This series of articles presents a build-it-yourself tutorial on two-port analyzers (TPAs), historically called curve tracers. A detailed design of a single-board TPA is presented that can be built for about $100 US - or be a “springboard” to your own TPA design.
Curve tracers are measurement instruments used to characterize devices such as transistors and diodes. New ones cost thousands of dollars. Even old Tektronix 575 transistor curve tracers, implemented with electron tubes and found on the surplus market, cost $75 or more. Their huge power dissipation and size, lack of computer interface, and difficulty to maintain make them largely undesirable, even at surplus prices. This article series draws on newer electronics to design a cheaper, simpler laboratory-quality TPA that can be built by a technician or engineer in little time and at low cost. (Note: for more background on TPAs and integration, see the series, 2-Port Analyzers on a Chip? Part 1: What Are TPAs?, by the author during 2Q14 titled “Two-port Analyzers (TPA) on a Chip” on Planet Analog.)
The TPA202 is the first of the TPA200 series and is the only one that is not computerized. It is simplest to build and understand and is a place to start with TPAs. The TPA202 Manual is available. (Request a copy from www.innovatia.com.) My one sloppily-built prototype - a kludge - exists but works and meets specs; see photos below.
The parts used in all the 200-series instruments are readily available from the major parts suppliers, and often are chosen because they are enduring “legacy” parts. For instance, low-cost LF353, LF347, LM324, and LM358 op-amps are sometimes used, though where there is a design advantage in using newer “neo-classical” parts (TLV2372, TLC2272A, TLE2022A), they are used instead.
The main difference between a TPA202 and a classic curve tracer is that it has a numerical display instead of a graphic display of a family of v-i curves. Though it lacks the larger view of a curve tracer, it allows for DVM accuracy of the plotted points on the curves, albeit only one at a time. By turning front-panel knobs and watching the display change, curves can be “traced out”. Furthermore, some parametric measurements are faster and more accurate. By setting the input-port (base-emitter) current to a round value such as 100 μA, the BJT β is read on the output-port (collector-emitter) 10 mA current range as the displayed number, and to
1 % accuracy.