential amplifier to operate from
±15V supplies with a differential gain of 20 and a tail current set close to 1 mA by a
current-mirror current source. The input should be able to swing by at least ±0.25V
around ground without saturating the output. The output only has to be able to drive
a high impedance load (e.g. the base of another transistor). Estimate the commonmode range of your design.
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