Summer P123 HW 3: Bipolar Transistors I 1
Total Points : 15
DUE TUESDAY, June 30, 2010 in class.
1 Measuring Stray Capacitance (1 point) 1
2 Current sources: passive and active (2 points) 2
2.1 Passive . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2
2.2 Active (single-transistor version) . . . . . . . . . . . . . . . .
2
3 Follower, loaded (2 points) 2
4 Transistor Switch (2 points) 3
4.1 Collector current & switch power . . . . . . . . . . . . . . . .
3
4.2 Why is one better than the other? . . . . . . . . . . . . . . . .
3
5 High gain amplifiers (2 points)
6 Temperature stability (1 points)
7 Differential Amplifier (5 points, total) 5
7.1 Maximize Gain (2 points) . . . . . . . . . . . . . . . . . . . .
5
7.2 Stable?
(1 point) . . . . . . . . . . . . . . . . . . . . . . . . .
5
7.3 Tail of Diff amp (2 points) . . . . . . . . . . . . . . . . . . . .
5
4
4
Tom & Paul made different guesses, a while ago, at the value of capacitance between two adjacent columns on the breadboards. We wondered, because of the demo in which we purposely missed a connection by one column, and then found we had made an “accidental” differentiator. You may recall the scheme:
Figure 1: If you just miss a breadboard connection, you have put a small capacitance in the signal path
Describe how you would go about measuring this small capacitance, given only a function generator with BNC cable, and a 2-channel scope, with probes. Assume a scope probe looks like 10pF parallel 10M (this is what it looks like when connected to scope); a BNC cable of 3 feet shows about 100pF capacitance to ground. Invent some hypothetical readings that would reveal the breadboard’s stray capacitance, assuming the value is in the range 0.5pF
to 5pF. Show how you infer C from your hypothetical readings.

Summer P123 HW 3: Bipolar Transistors I 2
We’d like to ask you to make a silly passive current source, along with a couple of 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 current “sink”) that will sink current from a load connected to a positive voltage supply of +10V. The circuit should hold the load’s current constant at 1mA, ± 1%, as the voltage at the foot of the load varies between close to ground and +10V. Thus:
Figure 2: current sink
Sketch a circuit for a (ridiculous-) passive current source using only resistors, that would do this job. You may use a negative power supply.
Sketch a circuit for a single-transistor current source that would do this job. This time, use no negative supply, only ground. How close to ground can your circuit’s output go, while the current source still functions well?
Text Book Fig 2.8 and 2.9 show that an emitter follower can clip when loaded, despite its low R out circuit. Draw V out
. Here is a similar
. Explain at what voltage this clipping occurs, and tell us why it occurs at that voltage.
Figure 3: Clipping follower

Summer P123 HW 3: Bipolar Transistors I 3
An ideal switch ought, when ON , to put maximum power into the load, while dissipating none itself. Here are two possible switch circuits, and some questions about them:
Figure 4: 2 possible “switch” circuits
What is approximate load current in the two cases? Assume that β is 100.
What power is wasted in the two cases? Wasted power is power that doesn’t get dissipated in the load. Assume that
V
CE − saturation is about 0.2V. Remember to include power in the base resistor. Remember, power dissipated in the transistor comes from two places: The current from Base to Emitter (times its voltage drop) and the current from
Collector to Emitter (times its voltage drop).
Explain briefly why one is the preferred switch configuration.

Summer P123 HW 3: Bipolar Transistors I 4
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)” is extra credit. We 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 V
BE
’s are the same. This is the scheme that sets up the ’biasing:’ puts Vout, quiescent, at about V+/2, as usual.
Enough said?
Which of the circuits just above are (is?) protected against temperature effects, and how? Explain the mechanism or mechanisms.

Summer P123 HW 3: Bipolar Transistors I
Some questions about the design of a differential amplifier:
5
Lab 5 begins with a diff amp of modest gain (collector resistor is 10k, emitter resistors are 100 Ω ). 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?
When you have maximized gain, does your circuit remain temperature stable? (Explain your answer.)
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 R
TAIL
Explain your answer.
—say, to 100k—improve CMRR, relative to the circuit of fig. 1 of lab 5?
(hw2 feb10.tex;June 27, 2010)