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Bipolar Junction Transistors in Active Mode
23 questions By Tony R. Kuphaldt
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Question 10 of 23
Explain why a bipolar junction transistor tends to regulate collector current over a wide range of collector-to-emitter voltage drops when its base current is constant. What happens internally that makes the BJT’s collector current relatively independent of collector-to-emitter voltage and strongly dependent on base current?
Reveal answerBecause the BJT is a minority carrier device, the vast majority of collector current is the result of charge carriers injected from the emitter into the base region. Since this rate of charge carrier injection is a function of base-emitter junction excitation, base current (or more properly, base-to-emitter voltage) primarily determines collector current with collector-to-emitter voltage playing a relatively minor role.
Notes:The current-regulating nature of a BJT is made more understandable by analyzing an energy band diagram of the transistor in active mode.
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Question 11 of 23
Many technical references will tell you that bipolar junction transistors (BJTs) are current-controlled devices: collector current is controlled by base current. This concept is reinforced by the notion of “beta” (β), the ratio between collector current and base current:
$$\beta = \frac{I_C}{I_B}$$
Students learning about bipolar transistors are often confused when they encounter datasheet specifications for transistor β ratios. Far from being a constant parameter, the “beta” ratio of a transistor may vary significantly over its operating range, in some cases exceeding an order of magnitude (ten times)!
Explain how this fact agrees or disagrees with the notion of BJTs being “current-controlled” devices. If collector current really is a direct function of base current, then why would the constant of proportionality between the two (β) change so much?
Reveal answerSit down before you read this, and brace yourself for the hard truth: bipolar junction transistors are technically not current-controlled devices. You were sitting down, right? Good.
Follow-up question: if BJTs are not controlled by base current, then what are they controlled by? Express this in the form of an equation if possible. Hint: research the “diode equation” for clues.
Notes:For discussion purposes, you might want to show your students this equation, accurate over a wide range of operating conditions for base-emitter voltages in excess of 100 mV:
$$I_C=I_{ES}(e^{V_{BE}/V_T}-1)$$
This equation is nonlinear: increases in VBE do not produce proportional increases in IC. It is therefore much easier to think of BJT operation in terms of base and collector currents, the relationship between those two variables being more linear. Except when it isn’t, of course. Such is the tradeoff between simplicity and accuracy. In an effort to make things simpler, we often end up making them wrong.
It should be noted here that although bipolar junction transistors aren’t really current-controlled devices, they still may be considered to be (approximately) current-controlling devices. This is an important distinction that is easily lost in questions such as this when basic assumptions are challenged.
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Question 12 of 23
A common term used in semiconductor circuit engineering is small signal analysis. What, exactly, is ßmall signal” analysis, and how does it contrast with large signal analysis?
Reveal answerSmall signal analysis is where the signals are presumed to be small enough in magnitude that the active device(s) should respond in a nearly linear manner. Large signal analysis is where the signals are presumed to be large enough that component nonlinearities become significant.
Follow-up question: why would engineers bother with two modes of analysis instead of just one (large signal), where the components’ true (nonlinear) behavior is taken into account? Explain this in terms of network theorems and other mathematical “tools” available to engineers for circuit analysis.
Notes:When researching engineering textbooks and other resources, these terms are quite often used without introduction, leaving many beginning students confused.
