Investigating physics and engineering students' understanding of AC biasing networks

Abstract

This research paper describes a targeted investigation of student understanding of ac biasing networks, which has been conducted as part of a larger, ongoing project focused on the learning and teaching of canonical bipolar junction transistor (BJT) circuits (e.g., the commonemitter amplifier). Biasing networks are critical for signal processing via BJT circuits, yet coverage of such networks in physics and electrical engineering courses and texts is sparse and frequently secondary to coverage of the amplifier circuits themselves. In this cross-disciplinary project, we have been examining how students reason about biasing networks across a variety of courses including a junior-level electronics course for Electrical and Computer Engineering (ECE) majors, a junior-level electronics course for physics majors, and two introductory ECE courses on circuits for majors and non-majors. In this investigation, which was designed and conducted through the lens of the specific difficulties empirical framework, free-response questions were administered to introductory students after all instruction on RC circuits in both time and frequency and to junior-level students after all instruction on BJT amplifier circuits. This paper focuses on a particular task that requires students to predict both the dc and ac behavior of the biasing network. Written responses to the task were subsequently coded and analyzed according to the reasoning articulated, using modified grounded theory. The findings from the investigation indicate that many students fail to apply foundational circuits concepts (e.g., Kirchhoff's current law) when analyzing ac biasing networks. In this paper, we describe the most prevalent conceptual and reasoning difficulties identified. In addition, we document variations in student performance between courses, and examine the extent to which observed trends may be ascribed to specific populations or generalized across disciplines and course levels. Specific examples will be used to illustrate the implications for instruction emerging from this investigation.