Dick Cappels' project pages http://www.projects.cappels.org
Return to HOME (more projects)

Making Matched Pairs And Sets Of  Transistors
A fixture and a method of matching the VBE of transistors

The hardware is pretty simple: A power supply, a few resistors and some sockets.
 The magic is in the technique, and its all pretty simple and straight forward.
The schematic shows two pairs of sockets, one for NPN and the other for PNP.
Realizing that I will also need mathced PNP transistors for a current project,
I added the PNP sockets after the photographs on this page were taken.

Find updates at www.projects.cappels.org


I needed an AC wattmeter, and while collecting bits an pieces for something based on a commercial wattmeter chip, I came across and elegant design in a National  Semiconductor application note, Application Note 222, "Super Matched Bipolar Transistor Pair Sets New  Standards for Drift and Noise", July 1979, featuring the LM194 dual transsitor. That was a long time ago, and unfortunately,  these parts have been out of production for a long time.  According to their website, Analog Devices still makes a matched pair of transistors, but I was unable to locate a source of them at a reasonable price, and this is important because I buy lots of spares because I tend to destroy a lot of parts, especially in power circuits, and therefore decided to make my own matched sets.   The wattmeter project is described on this web page.

Bob Peas, at National Semiconductor expanded on the design of the original wattmeter, and recognizing the problem of finding matched pairs, wrote a short article,  How To Make Your Own Matched Transistors, which unfortunatley is not available on the internet at the moment, possibly lost when the National Semiconductor website was moved to Texas Instruments. The articles referenced within Mr. Peas' article have also disappeared from the web, but I think I found them and have duplicated the information on this web page. Most of this article is a rehash of the information provided by National Semiconductor with little original of my own added other than some pictures and an account of my experience.    

The transistors I decided to use for this is Fairchild's 2N3904. This also dates back to the 1970's, but its still being made and is very inexpensive. I bought 100 2N3904's for $US2.47. From this 100 parts, I screened approximately 80 of them, and from those 80,  I was able to match the VBE of more than 60 of them to another transistor to within 100 microvolts.  If I were to throw away the unmateched transistors, each matched pair would cost me only 8.2 cents (US) each, not counting the labor.

After matching a batch of 2N3904's, I decided to make some matched set of 2N2906's, so I added a second set of sockets to accomodate them.

This simple fixture can also be used to compare and match the forward drop of diodes at 1 milliamp. Of course if you want to test at a different current, change the resistors but remember to take the diode drop into account.

The Circuit

The only purpose for the LED is to let me know when that the power is on.

The circuit is about as simple as it can get. +8 to +24 volts is applied to the power input connector. A 78M05 regulator reduced and regulates the voltage to + 5 volts. A lower power regulator could be used, but I have many 78M05 regulators and size was of no particular concern. The transistors under test are diode-connected (that is to say that their bases are connected to their collectors). In the case of the NPN transistors, the emitters grounded, the collectors and bases of each transistor are connected to +5 volts through a 4.32k 1% resistor. The Base-Emitter voltages of the 2N3904 transistors that I was sorting was approximately 0.68 volts at 1 milliamps, so the current through the transistors is (5.0 volts - 0.68 volts) / 4.320 k Ohms = 1 milliamp exactly.

Why didn't I use a constant current source? The voltages of the junctions are all similar and the slight mismatch would result in a very small difference in total voltage across the resitor, so the 1% tolerance of the resistors will dominate the current difference. Since the voltage drop across the transistors is a function of the log of the current, the difference in the voltage resulting from the mismatched current is insignificant when matching to within 100 microvolts.

The 7805 that I used provides 4.98 volts, making the current pretty accurate. If you have a strong desire for more accuracy, you can select a regulator that is closer to 5.00 volts or use an adjustable regulator such as an TL431 or LM317. Accuracy of the test current is not very important. What is important is that the currents though and the temperatures of the two transistors under test be as close to being the same as possible, so matching the 1% resistors might be a better investment of time.

Rather than measuring the voltage across each transistor, which would require one more digit on my voltmeter to obtain 100 microvolt resolution, one reference transistors is arbitrarily selected and put into the "Reference" socket and then the transistor being sorted is put into the TUT (Transistor Under Test) socket. The voltmeter is connected between the two transistors with the negative lead connected to the base and collector of the reference transistor and the positive lead connected to the base and collector of the TUT. Thus, if the meter reads 1.2 millivolts, it means that the base-emitter voltage of the TUT is 1.2 millivolts higher than that of the transistor under test.

The ground test point is there to allow measurement of the actual base-emitter voltage. Sometimes that is interesting or helpful to know that, such as when diode-connected transistors are used as temperature probes, direct measurement of that voltage is necessary.

Building The Fixture

I did not bother with any fancy packaging -not even a box. After all, the two transistors need to be in the open for access to the sockets and so the fan can blow on them.

The sockets I used are some that I found in a surplus store with female contacts 2.54 mm (100 mill for Americans) centers. They have leaf contacts and though I don't know what for what use they were originally intended, I use them with Berg Connectors. You can also use transistor sockets. I cut down the connector I used so there were two rows of three pins. The spacing for Berg Connectors is idea for mounting two TO-92 transistors with their flat faces facing one-another. I think that when I use matched pairs in a circuit, they will be mounted this way; with the flat faces toward each other, most likely coupled with high therma conductivity epoxy, so arranging the sockets in this orientation will make it easier to test pairs of transistors that are glued together.

Using The Fixture

The setup as on the seat of a chair, which makes a clear and convenient
workspace in the midst of workbenches overflowing with junk.

To sort the transistors, set up on table reasonably free of clutter to allow for free airflow over the surface.

