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A Simple and Inexpensive Way to Match Transistors

A Simple and Inexpensive Way to Match Transistors



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Tags: transistors

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31 Comments

  1. Date: February 17, 2018 at 06:07

    I have seen the back to back diode before. To me it seems like there is no way for current to go in either direct, like if you put to check valves back to back would prevent water from flowing in either directions. This has always baffled me. Could someone explain how 2 diodes that are back to back and facing opposite direction let anything through. Was thinking if the diodes were separated you qould get the .6 drop. But back to back either one would block not letting anything through. Iam sorry if iam missing something, iam still learning. So could you please explain how any current gets through when there back to back? Great Video and thanks in advance.

  2. Date: February 17, 2018 at 10:01
    Author: tompas11

    Very nice little test circuit you came up with. I will for sure build myself one. Many thanks for the video and the good idea.

  3. Date: February 17, 2018 at 10:02
    Author: 1959Berre

    Accuracy would improve if the transistors were thermally coupled with some thermal compound and a clamp.

  4. Date: February 17, 2018 at 10:06
    Author: mamoynas

    Simple and inspiring. Just sent you an email.

  5. Date: February 17, 2018 at 10:38
    Author: fabio eleuterio

    Very simple and best of all accurate and inexpensive. thanks for sharing!

  6. Date: February 17, 2018 at 11:09
    Author: M.

    This circuit willnot give any better match than this Chinese transistor tester (actually tester was designed by mate from Germany). It is actually better and simpler to set base current (by power supplyand trim by plus) and set collector volatage to operating point in circuit (be second power supply ) and mesaure collector current. That way you will match transistors for operating point and one can see if this match will be preserved by varying base current.

  7. Date: February 17, 2018 at 11:20

    Problem is, you measure at a very low current. This will probably work with small transistors, but larger power transistors must be tested at much higher C-E currents. Also the series back to back diodes will not drop any voltage, unless you exceed the reverse breakdown voltage of one of the diodes plus the forward voltage of the second one. That is around 700V rms for the 1N4007.

  8. Date: February 17, 2018 at 11:43
    Author: Stephen Morton

    nice demo.
    you are balancing two transistors not matching two transistors.
    the DC current gain with the tester at the beginning of your vid is all you need to do.
    match the DC Hfe and then drop them into an amp.
    what you have built is a bias demonstration.
    matching tubes and transistors is just finding two devices with identical gain and hopefully dynamic response. NPN to NPN or PNP to NPN for bipolar topologies.
    there are sometimes discrepancies across rated devices!

  9. Date: February 17, 2018 at 14:04
    Author: Eduardo Artigas

    The diodes in series will never conduct as one will be blocked always. Am I wrong?

  10. Date: February 17, 2018 at 14:08
    Author: Cliff Matthews

    Nice video! Along with the 3.3k current limit, those back to back diodes should be 1-watt ~6.3v zener type (this establishes a Vref.) For TO-126 / 220 devices, the resistors can go 3 to 5 times smaller for better matching at the higher currents.

  11. Date: February 17, 2018 at 15:00
    Author: Nathan Grant

    Tony,
    I like the disclaimer/warning about the circuit. LMAO! You should have added… "No Transistor was harmed in the making of this circuit."

  12. Date: February 17, 2018 at 17:45
    Author: Eric Schultz

    Unless one of your two diodes are a zener would they do anything?? One would have to have a breakover voltage lower than 12 volts.

  13. Date: February 17, 2018 at 18:30
    Author: esmaeil nam

    A question of curiosity: Was it any chance to compare the matched transistor pairs found with this nice little bridge and your Atlas or curve trace please? As the circuit accuracy is limited to about 0.5 meV voltage difference, it would be interesting to know the corresponding gain difference for a matched pair. Anyway thank very much for the nice videos.

  14. Date: February 17, 2018 at 18:39
    Author: t lech

    Can the same thing be made up for MOSFETs and can you build one of those and show us I’m starting construction on this little unit you just spilled but I don’t have the knowledge to create from nothing

  15. Date: February 17, 2018 at 19:18
    Author: gd2329j

    Hi this may help .
    You have 0 V , 12 V & 24 V input ( -12 0 +12 V on the map ) .
    The base of the 2 transistors are on the 12 volt line
    ( or your 0 volt line ).
    So the 2 diodes do nothing & could be removed !

