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.
A two-port analyzer consists of two drive-sense or stimulus-measure units (SMUs). Each SMU drives a port.
A port is a pair of interface nodes or terminals of a network (circuit) or device, shown above as a box or block, with a voltage across them and current into or out of them. The + terminal current is defined by port convention as positive going into the block. The terminal currents flowing into or out of the block are equal in magnitude. (Otherwise, a third terminal would need to be included and this defeats the definition of a port.) The unconventional source symbols are generalizations of either a Thevenin or Norton equivalent source, where the x variables are either voltages or currents, written in functional notation, f (x) .
The most general SMU can be configured as either a voltage or current source, and both output port voltage and current are measured. A general SMU block diagram is shown below.
The device to be tested, or device under test (DUT) can have one or two ports. An active device such as a transistor has input and output ports where the input port is across the base-emitter or gate-source terminals and the output port is across the collector-emitter or drain-source terminals. The two ports have a negative terminal in common, the common (COM) terminal. COM can either be grounded to the system ground or be floating. In the TPA202 it is grounded.
SMUs can be less general than the one shown above. Useful functionally-reduced SMUs are V source, I sense and I source, V sense. Abbreviations for the current and voltage drive (DR) and sense (SN) circuits or their output variables for the two ports are given in the following table.
The SMU shown above has a current sense circuit (ISN) that is a high-side circuit (ISN-H) because the sense circuit is in series with the positive or high-side terminal. The TPA202 input-port (INP) SMU has high-side current sensing and the output port (OUT) has low-side current sensing (ISN-L), where the OISN circuit is between the negative port terminal and the amplifier or source ground. The TPA202 output source (ODR) is a voltage source in series with the ISN sense resistor which also functions as a current-limiting resistor. The input source can be switched to be either a voltage or current source – voltage for FET gate-source VGS and current for BJT base IB . It too has the ISN-H sense resistors in series with the drive amplifier. Each drive amplifier consequently has a Thevenín equivalent circuit output consisting of a voltage source in series with a sense resistor, but feedback results in close to an ideal voltage or current source at the port terminals.
The TPA202 has a 3.3 digit DVM with 7-segment LED display and three units LEDs (nA, μA, mA) to its right. Voltage is measured in units of volts only. For the input port, it is resolvable down to 10 mV and for the output port, to 0.1 V. Port voltage or current can be displayed for either port. Source or drive voltages are varied over a decade range (or 1.5 decades for output voltage) using input and output port front-panel 10-turn potentiometers. They control the voltage of either port or input-port current. Transistor parameters are measured by sequencing through port measurements and calculating the parameter from the displayed numerical values. Some static or quasistatic parameters that are capable of being measured using a TPA202 are β0 , α0 , r0 , VA , rm , and re .
Specified full-scale (fs) values are given in the following table. Measurement resolution decreases linearly for values less than fs. Calibrated accuracy depends on op-amp input characteristics (offset voltage and its drift with temperature). Inaccuracy over a temperature range of 20 o C, +/-15 o C depends on your selection of op-amps, but for the ones on the circuit diagrams (given in future parts of this series) and when calibrated, can range from about 0.2 % to over 1 %. For the design as given, op-amps are selected that result in a measurement inaccuracy of around 0.5 to 2 %. By selecting more precise op-amps or resistors, inaccuracy can be reduced. Other causes for inaccuracy, such as leakage currents, are insignificant for the specified minimum current range.
How did this project originate? Over the years I have developed various microcomputer (μ C) and instrument subsystems and some prototypes of them have been built. Since I am not a “thower-outer”, they languish in my laboratory, awaiting a time of functional fulfillment. Then it finally came to me about three years ago that I can apply all of this shelved technology by using it to build limited-edition (LE) measurement instruments. My instrument quest zealously began. So many seemingly good ideas for mainstream-performance, “workhorse” instruments came to mind that I have summarized them in tabulated form at www.innovatia.com under Instruments. I am working my way through the list, project by project, and in the spirit of Linux, invite other “hackers” to join the quest of building up a base of open-source, medium-performance instrument designs that do not involve some of the very subtle complications of high-performance instruments and that can be built and made to work by most electronicists without undue grief. The TPA202 design is open-source and these articles are chatty explanations of it.
For total ownership status – that is, buying into the design as well as the parts – building and making an instrument perform puts you in a position of knowing it in depth. You buy not only its parts but also buy into its concepts and can thus calibrate, repair, modify, and even possibly evolve it indefinitely. It becomes yours, both physically and conceptually. These open-source instruments can even be made and sold without anything beyond their Creative Commons license. A cottage instrument industry can possibly spring up just as Linux suppliers have.
Whenever at least three of you indicate that you want a TPA202 (my email address is at www.innovatia.com/Inquiry.htm), I will finish the board layout and provide blank boards for close to my cost – probably about $40 US each.
The only unusual part of the TPA202 is the 2P7T rotary switch. I have a stock of them that should be adequate in supplying anyone who buys a circuit-board. Ten-turn pots are somewhat expensive ($12 US each) but worth the higher resolution for precise settings. In the next part, we’ll go “under the hood” and look at TPA202 design and operation.