• Place a fan so that it constantly blows room temperature air across the Reference transistor and the Transistor Under Test. This helps both transistors settle to the same temperature quickly.

• Attach the test leads to the + and - terminals on the fixture so that you will not have to hold the leads in place.

• Keep the transistors that are about to be tested in the same room temperature stream of air from the fan so that they are kept at close to the same temperature as the Reference Transistor, making settling time shorter.

• Use a pair to tweezers, and your warm fingers as little as possible, to pick up the Transistor Under Test and place it in the test position. Keep the tweezers in the fan's output stream to help keep it at the same temperature as the Reference Transistor.

• Wait as long as you can stand to, to make sure that the voltage across the Transistor Under Test has stabilized. This will be indicated by the voltage on the voltmeter settling down.

I the case of my first use of this fixture, the air temperature in the room was 24.2° C, Whenever I picked up a transistor with my fingers, if even for a one or two seconds, the temperature of the transistor would increase by two or three degrees. It would take a painfully long time for the transistor to settle down after touching it. Keeping the tweezers laying in front of the fan instead of in my hand and keeping the transistors spread out in front of the fan, rather than in a pile as shown in the photograph, often reduce the settling time to a few seconds.

If you see this webpage on a site other than cappels.org, please email me at the address below. Thankyou.

This picture was taken after I had already bagged and labeled ten transistors
matched to within 100 microvolts ofthe Reference Transistor - 0.1 mv bins.

I decided to bin the transistors at 100 microvolt intervals, which was the resolution limit of my digital voltmeter, even though my initial application can tolerate up to approximately 400 microvolts of difference because there were enough matches to have paris more closely matched, and sometime in the future, this closer matching might be useful. After binning them, I further sorted two of the bins into "upper part" and "lower part", by picking one transistor from each bin and comparing the other transistors from that bin. That allowed me to be assured that those that went through secondary screening are within 100 microvolts of one-another.

It was both surprising and pleasing to see that in this batch of Fairchild 2N3904 transistors, the all matched one-another within two or three millivolts at the most, and by far the bulk of them were within 1 millivolts of the arbitrarily chosen Reference Transistor.

Using Matched Transistors In Pairs

Back in the late 1970's I designed video camera deflection circuits that used a pair of 2N3904 transistors as a wide bandwidth differential amplifier. At that time, I thought the best method of thermally coupling the transistors was to apply Wakefield thermal coupling compound to the faces of the transistors and then fasten them with a Panduit Tie Wrap.  Today,  having the transistors face each other is still a good idea and a plastic wire turns out to be a very good idea.

To couple two transistors together, I put two transistors from one ±100 uV bin into the base-emitter voltage test fixture and let the temperatures of the transistors stabilise to verify that the transistors are indeed matched. This precaution is taken because sometimes mistakes are made or an error in the original measurement could have been made, and it seemed prudent to verify the quality of the match before investing a lot more time and resources in using the pair.

Once it is apparent that the transistors are matched, keeping the transistors in the matching fixture, add a drop a fingernail varnish to the face of one transistors, and then fasten the two transistors together with a plastic wire tie. Put more fingernail varnish over the top of the transistors and to where the wire tie touches the transistors to keep the whole thing from slipping apart. I used green fingernail varnish to remind me that these are NPN transistors. I used red for the PNP transistors. Allow the varnish to dry and the temperatures to stabilise once again, and check again to verify that the base-emitter voltage drops of the two transistors are still sufficiently matched.

A useful side effect of using a plastic wire tie is that it offers a little bit of thermal insulation so as to reduce thermal gradients generated by air currents. In use, copper foil was wrapped around the pair to give the device more themal mass so that the temperature would not change as quickly, the pairs are placed inside a small circuit enclosure or embedded in a block of plastic foam to reduce the effects of air currents on the offset. If the leads are kept short, the leads won't be affected by air currents and the circuit board can act as additional thermal coupling between the two junctions.

Note: Thanks to Brian in Pittsburg, PA for letting me know about an error in the text.

HOME (More Projects)
Contents ©2011 Richard Cappels All Rights Reserved. Find updates at www.projects.cappels.org

First posted in January, 2011 Updated December 28, 2011.

You can send  email to me at projects(at)cappels.org. Replace "(at)" with "@" before mailing.

Use of information presented on this page is for personal, nonprofit educational and noncommercial use only. This material (including object files) is copyrighted by Richard Cappels and may not be republished or used directly for commercial purposes. For commercial license, click here.

  Liability Disclaimer and intellectual property notice
(Summary: No warranties, use these pages at your own risk. You may use the information provided here for personal and educational purposes but you may not republish or use this information for any commercial purpose without explicit permission.) I neither express nor imply any warranty for the quality, fitness for any particular purpose or  user, or freedom from patents or other restrictions on the rights of use of any software, firmware, hardware, design, service,information, or advice provided, mentioned,or made reference to in these pages. By utilizing or relying on software, firmware, hardware, design, service,information, or advice provided, mentioned, or made reference to in these pages, the user takes responsibility to assume all risk and associated with said activity and hold Richard Cappels harmless in the event of any loss or expense associated with said activity. The contents of this web site, unless otherwise noted, is copyrighted by Richard Cappels. Use of information presented on this site for personal, nonprofit educational and noncommercial use is encouraged, but unless explicitly stated with respect to particular material, the material itself may not be republished or used directly for commercial purposes. For the purposes of this notice, copying binary data resulting from program files, including assembly source code and object (hex) files into semiconductor memories for personal, nonprofit educational or other noncommercial use is not considered republishing. Entities desiring to use any material published in this pages for commercial purposes should contact the respective copyright holder(s).