Exam #1Review
(Last Modified: 04 November 2010 06:11:46 PM )
Date of exam: 27 Feb 03
Topics Covered
This exam will cover those sections of Chapter 6 covered in this course, namely 6.1 thorugh 6.6 and 6.9. You should be familiar and comfortable with the following:
The basic exponential equation that describes the collector current as a function of base-emitter voltage in a BJT - a.k.a. the transistor equation.
The meaning and use of the "alpha" and "beta" coefficients of a transistor.
The meaning and derivation of the BJT transconductance, input resistance and emitter resistance.
The verification of the common Rule of Thumb that a 60mV increase in Vbe leads to an order of magnitude change in Ic.
The Early Effect in so much as how it affects the BJT characteristic equations and how the basic BJT exponential equation can be modified to account for it.
The meaning and derivation of the BJT output resistance in light of the Early Effect.
The two principle small-signal models for a BJT.
The basic topology of the BJT differential pair (BJT-DP).
The meaning and use of the concept of "common-mode" and "differential-mode" signals.
The determination of the input resistance, gain and output resistance of simple BJT-DP based circuits.
The meaning and determination of Common-Mode Rejection Ratio (CMRR).
The meaning and causes of a BJT-DP's input offset voltage, input bias current and input offset current.
The principle of operation and derivation of the I(V) equation for a diode-connected BJT.
The principles of operation for the basic current mirror - the "why" behind how it works.
The primary shortcomings of the basic current mirror.
How these shortcomings are addressed in the three improved current mirrors and sources studied.
The determination of the current transfer ratio for the basic current mirrors and sources.
The determination of the output resistance of the basic current mirrors and sources.
The advantages of using current sources and mirrors as active loads for BJT-DP's.
The principle differences between, including relative strengths and weaknesses of, MOSFET and BJT based DP's.
The principle roles of the input, middle and output stages of a typical multistage amplifier.
The determination of the DC bias point (operating point) of relatively complex multistage circuits (such as those found in P6.51 or P6.122).
The determination of the input resistance, gain, output resistance and CMRR of simple multistage amplifiers.
Clearly we have covered a lot of material in this chapter. It would probably be very productive, both in preparing you for this exam as well as giving you a concise review sheet for future reference, to spend some time writing a couple of short sentences (or equations or procedural steps as appropriate) for each of the above topic items. Most of the above topic descriptions can be turned into exam questions very easily. Hint. Hint.
Exam Format and Guidelines
The exam will be closed-book and closed-note. At least 1/4 of the points on the exam will be from the following problems verbatim. Not all of these problems are trivial and might take quite a bit of time to solve the first time. But if the exam is the first time you are attempting to solve the problem, that will be the result of a choice you made with full knowledge of the potential consequences. In other words, ignore this review sheet at your own risk.
The exam will consist of a number of multiple choice or short answer questions followed by problem solving questions. The focus is on the fundamental concepts and your grasp of them. I am not looking for you to be able to solve complex problems involving lots of algebra within the framework of an hour exam. I am looking for you to be able to apply the fundamental concepts to simple problems in a manner that demonstrates that you would know how to proceed if faced with a more formidable circuit.
Unless specifically stated otherwise, assume that all answers are to be supported. This may be in the form of equations and mathematical manipulation or, if appropriate, through a logical line of reasoning. Just writing down a memorized result will not get full or even most of the credit - though certainly it will get considerably more credit than if no answer at all is given.
Review Problems
1) All homework problems. Be sure that you understand the principles behind the solutions for each. Many are of reasonable difficulty for exam questions and you can count on some of the exam problems either being homework problems or being closely related to homework problems.
2) There is a Rule of Thumb which states that the collector current of a BJT will increase by an order of magnitude for every 60mV increase in the base-emitter voltage. At what temperature is this Rule of Thumb actually true?
4) The transconductance of a BJT transistor is the ratio of the changes in what two parameters? Derive the equation which expresses the transconductance of a transistor in terms of its DC operating point values.
5) The input resistance of a BJT transistor is the ratio of the changes in what two parameters? Derive the equation which expresses the input resistance of a transistor in terms of its DC operating point values.
6) The emitter resistance of a BJT transistor is the ratio of the changes in what two parameters? Derive the equation which expresses the emitter resistance of a transistor in terms of its DC operating point values.
7) Describe the Early Effect. In particular, how does the Early Effect manifest itself on the plot of collector current versus collector-emitter voltage for a particular transistor? How is the Early Voltage determined on such a plot? How can the basic transistor equation be augmented to incorporate the Early Effect?
8) The output resistance of a BJT transistor is the ratio of the changes in what two parameters? Derive the equation which expresses the output resistance of a transistor in terms of its Early Voltage.
9) Draw the basic current mirror circuit and explain how it achieves its current mirror behavior.
10) Explain what a diode-connected BJT is and show that the equation governing it's I-V characteristic is the same as that of a diode and that it's incremental resistance is simply the transistor's emitter resistance.
11) In a multistage amplifier having an input stage, an inner stage and and output stage, what is typically the primary purpose of each stage?
12) Draw a basic current mirror that has base current compensation. What is the current transfer ratio of the mirror if the transistors all have a beta of 200?
13) Draw a Wilson current mirror. Explain how the circuit improves the output resistance of the mirror.
14) Draw a Widlar current source. What is the current transfer ratio of this source?
15) A signal of -2V is applied to one input of a DP and +5V is applied to the other. What is the common mode component of this signal?
16) For a basic BJT-DP, show that the input offset voltage due to a mismatch in collector load resistance is equal to the percent mismatch times the thermal voltage.
17) For a basic BJT-DP, show that the input offset voltage due to a mismatch in transistor scale current is equal to the percent mismatch times the thermal voltage.
18) Problem #6.68.