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Bipolar Junction Transistors as Switches
34 questions By Tony R. Kuphaldt
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Question 19 of 34
The following circuit used to work fine, but does not work as it should anymore. The lamp does not come on, no matter what is done with the switch:
The very first thing you do is use your multimeter to check for source voltage, TP4 to ground. Between those points you find 24 volts DC, just as it should be. Next you measure voltage between TP2 and ground with the switch pressed and unpressed. With the switch pressed, there is no voltage between TP2 and ground, but there is voltage (about 23.5 volts) when the switch is unpressed.
From this information, determine at least two possible failures in the circuit which could cause the lamp not to energize, and also account for the voltage measurements taken.
Reveal answerPossible circuit faults:
- Lamp burned out
- Transistor failed open at collector terminal
- Bad (open) connection between lamp and transistor
- Bad (open) connection between lamp and ground
Follow-up question: explain why we may say with confidence that there is no problem with the resistor or the switch.
Notes:Discuss with your students how to reason from the empirical data to a set of possible faults in the circuit, and also how certain areas of the circuit (or individual components) may be safely eliminated from the list of possible faults.
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Question 20 of 34
What is the significance of a transistor’s beta (β) rating? What, exactly, does this figure mean? Explain how you might set up an experiment to calculate the beta of a given transistor.
Obtain a copy of a datasheet for a small-signal transistor such as the 2N2222 and see what it has to say about that transistor’s beta. Note: the beta of a transistor may be represented by the Greek letter (β), or by a completely different symbolism known as a hybrid parameter: hFE
Reveal answer$$\beta_{DC} = \frac{I_C}{I_B}$$
$$\beta_{AC} = \frac{\Delta I_C}{\Delta I_B}$$
Notes:The “beta” rating of a transistor is a very important, yet very elusive parameter. Discuss the meaning of this specification with your students, and also their experimental setups for empirically determining beta. Then, compare the results of this discussion with the figures obtained from their datasheets. What do the datasheet figures indicate about the nature of beta as a parameter?
After sharing the datasheet results, re-discuss the experimental setups for measuring beta. Would a single test be sufficient, or would it be advisable to test beta more than once for a given transistor?
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Question 21 of 34
Explain what β means for a bipolar junction transistor, in your own words.
Reveal answerI’ll let you figure this one out for yourself!
Notes:Many written definitions for β may be found in various textbooks and lexicons. What I am looking for here, as best as possible, is a definition cast in students’ own words.
