Physics 361 - Electronics Laboratory I

Dr. Stuart Elston

Nielsen Physics 515

974-7818

selston@utk.edu

Course Description

3 Credit hours; 6 hours laboratory/discussion per week.

Course Objective: To provide instruction and acquaintance with electronic devices and instrumentation techniques important in the modern physics laboratory.

Achieving this course objective in a single semester is an ambitious goal and requires a tight focus. This course is thus organized around a more specific goal: to provide, in five carefully chosen units, or cycles of lab exercises, the skills and understanding needed to successfully complete a sixth, and final, "capstone" exercise. The sixth exercise involves using a personal computer-based data acquisition system to measure the characteristic curves of a variety of semiconductor diode devices, including a photodiode under dark and illuminated conditions. The photodiode is a widely-used element in imaging devices and in laboratory applications of lasers. It also serves as an example of a much broader range of radiation detectors, such as surface-barrier particle detectors and Si-Li (lithium-drifted silicon, pronounced "silly") and Ge-Li (lithium-drifted germanium, aka "jelly") X- and gamma-ray detectors.

This course is heavily hands-on in orientation, because . . .

In a little more detail, the six units include:

Unit 0 - Underpinnings. A hand's on review of prerequisite material: can you apply what you've learned?

Unit 1 - The Source-Load Paradigm. Sources, sinks (loads), and Thevenin Equivalents; Basic Instrumentation.

Unit 2 - Filters. Simple Passive RC Filters; Resonant RLC Filters; Frequency Response; Bode Plots.

Unit 3 - Non-linear Elements. Characteristic Curves; Semiconductor Diode Characteristics; Applications of Diodes.

Unit 4 - Operational Amplifiers. Introduction to Operational Amplifiers; Frequency Response; Input and Output Impedance.

Unit 5 - Data Acquisition (Capstone). Introduction to Data Acquisition Techniques; Photodiodes.

In each of these units, there are three threads of discussion and development:

A Physics Thread - Electronic circuits and components are physical systems subject to the laws of physics.

A Lab Skills Thread - The effective use of basic electronic instruments (oscilloscopes, signal generators, power supplies, amplifiers, data acquisition systems, etc.) is a survival skill important in the modern laboratory.

A Design Thread - The use of electronic circuits and components to tailor basic instruments to specific needs is important because it is often the case that off-the-shelf instruments don’t quite do exactly what the lab scientist has in mind.

An example of the thread idea is illustrated in the following more detailed description of the first unit, or cycle of lab activities.

Unit 1 - We study how basic laws of circuits lead to Thevenin’s theorem (and generalizations thereof) and apply this theorem to a loaded D.C. voltage divider. Thevenin’s theorem provides us with a source-load paradigm, which permits simplifying a complex circuit into two parts (the source and the load) connected at two points (terminals). Thevenin’s theorem is more specific than this; applying the specifics and through measurements we see whether a particular circuit (the loaded voltage divider) behaves the same as one which the theorem asserts is equivalent to it. Thevenin’s theorem is then generalized to an A.C. source connected to a resistive load, and we apply it to determine the output impedance (claimed by the manufacturer to be 50 ohms, resistive) of the signal generators we will use throughout the semester. Finally, we examine the extent to which the output of the digital-to-analog converters (that are part of the data acquisition boards in our lab computers) can be modeled as Thevenin sources.

The physics thread is embedded in the circuit behavior underlying Thevenin’s theorem and in testing the theorem itself. The basic laws of circuits (Kirchhoff’s laws) are essentially consequences of conservation laws (conservation of energy and conservation of electric charge corresponding to Kirchhoff’s voltage or loop law and Kirchhoff’s current or node/branch law).

The lab skills thread involves constructing physical realizations of circuits from circuit diagrams and practicing the use of digital multimeters, oscilloscopes, power supplies, signal generators, and computer-based data acquitision systems. As complicated as a digital to analog converter interfaced to a PC really is, it can still be treated (to an extent that will be discovered in the final lab activity of this cycle or unit) as a simple Thevenin source, much as a battery can. The trick is in learning the limits.

The design thread is present in the idea that a voltage divider can be used to tailor a measurement instrument (the analog to digital converter (ADC) of a PC-based data acquisition system, for example) to a different measurement range than the instrument is capable of by itself. As an example, one can use an ADC designed for -10 Volts to +10 Volts to measure voltages in the range from -10 kilovolts to +10 kilovolts by using an appropriately designed votage divider between the high voltage source and the ADC. Better yet, we will see (in unit 4) how the combination of a voltage divider and an operational amplifier can be used to measure -10 millivolts to +10 millivolts with the same ADC, thanks to the "magic" of negative feedback.