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Bipolar Junction Transistor (BJT) theory
28 questions By Tony R. Kuphaldt
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Question 16 of 28
Bipolar junction transistors are classified as minority carrier devices. Explain why.
Reveal answerConduction through a BJT depends on charge carriers being “injected” into the base layer of the transistor, and these charge carriers are always the “minority” type with respect to the doping of the base.
Notes:Note to your students that there is such a thing as a majority carrier type of transistor, but that it does not resemble the BJT in either construction or operational characteristics.
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Question 17 of 28
Transistors act as controlled current sources. That is, with a fixed control signal in, they tend to regulate the amount of current going through them. Design an experimental circuit to prove this tendency of transistors. In other words, how could you demonstrate this current-regulating behavior to be a fact?
Reveal answer
Procedure: measure the voltage dropped across RC while varying VCC, for several different values of IB (inferred by measuring voltage drop across RB).
Notes:Here, students must think like an experimental scientist: figuring out how to prove the relative stability of one variable despite variations in another, while holding the controlling variable constant. Encourage your students to actually build this circuit!
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Question 18 of 28
Compare the relative magnitudes of each current in this bipolar transistor circuit:
Which current is the smallest and which is the largest? Are there any two currents that are closer in magnitude than with the third? If so, which currents are they?
Reveal answerIE > IC >> IB Notes:Note the brief nature of the answer. This mathematical expression says it all, and it is a good review of inequality symbols.
What surprises me is the following: I did not see any mentioning of the most important BJT parameter: Transconductance gm.
(Instead, you are stating that the beta-value would be a very important parameter - which is NOT the case!).
Like some other low-level electronic books and papers you treat the BJT as a cittent-controlled device - this is simply wrong.
There is not a single proof that the BJT would be a CCCS !
However, there are many theoretical explanations, measurements and observable facts which clearly show that the BJT is - of course - voltage-controlled. Who can deny it ? With which arguments? Ic=beta x Ib is just a formula - a misinterpretation of Ib=Ic/beta.