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Help with transistors

Discussion in 'General Electronics Discussion' started by dbyrd26, Feb 6, 2014.

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  1. dbyrd26


    Sep 16, 2013
    Can someone point me in the right direction to a book or a link or something where I can learn about transistors in general. I have just about every component down except this one, which I feel is probably the most important. I have used them when putting together circuits from a schematic but I cannot figure out how to put one into my own designs. This is terribly frustrating to me. Any help is appreciated.
  2. mursal


    Dec 13, 2013
    To get started bipolar transistors (NPN and PNP) are just glorified switches. Click NPN

    We usually place a control switch in series with a resistor on the base pin to Vcc. Closing the switch will allow a small current to flow into the base (amount of current controlled by the resistor). This will allow a higher current to flow through the collector, emitter circuit.

    The ratio between base current and collector current will be about 10 when used as a switch. So we say the gain or Beta (sometimes referred to as hfe) = 10.

    The PNP is exactly the same only we place it on the supply to the load (on the collector circuit) instead of on the return from the load in the case of the NPN. We also pull the base low to switch it on, instead of pulling it high in the case of the PNP.

    If we want to use transistors as amplifiers, then we must set then up slightly differently (bias then differently) or use an operational amplifier :)

    Hope this helps ........

    PS I'm sure I have overlooked something so best to wait and see what the other members have to say ..........
    Last edited: Feb 6, 2014
  3. Harald Kapp

    Harald Kapp Moderator Moderator

    Nov 17, 2011
    While you can use a transistor as a switch, it is much more. Basically, a transistor is an analog element where an output current is controlled by either an input current (in the case of a bipolar transistor) or by an input voltage (in the case of a FET transistor).
    To call it a "glorified switch" doesn't do justice to the transistor.

    Google "transistor fundamentals" to get lots of useful results like this one or this one.
    Another set of hits you get when Googling "transistor basics", e.g. this one or this one.

    "Slightly differently" is an understatement. Operational amplifiers are complex circuits made of transistors. It helps to know how a transistor works to understand the bahaviour of an OpAmp.
    Last edited: Feb 6, 2014
  4. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    Jan 21, 2010
    I think it may be best to realise that there are certain models which describe how a transistor works.

    The simplest possible model treats it as a switch. It is either on or off. You'll find it's not a perfect switch, but good enough for many purposes.

    However, the transistor does not always act as a switch, there's some middle region where other stuff goes on (essentially it doesn't suddenly turn on or off, but has some sort of transition).

    More complex models will consider this behaviour (and it is this region in which the transistors within an amplifier operate).

    The models will allow you to do calculations, and depending on the way you think, may even give you a feeling for how they operate.

    Another option is to ignore the theoretical approach and learn by example, starting with very simple circuits and working up.

    For the best understanding, you really need to do both (well, I did).
  5. dbyrd26


    Sep 16, 2013
    Thanks for the links and advice. Steve would you happen to have any sample schematics I could use as a learning tool. I've been browsing and experimenting today with some I've found online. It's more my learning style to make a circuit from the schematic and figure out how it works.
  6. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    Jan 21, 2010
    Many years ago you could pick up books containing 101 transistor circuits and the like. Then it moved to 101 op-amp circuits, then 101 i don't know, PIC circuits for the evil genius.

    Unfortunately it's now hard to find printed material that covers the practicality of transistor circuits without going into the blindingly mathematical treatment of them.

    There are web sites with simple transistor circuits, but what you really need is the technical (but not mathematical) description of what they do.

    I could fall back to my generic recommendation of "The art of electronics" which (other than the first chapter) does a great job of covering transistors (in fact far more than just transistors) in a way that's not too mathematical.

    Ooh, I just found something. Good old Colin Mitchel has placed some great material on the web. Here:

    It's still a bit light-on on technical detail, but I trust him as a source of information :)

    If you're interested in who he is, see here:
  7. dbyrd26


    Sep 16, 2013
    Thank you! This is greatly appreciated! I feel that this will really help me learn.
  8. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    Jan 21, 2010
    The Talking Electronics site... Look at the rest of the links on his page as well, there is some great stuff there.

    Here is an explanation of how transistors work from the perspective of the physics. This explanation is pretty much contrary to anything else you'll read, but if you understand it you'll see that most explanations talk about secondary effects.

    Think of the two explanations of how to make a car go faster:

    (1) Push on the accelerator and the car will go faster.

    (2) place more fuel into the cylinders of an engine and it will develop more power. This will cause the car to accelerate.

    The first explanation only talks about causes and effects which are once removed from what is actually going on (The accelerator causes more fuel to get to the engine, and the increase in power causes acceleration which increases the speed) whereas the second is more direct.

    These explanations are akin to the explanation of transistors. (1) is the normal explanation, (2) is the one I have pointed you to.

    Once you understand the second explanation, you can use the first as shorthand, with a more full understanding of what's going on.

    So, when people say "the increased base current causes a corresponding and larger increase in the collector current", you'll understand that what is actually happening is that increased current through the base causes an electric field which extends into the collector, allowing electrons to pass through the junction.

    You'll normally read that the current through a diode causes a change in the voltage across it, but the real reason is exactly the opposite. However, since the cause and the effect cannot be separated, we can treat it as though the effect is the cause and nothing (apart from our understanding) suffers.

    Before you read this, you've got to decide if you're the "press the accelerator to make the car go faster" kind of person, or a "push accelerator -> more fuel -> more power -> acceleration -> higher speed" kind of person.
  9. sndscientist


    Jul 10, 2013
    not much but possibly a little help

    few years ago (more like 15) i came across a list of schematics i used to go through it all the time, there were tutorials and schematics for lots of stuff. now i'm nt sure what links work anymore and what don't but i saved the html and have been hosting it for over a dozen years. hopefully you can find something that still works and it helps you out a bit.

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