P123 HW 2: Bipolar Transistors I
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Physics 123: Homework 2: Bipolar Transistors I
REV 0; February 8, 2015.
DUE Monday, Feb. 16, 2015, 5pm
Total Points: 24
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Contents
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1 Measuring ZIN (3 points)
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2 This waveform from That (3 points)
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3 Current sources: passive and active (2 points)
3.1 Passive . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3.2 Active (single-transistor version) . . . . . . . . . . . . . . . .
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4 Bad Circuits (5 points)
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5 High gain amplifiers (2 points)
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6 Temperature stability (2 points)
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7 Differential Amplifier (5 points, total)
7.1 Maximize Gain (2 points) . . . . . . . . . . . . . . . . . . . .
7.2 Stable? (1 point) . . . . . . . . . . . . . . . . . . . . . . . . .
7.3 Tail of Diff amp (2 points) . . . . . . . . . . . . . . . . . . . .
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8 Op Amp Level Shifter (2 points)
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Measuring ZIN (3 points)
Describe a procedure for measuring RIN and CIN for the device shown below. Note that your procedure should work
even if the foot of the resistor shown to model RIN is not tied to ground. (This last requirement makes the problem
harder than it otherwise might be).
Figure 1: Input impedance problem
P123 HW 2: Bipolar Transistors I
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2
This waveform from That (3 points)
Show skeleton circuits (which means just “a circuit without component values”, except that if relative component
values matter, please indicate these (for example, “R,” “10R”).) Show a circuit that would deliver the output shown,
given the input shown. You may invoke power supplies, if you like. Try for the simplest circuit that will do the job.
Figure 2: This from that...
P123 HW 2: Bipolar Transistors I
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3
Current sources: passive and active (2 points)
We’d like to ask you to make a silly passive current source, along with an active sources, so that you can appreciate
what’s handy about the transistor versions.
Suppose that you want to make a current source (strictly, a “sink”) that will sink current from a load returned to a
positive voltage supply of +10V. The circuit should hold the load’s current constant at 10mA, ±1%, as the voltage
at the foot of the load varies between close to ground and +10V. Thus:
Figure 3: current sink
3.1
Passive
Sketch a circuit for a (ridiculous-) passive current source (no transistors), that would do this job. You may use a
negative power supply.
3.2
Active (single-transistor version)
Sketch a circuit for a single-transistor current source that would do this job. This time, use no negative supply. How
close to ground can your circuit’s output go, while the current source still functions well?
P123 HW 2: Bipolar Transistors I
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4
Bad Circuits (5 points)
Fix whatever’s wrong with these circuits. If the repair doesn’t make the defect evident, write a note telling us what’s
wrong.
Figure 4: bad circuits
P123 HW 2: Bipolar Transistors I
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5
High gain amplifiers (2 points)
How do these amplifiers compare with respect to “linearity” or constancy of gain over the output swing? Explain
your conclusion, briefly. Assume that each amplifier is fed by a properly-biased input.
Figure 5: High gain amplifiers
For case “b)” we should provide some hints, because this circuit is very similar to a “current mirror,” a circuit we
didn’t require you to learn. This circuit, mirror-like, will sink a current through Q1 of about V+/2R. That same
current will flow in Q2, if we don’t disturb things; Q2 is said to “mirror” the current passed by Q1, because the two
VBE ’s are the same. This is the scheme that sets up the ’biasing:’ puts Vout , quiescent, at about V+/2, as usual.
Enough said?
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Temperature stability (2 points)
Which of the circuits just above are (is?) protected against temperature effects, and how? Explain the mechanism or
mechanisms.
P123 HW 2: Bipolar Transistors I
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6
Differential Amplifier (5 points, total)
Some questions about the design of a differential amplifier:
7.1
Maximize Gain (2 points)
Lab 5 begins with a diff amp of modest gain (collector resistor is 10k, emitter resistors are 100Ω). Modify the circuit
so as to maximize gain (without use of current sources). (Please draw the modified circuit.) What is the modified
circuit’s differential gain? Common-mode gain?
7.2
Stable? (1 point)
When you have maximized gain, does your circuit remain temperature stable? (Explain your answer.)
7.3
Tail of Diff amp (2 points)
In Lab 5, we suggest that you replace the 10k resistor in the tail with a current sink. Why? What is the argument
that suggests a current sink will improve CMRR?
Would increasing the value of RTAIL —say, to 100k—improve CMRR, relative to the circuit of fig. 1 of lab 5?
Explain your answer.
P123 HW 2: Bipolar Transistors I
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Op Amp Level Shifter (2 points)
Show a circuit that converts an input voltage in the range 0 to +5V to an output voltage in the range -10V to +10V.
0V in should generate -10V out; +5V in should generate +10V out, and input values between 0 and +5V should
generate outputs continuous between -10V and +10V.
ROU T for the signal source is unknown. The circuit should work for signal frequencies down to zero (DC).
(hw2 feb15.tex;February 8, 2015)