Physics S123 HW 3: Bipolar Transistors I

Size: px

Start display at page:

Download "Physics S123 HW 3: Bipolar Transistors I"

Dwight Welch
5 years ago
Views:

1 S123 HW 3: Bipolar Transistors I 1 Physics S123 HW 3: Bipolar Transistors I Total Points: 18 REV 0; June 27, DUE Thursday, July 3, 2008 If a question baffles you, one of us. The fault may lie in the wording of the question, and we re happy in any case to help you get un-stuck. Contents 1 Measuring Z IN (3 points) 2 2 Current sources: passive and active (2 points) Passive Active (single-transistor version) Transistor Switch (2 points) Collector current & switch power Why is one better than the other? High gain amplifiers (2 points) 5 5 Temperature stability (2 points) (this may be harder than the other questions) 5 6 Push-pull (2 points) What is annoying about the behavior of this circuit? What is good about the circuit? Show a remedy for the annoying behavior of the circuit Differential Amplifier (5 points, total) Maximize Gain (2 points) Stable? (1 point) Tail of Diff amp (2 points)

2 S123 HW 3: Bipolar Transistors I 2 Old Stuff 1 Measuring Z IN (3 points) Describe a procedure for measuring R IN and C IN for the device shown below. Note that your procedure should work even if the foot of the resistor shown to model R IN is not tied to ground. (This last requirement makes the problem harder than it otherwise might be). Figure 1: Input impedance problem

3 S123 HW 3: Bipolar Transistors I 3 2 Current sources: passive and active (2 points) 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 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 1mA, ±1%, as the voltage at the foot of the load varies between close to ground and +10V. 2.1 Passive Sketch a circuit for a (ridiculous-) passive current source (no transistors), that would do this job. You may use a negative power supply. 2.2 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?

4 S123 HW 3: Bipolar Transistors I 4 3 Transistor Switch (2 points) 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 2: 2 possible switch circuits 3.1 Collector current & switch power What is approximate load current in the two cases? Assume that β is 100. What power is dissipated in the transistor switch in the two cases, assuming that V CE saturation is about 0.2V? (Please include power in the base resistor.) 3.2 Why is one better than the other? Explain briefly why one is the preferred switch configuration.

5 S123 HW 3: Bipolar Transistors I 5 4 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 3: 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? 5 Temperature stability (2 points) (this may be harder than the other questions) Which of the circuits just above are (is?) protected against temperature effects, and how? Explain the mechanism or mechanisms.

6 S123 HW 3: Bipolar Transistors I 6 6 Push-pull (2 points) Draw a push-pull voltage follower. Use power supplies of±15v. Push-pull follower 6.1 What is annoying about the behavior of this circuit? 6.2 What is good about the circuit? 6.3 Show a remedy for the annoying behavior of the circuit. Make sure your remedy is thermally stable.

7 S123 HW 3: Bipolar Transistors I 7 7 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; this is quite subtle.) 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 R TAIL say, to 100k rather than 10k improve CMRR, assuming you made other changes required to keep V OUTquiescent centered at V+/2, as usual? Explain your answer. (hw3ss july08.tex;june 27, 2008)

EXPERIMENT 12: SIMULATION STUDY OF DIFFERENT BIASING CIRCUITS USING NPN BJT

EXPERIMENT 12: SIMULATION STUDY OF DIFFERENT BIASING CIRCUITS USING NPN BJT AIM: 1) To study different BJT DC biasing circuits 2) To design voltage divider bias circuit using NPN BJT SOFTWARE TOOL: PC