Guide to Tektronix Transistor Curve Tracers

From time-to-time, we see various models of Tektronix curve tracers for sale on eBay. The model numbers mean little, and the prices vary enormously. What are curve tracers, why would you want one, and what are the various types and models all about?

A curve trace is a device to apply varying operating parameters to any device, and display chosen parameters for the operator. The display may be in any form, TV, plotter, printer, crt. A Tektronix curve tracer contains a display section very similiar to their oscilloscopes, and this is what we will discuss here. This display device may be attached to observe two different parameters, and plot them against each other, one on the vertical access, and one on the horizontal. The display is accurately calibrated, and the inputs and outputs from the device under scrutiny can be accurately measured.

How They Work

The way this is done is to use calibrated STEP GENERATORS and an accurately adjusted SWEEP GENERATOR. The steps are applied to one part of the device under test, say, the base-emitter junction of a transistor. The sweep is applied to a different junction, say, the collector. As the collector voltage is varied, the base current is stepped progressively. By being able to adjust and measure these various test currents and voltages, other parameters can be calculated, such as gain and impedence.

"Family" of curves on a Tektronix curve tracer

The most commonly published and probably most useful families of curves are the Collector Family and Input Characteristic curves in common-emitter and common-base configurations. . A given parameter may be measured from one of several displays of the current and voltage relationships involved in the parameter. The display chosen depends upon the particular parameter and the charactristics of the transistor under test. The display that should be used is, of course, the one which yields the most accurate measurement or vivid display of that parameter. The user may sometimes prefer to measure other parameters that may be combined in a formula to produce a more accurate result.

A word may be in order regarding "small signal hybrid" or, "h" parameters. In small-signal operation, linearity should exist over the operating range of the parameter and the value of the parameter should be independant of the the signal amplitude. Ordinarily, the active region of a transistor without defects is quite uniform, and a cursory examination of the family of characteristics will verify that linearity exists over the operating range of the parameter. The versatility of the 575 provides for making "small signal" measurements as well as "large signal" ones. In order to verify that the parameter value, or operating region of the transistor, is independant of the signal amplitude, additional measurements of the parameter may be made adjacent to, or within the intended operating region and the resulting values compared. For example, the base step values may be reduced and the number of steps increased to examine a smaller segment within the same operating region, etc. (from "Some Transistor Measurements Using the Type 575" by Robert N. Browning).

Testing Vacuum Tubes

I have been talking a lot about transistors, but what about vacuum tubes. Yes, Tektronix made a curve tracer for vacuum tubes, called the 570. It stepped the grid voltages while sweeping the plate, and plotted a family of curves. However, the usefulness of these devices has just about disappeared, other than as a curiosity or a collectors item. By all means, if you can get your hands on a 570, and you have an interest in tubes, they are fun and extremely educational. But for this guide, I will deal for the most part with testing transistors and other semiconductors.

Tektronix 570 Vacuum Tube Curve Tracer

Why Get One?

Now, why would I want to do this? Well, a curve tracer can be used for the following purposes, and probably a bunch of others as well:

- Learning how semiconductor devices work. Since such a device is DYNAMIC, unless you can see what it does as inputs vary, you cannot really understand how it works.

- Teaching someone else transistor theory.

- Testing all semiconductor devices for open and shorted junctions, as well as gain.

- Comparing and matching transistors.

- Testing transistors under stress, such as plotting failure modes.

- Determining if a transistor is NPN or PNP.

- Comparing transistor curves to those in a manual.

- Testing other devices such as inductors for hysterisis.

The visual display of a family of transistor characteristic curves using oscilloscope techniques employing synchronously stepped and swept votage-current sources offers several advantages over DC point-by-point measurement techniques:

- Small irregularities in the characteristics are visible which might escape observation by the point-by-point method.

- The extremes in the variation of a parameter value may be observed without altering the operating conditions.

- The changing magnitudes ot two parameters may be observed simultaneously, as well as the dependance of one upon another.

- The short duration and lower duty cycle of the peak sweeping voltages or currents applied to the transistor produces less thermal rise than does the steady-state DC condition which occurs in point-by-point measurements. Inaccuracies due to thermal gradients are thereby minimized.

- Maximum ratings of transistors may be observed without exceeeding the the safe limit of its power dissipation. Permanent records may be made by photographs.

The Tektronix 575 Curve Tracer

Tektronix 575 Transistor Curve Tracer

The very available and useable Tektronix curve tracer for transistors was the 575. These come up on eBay, and usually sell for very reasonable prices. The down side is they are very heavy, and can be expensive to ship. But they can be worth the cost if they are what you need. The 575 provides a collector sweep of 0-20 volts at a maximum of 20 amps, or 0-200 volts with a maximum of 1 amp. It has 17 current ranges for base steps from 1 ua/step to 200 ma per step. It also has 5 voltage steps from .01 volt/step to .2 volts/step. Collector current can be plotted from .01 ma/div to 1000 ma/div. Switching is provided so two seperate devices can be easily compared.

Other Tektronix Curve Tracers.

                                       576                                                              577                                                                  370

Accessories

A number of accessories for Tektronix curve tracers also show up on eBay. These are mostly TEST ADAPTERS, to allow easier testing of various devices. These are little rectangular blocks with some type of clips or sockets on top, and either 3 or 5 bananna plugs on the bottom. The bananna plugs are inserted into the binding posts on the instrument, and then the device under test is inserted in the clips or sockets. The 575 uses the ones with three plugs, although the ones with 5 will plug into a 575 and work fine. The ones with 5 plugs have a thermal sensor that allows the curve trace to compensate for changes in temperature at the junction of the device under test. This is only implemented in the 576 and later models. These tend to get a little pricey, as all accessories do, because most of them were lost when the instrument was taken out of service. Make sure you get only ones you need, as they are expensive to collect.

Diode Adapter for 575

Another accessory for the 575 is the 175 high-current adapter. This device sits under the 575 curve tracer and plugs into it, replacing many of its internal generators. This provides very high currents for plotting power transistors. It is very heavy and expensive to ship, and not required for most applications. Still, if you run across one at a hamfest or flea market ...

Parts and repair

The 575s main weakness is the crt. The crts developed for this instrument came from the 532 and the 570. They are not readily available, and if you get an instrument that has a broken or defective crt, you will have trouble finding a replacement.There are also a number of high-precision resistors of precise values, and specialized power supply components, such as transformers and rectifiers. For serious use of a 575, buying a parts doner is a good idea. But this is a rugged and tough instrument, which should provide years of service once it is working.

The 576 and Beyond

The models after the 575 began to offer many new features. Automated programmable testing, higher currents, more test fixtures, and IC testing became available. Testing is more precise, with magnification and precision calibration to give ever-more accurate displays.And as the features rose, so did the price. These newer units command 10-40X more than the venerable old 575, and then some. But for reearch and industrial use, the 575 is pretty dated, though still useful.

The 576, next model in the line-up, is a much more modern instrument both built from the then-current semiconductors, and able to test them. Featuring an AC or DC collector supply of from plus/minus 0-1500 volts, it has a base step range from 5 na in the current mode and from 5 mv to 40 V in the voltage mode. The steps may also be produced as short duration pulses. In addition to the normal measurements, leakage current may be measured from 5 na.

In passing, it is worth mentioning the 5CT1N and the 7CT1N, plugins for the 5000 and 7000-series oscilloscope mainframes. The little buggers provide curve tracing without buying a curve tracer. Snap one up when you see one, they are fairly scarce. At power levels only to .5 watts, they don't scream like the larger models do, but can still perform all the functions on small-signal transistors that the big jobs can.