  16. Date: February 18, 2018 at 13:19

    Good video! I'll use this circuit. But in 14:10, 'how to calibrate' is wrong. If we want the same current thru the transistors, the voltage in the potentiometer will be 0V only in the middle, but the reason to the potentiometer be there is to compensate the difference between the resistors, therefore the potentiometer shouldn't be in the middle. I'm waiting for the next videos.

  17. Date: February 18, 2018 at 19:17
    Author: MrBrymstond

    How much does that KEITHLEY 197 cost? Can you just use a multimeter?

  18. Date: February 19, 2018 at 23:59
    Author: Airgead

    Sorry to say, but you got it all wrong. You are matching now Vbe instead of Hfe.

  19. Date: February 20, 2018 at 21:16

    The way the diodes are connected at the + rail, they do nothing. Maybe you wanted to make then ANTIPARALLEL. That way they would always produce about 0.6 – 0.7 V, positive or negative. The tester would work fine to determine the V_BE balance even at this reduced collector voltage.
    But another issue is the -2 mV/degC temperature dependency of silicon transistors. You really should do something to match the temperatures and especially avoid adding your finger heat to the result. Maybe using tweezers or some other tool. But best to mount them on a single heat sink.
    The operation at 120 microampere current is typical for high gain low noise transistors in front end differential pair.. Other transistors have their optimum (highest gain) points at as much as 100 mA and even higher for actual power transistors. But this device does not try to match the gains. And that is fine, the V_BE match is the essential parameter for use in differential amplifiers.

  20. Date: February 21, 2018 at 10:04
    Author: the navigator

    About time, I've been wanting this tutorial for ages. Your about a week to late as I got Ian Fritzs transistor matcher which you can get from Oshpark. I used 0.1 resistors because its a pcb and I couldnt be bothered to measure and match resistors and I want to match them as close as possible. There is another circuit from Music From Outer Space web site from Ray Wilson (R.I.P) but it is a lot more complicated than this circuit but allows you to test several at once.
    Great video wish it would of been a little earlier but hey ho.
    At least somebody else can have an acurate circuit now.

  21. Date: February 22, 2018 at 14:20
    Author: Zaph Hood

    Awesome stuff! keep it coming. This is a wondermethod for us poor peons.

  22. Date: February 23, 2018 at 10:11
    Author: Vyl Bird

    I built my own solution – I call it the 'sillyvolt' meter. It combines a precision ultra-low-voltage power supply (1mV-4.5V) with similarally precise dual-channel ammeters and voltmeters. It's also a curve tracer, if you plug it into a computer to record the readings and draw the graph. It can be used to very accurately measure component characteristics. I actually built it to test suspicious parts off eBay and detect counterfeits. Happy to share the design if anyone wants it.

  23. Date: February 24, 2018 at 19:54
    Author: Petr Brazda

    If the first diode block than there is no way for the second diode conduct to ground.

  24. Date: February 25, 2018 at 06:10
    Author: Carl Franz

    Would using some sort of heater help? Hair dryer or something?

  25. Date: March 1, 2018 at 01:20
    Author: Jan Verschueren

    If one can read German, one can build a better transistor tester than the Peak DCA55 for around 35,- Euro (given one already has a means of programming AVR microcontrollers, that is). It's on mikrocontroller.net, iirc. I've compared it to the curve tracer I built way back in the day one had to buy an electronics magazine in order to get circuits and, for small signal transistors, it yields comparable, good results (i.e. matches) and is good at identifying unmarked or custom marked components (car manufacturers expect people to buy $400,- ECUs instead of replacing one faulty SMD equivalent of a 2N3904).

    Anyhow, I'm intrigued to watch on to see what method you're about to propose.

  26. Date: March 31, 2018 at 14:47
    Author: vargasmongo3435

    a good PCB making yes definitely, don't worried we all know that we make first a "working circuit first" then later to be transfer to layout design is all good sir 🙂

  27. Date: April 3, 2018 at 12:45
    Author: vargasmongo3435

    question sir, the supply voltages are +12 DC GND -12V DC similar to a supply of a simple class AB audio amplifier kind like positive rail 0V GND reference and a negative rail right ? like this +12/0/-12 DCV

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