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JFET Amplifiers
15 questions By Tony R. Kuphaldt
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Question 13 of 15
It is a well-known fact that temperature affects the operating parameters of bipolar junction transistors. This is why grounded-emitter circuits (with no emitter feedback resistor) are not practical as stand-alone amplifier circuits.
Does temperature affect junction field-effect transistors in the same way, or to the same extent? Design an experiment to determine the answer to this question.
Reveal answerDid you really think I would tell you the answer to this question? Build the circuit(s) and discover the answer for yourselves!
Notes:The purpose of this question is to get students thinking in an experimental mode. It is very important that students learn to set up and run their own experiments, so they will be able to verify (or perhaps discover!) electronic principles after they have graduated from school. There will be times when the answers they seek are not to be found in a book, and they will have to “let the electrons teach them” what they need to know.
Remind your students that proper scientific experiments include both experimental and control subjects, so that results are based upon a comparison of measurements.
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Question 14 of 15
Identify what type of amplifier circuit this is, and also what would happen to the output voltage if Vin2 were to become more positive:
Reveal answerThis is a differential amplifier circuit. If Vin2 were to become more positive, Vout would become more negative.
Notes:Students should be able to relate this circuit to its bipolar transistor counterpart. Ask them to explain what advantages or disadvantages this circuit holds over a bipolar differential amplifier circuit.
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Question 15 of 15
The following circuit is a “multi-coupler” for audio signals: one audio signal source (such as a microphone) is distributed to three different outputs:
Suppose an audio signal is getting through from the input to outputs 2 and 3, but not through to output 1. Identify possible failures in the circuit that could cause this. Be as specific as you can, and identify how you would confirm each type of failure using a multimeter.
Reveal answerGiven the existence of multiple answers for this question, I will defer the answer(s) to your instructor, to review during class discussion.
Notes:Always be sure to spend plenty of time discussing troubleshooting scenarios with your students, because diagnostic skills are the highest level (and the most valuable) to develop.
Some of your students may be unfamiliar with the symbols used for the input and output jacks. Elaborate on this symbolism, if necessary.
Ask your students to identify the configuration (common-source, common-drain, or common-gate) of each JFET in this circuit, and how these respective configurations relate to the voltage gain (AV) of each amplification stage.
