Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Understanding how transistors work

  1. Jan 3, 2009 #1
    I am not fully understanding how transistors work. It would be great if someone could explain to me more thoroughly. As I understand it and have read transistors can be used as switches or to amplify voltages. Say then I needed to amplify 5v into 12 volts. How would I do this with a transistor, and how would I calculate what type of transistor I need? (Is that what it means when I keep reading amplify?)
  2. jcsd
  3. Jan 3, 2009 #2
    Re: Transistors

    When you read amplify, think control.

    Transistors replaced vacuum tubes and tubes were sometimes called valves. They kind of do act like a valve.
  4. Jan 3, 2009 #3


    User Avatar
    Science Advisor
    Homework Helper
    Gold Member

    Re: Transistors

    A transistor can't turn 5V into 12V, but it could modulate a 12V source to turn an input 5V (peak-to-peak) signal into an output 12V (peak-to-peak) signal. I like Horowitz and Hill's (The Art of Electronics) model of "transistor man;" he can look at the base-emitter current, and he can adjust the effective internal resistance of the transistor to give a proportionally larger collector-emitter current, but he can't turn the resistance below 0 ohms (or even that far). Does this make sense?
  5. Jan 4, 2009 #4
    Re: Transistors

    I would study the triode valve first. Field Effect Transistors (FETs, MOSFETs) work in a similar way.

    Then PN junctions.. diodes.

    And then bipolar junction transistors.. PNP and NPN.. Which are current amplifiers really. A small base current produces a much larger collector cuurent.
  6. Jan 4, 2009 #5


    User Avatar
    Homework Helper

    Re: Transistors

    Well it depends on what you need to understand. If you want the details of how they work, then you should grab a book on the physics of electronic devices. I used Semiconductor Physics and Devices 3rd Edn by Donald Neamen. Otherwise if you simply want to know how they are used in the construction of micro-circuit amplifiers, then a book like Sedra & Smith's Microelectronic Circuits would do fine for you.
  7. Jan 4, 2009 #6


    User Avatar

    Staff: Mentor

    Re: Transistors


  8. Jan 4, 2009 #7
    Re: Transistors

    The operation of transistors, both FET & bjt, have been thoroughly discussed in this forum. If you search through my posting history, you will find numerous threads I've been involved in regarding the subject. The links below are all of the threads I've posted in regarding transistors. If you then have specific questions, I can help.


  9. Jan 5, 2009 #8
    Re: Transistors

    Sounds like you need a basic text book on transistors. I think this is the book that I study in the 70s, I got the book using the author "Malvino" and this came up. I looked through whatever they show in the book. Looked like it is the same book I thought. THis is a very good book for Bi-Polar transistors. I learn all the introduction of transistors from this book and I did move onto even bi-polar IC design in the 80s. I never even went to school except studying on my own. I see they sell this used as low as $7.00 on Amazon. You might want to take a chance.


    Good luck. After this book, you can move onto more advanced and modern books.
  10. Jan 6, 2009 #9
    Re: Transistors

    I will try to answer one question only, as simply as possible. "how transistors work".
    Think of a transistor as a "voltage controlled semi-conductor, with 3 terminals B, C and E, whose conductivity between C and E varies in response to a (small) voltage applied to B".

    How this conductivity is changed is what is the semi-conductor physics all about. That you can read in your text books - all the PN junction jazz, field effect, electron tunnel or whatever.

    But basically, it boils down to altering the C to E conductivity from B's terminal voltage.

    Say you have a typical NPN transistor, with E grounded, C connected to a DC bias and signal input at B: The increasing B's voltage makes C to E "path" more conducting and and falling B voltage makes the path less conducting. If there is a large voltage source at C, that will make large or small flow from C to E. As the voltage varies at B, the large current from C to E varies as the same waveform as the voltage at B. Now if you make this large current flow thru a resistance external to the transistor, you will get a large voltage waveform across the resistor....this is amplification.

    If you push it to the extreme - using voltage at B if you make the transistor such that
    if there is some minimum voltage at B the C to E path is totally conducting and it the voltage at is less, that will make C to E path totally non conducting...this is switching.

    hope this can get you started to dig more deep...

Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?

Similar Discussions: Understanding how transistors work
  1. How Transistors Works? (Replies: 10)