Investigating student understanding of bipolar junction transistor circuits

The research reported in this article represents a systematic, multiyear investigation of student understanding of the behavior of bipolar junction transistor circuits using a variety of different tasks to isolate and probe key aspects of transistor circuit behavior. The participants in this study were undergraduates enrolled in upper-division physics electronics courses at three institutions, as well as undergraduates in upper-division engineering electronics courses at one of the institutions. Findings from this research indicate that many students have not developed a robust conceptual understanding of the functionality of bipolar junction transistors circuits even after all relevant instruction. Most notably, when asked to analyze the impact of a transistor circuit on input signals, students frequently applied reasoning appropriate for an analysis of the circuit’s dc bias behavior. However, students often displayed knowledge of fundamental transistor behavior when responding to more targeted questions. This article provides insight into student thinking about transistor circuits, describing the most prevalent conceptual and reasoning difficulties identified and discussing some important implications for instruction.

Figure 1

The canonical BJT common-emitter amplifier circuit. The base, collector, and emitter voltages have been labeled ($V_B$, $V_C$, and $V_E$, respectively) for clarity.

Figure 2

Canonical BJT common-emitter amplifier circuit used in the three amplifier comparison task. The base ($V_B$), collector ($V_C$), and emitter ($V_E$) voltages have been labeled for clarity.

Figure 3

Three amplifier comparison task. Note that the text has been slightly abridged and paraphrased and that explanations were required.

Figure 4

Follower current ranking task.

Figure 5

Follower voltage graphing task. Note that students were provided with both a plot of $V_{\mathrm{in}}$ vs time and a labeled grid in which to plot $V_{\mathrm{out}}$ vs time.

Figure 6

Common student responses to the follower graphing task (shown in Fig. 5). These responses shown include the following: (a) a quantitatively correct graph, (b) a linear graph with an offset of –0.6 V, and (c) a linear graph with no offset.

Figure 7

Transistor supply voltage modification task. Note that the text has been abridged and paraphrased.

Figure 8

Revised amplifier comparison task. Note that the text has been abridged and paraphrased.