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Info for 2N3904 and 2N3906

Discussion in 'General Electronics Discussion' started by spike1947, Mar 4, 2016.

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  1. spike1947


    Feb 4, 2016

    As I am a beginner into Electronics I am always asking questions that may seem obvious to other members on here so I am apologising in advance .

    I have a attached a schematic , can someone explain to me the difference to a 2N3904 and a 2N3906 apart from the different electron flow( ie could they be used for the same project but pinned opposite ) , and also in the pic I see it has a pic of the transistor, were the legs are numbered from the flat face E B C ,is that the same for both of these , in the diagrams below that they are shown opposite's ! .

    I did say I am new to electronics .

    ps: attached a pic of a 2N3906 !


    Attached Files:

    Last edited: Mar 4, 2016
  2. dorke


    Jun 20, 2015
    There is a long way for you to go learning.

    1. The pin-outs in this case are the same.
    you should always check the datasheet of the device(as you did) for that.

    2. "pinned opposite" is not at all defined !
    NPN and PNP can ,and are used in the same project.
    The way they are connected depends on the application needed.

    3. You should study the behavior and uses of BJTs ,there are many tutorials on the Web.

    4. Viewing the differences between NPN and PNP as being
    " different electron flow" is a good way to start.
    I would put it like that :
    " The difference being changing the polarity of the voltages and direction of the currents".

    5. Most beginner have more difficulties understanding
    PNPs than NPNs...
    Last edited: Mar 16, 2016
  3. shrtrnd


    Jan 15, 2010
    The term I've always heard for devices like this is that they are 'complementary'
    The NPN version of this transistor is the complementary PNP device.
    There are a lot of similar complementary devices used in electronics.
    As dorke points-out, you would use the NPN transistor in one application, or the complementary PNP device in another circuit application.
    The choice of one or the other is dependent on your circuit.
  4. dorke


    Jun 20, 2015
    Just to make things clearer:

    The term "complementary pair" is indeed refereed to
    NPN and the accompanying PNP BJTs.
    But only when they have the very same
    characteristic parameters,
    i.e. they are closely(not perfectly) matched in that aspect.

    The 2N3904 and 2N3906 are a "complementary pair",that is also stated in the manufacture's datasheet.
  5. BobK


    Jan 5, 2010
    And complementary pairs are often used, surprise, as pairs! In a push-pull audio amplifier, the PNP conducts on the positive half of the signal and the NPN on the negative half.

    They are also used to form H-bridges and half bridges, which allow you for example, to reverse the current to a motor. In this case one is on when the motor is running in the one direction and the other is on when it is moving int the opposite direction.

  6. hevans1944

    hevans1944 Hop - AC8NS

    Jun 21, 2012
    The 2N3904 (NPN) and 2N3906 (PNP) are my favorite go-to small-signal transistors. They have good gain and frequency characteristics for most analog or digital projects, but most significant to me: they are dirt cheap. I keep a hundred or so of each type on hand to play with. If I accidentally "let the smoke out" and destroy one or two it's not a big deal.

    Go learn some electronics from text books or on-line tutorials and then actually build some circuits using these transistors. A solderless breadboard, a few dozen of each of these transistors, a handful of assorted 1/4 watt resistors, capacitors, LEDs, some diodes of various types, a 9 V battery, and an inexpensive multi-meter will teach you more in a few days than weeks of reading about it.

    Learn Ohm's Law and Kirchhoff's Laws and how to apply them to analyze circuits, then verify what you learn by doing experiments, making measurements, and taking careful notes of what does and does not work. Years later, look back on your notes and marvel at how far you have come! Well, Spike, if you are as clever as your avatar implies, maybe only a few months later will suffice... most important: have fun!
  7. camere


    Dec 21, 2017
    As people said that the basic difference between 2N3904 and 2N3906 is that former is NPN transistor and later is PNP. The quick difference between NPN and PNP is that,
    NPN is off by default and when we apply small voltage (generally 0.7v) at the base of transistor, it allow larger current to flow from collector to emitter.
    And in PNP, it is ON by default means collector-to-emitter flow is ON by default and when we apply small voltage at its base, it stops the current flow from Collector to emitter.
    The best way to learn them is from their Datasheets if you want a brief overview and a simple practice use then check this 2n3906 and 2n3904 article.
  8. duke37


    Jan 9, 2011
    Voltages relative to the emitter.
    The description of the action of the NPN is correct. A positive voltage on the collector will result in a current when a small positive voltage is applied to the base. With no base voltage, the transistor is OFF.
    The PNP uses the reverse polarity. It needs a negative voltage on the collector and a slight negative voltage on the base to turn it on. Without any base voltage, the transistor is OFF.
  9. Audioguru


    Sep 24, 2016
    Pinouts were mentioned:
    1) Most small American transistors have a 2Nxxxx part number and have a pinout that is EBC looking at the flat face with the pins downward.
    2) Most small European transistors have a BCxxx part number and have a pinout that is CBE (see, the reverse of the American ones) looking at the flat face with the pins downward, but BFxxx high frequency radio transistors are different.
    3) Most small Oriental transistors have a 2SCxxxx or 2SAxxxx part number and have a different pinout to the American and European transistors, I think their center pin is the collector.
  10. AnalogKid


    Jun 10, 2015
    No, no, no.

    While both of your statements might be true in a very small number of very limited cases, neither is anywhere close to true as a general statement about types of transistors. There is much more to transistor operation than "off" and "on", and it is very bad advice to give to a beginner.

    Yes, when a transistor is sitting in a box on a shelf, it is "off"; but I don't think we need to insult the intelligence of the TS. There is no "default" transistor condition in a circuit. ANY transistor's operation is governed by the circuit elements around it.

  11. Ratch


    Mar 10, 2013
    It would be a good idea to first learn how a junction diode (JD) works before trying to understand how a bipolar junction transistor (BJT) works. Although you cannot make a transistor with off the shelf JDs, many of the same JD principles are in effect for BJT operation.

    You have two branches of learning; how a BJT works, and using a BJT in a circuit. It seems like many designers and experimenters are able to make circuits without knowing how a BJT really works. Instead, they know what a BJT will do and use that knowledge to make something. It is best to know both areas of knowledge.

    hevans1944 likes this.
  12. camere


    Dec 21, 2017
    yes @AnalogKid, I should not have generalized this Switch application of transistor.
    I just wanted to explain in very simple manner so that one should atleast start using it then later he can know more about it, as I started :)
    But yes I should not have generalized it.
  13. hevans1944

    hevans1944 Hop - AC8NS

    Jun 21, 2012
    This is the only true statement in this comment. Everything after PNP ... to the final period at the end, including most of the "information" in the two links is either wrong, misleading, or irrelevant. In this forum, misinformed opinions can do great harm to the understanding of basic electronics theory by beginners. The advice given by @Ratch in post #11 is good, although perhaps moot by now because @spike1947 has not been seen on EP since two days after their original post on March 4, 2016. Another drive-by poster who wrote, read, and decided we weren't worth their effort.
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