Facebook
Google
GitHub
LinkedinDiscrete Semiconductor Devices and Circuits
Bipolar Junction Transistors as Switches
34 questions By Tony R. Kuphaldt
-
Question 1 of 34
Solid-state switching circuits usually keep their constituent transistors in one of two modes: cutoff or saturation. Explain what each of these terms means.
Reveal answer“Cutoff” refers to that condition where a transistor is not conducting any collector current (it is fully off). “Saturation” means that condition where a transistor is conducting maximum collector current (fully on).
Follow-up question: is there such a thing as a state where a transistor operates somewhere between cutoff (fully off) and saturation (fully on)? Would this state be useful in a switching circuit?
Notes:In all fairness, not all transistor switching circuits operate between these two extreme states. Some types of switching circuits fall shy of true saturation in the “on” state, which allows transistors to switch back to the cutoff mode faster than if they had to switch back from a state of full saturation. ECL (Emitter-Coupled Logic) digital circuits are an example of non-saturating switch circuit technology.
-
Question 2 of 34
Explain the function of this light-switching circuit, tracing the directions of all currents when the switch closes:
Reveal answer
Notes:Ask your students to explain what possible purpose such a circuit could perform.
-
Question 3 of 34
Trace the directions of all currents in this circuit, and determine which current is larger: the current through resistor R1 or the current through resistor R2, assuming equal resistor values.
If switch SW2 were opened (and switch SW1 remained closed), what would happen to the currents through R1 and R2?
If switch SW1 were opened (and switch SW2 remained closed), what would happen to the currents through R1 and R2?
Reveal answerI’ll let you determine the directions of all currents in this circuit! Although it is impossible to tell with absolute certainty, the current through R1 is likely to be much greater than the current through R2.
If SW2 opens while SW1 remains closed, both currents will cease. If SW1 opens while SW2 remains closed, there will be no current through R1, but the current through R2 will actually increase.
Follow-up question: what does this indicate about the nature of the two currents? Which current exerts control over the other through the transistor?
Notes:The most important principle in this question is that of dependency: one of the transistor’s currents needs the other in order to exist, but not visa-versa. I like to emphasize this relationship with the words controlling and controlled.
