Connect with us

transistor biasing???

Discussion in 'Electronic Basics' started by andrew_h, Feb 12, 2006.

Scroll to continue with content
  1. andrew_h

    andrew_h Guest

    Hi,

    Just wanted to know if someone out there could explain, fairly simply,
    exactly what transistor biasing is, and why it is needed. I.e. having a
    resistor in series with the base, and also even a resistor in series
    with the collector etc... what is the purpose of this?

    I understand when a resistor would be used before the base of the
    transistor, to limit the current to a known quantity (v=ir), but
    biasing didnt make full sense.


    Thanks,
    Andrew
     
  2. The exact function of any particular resistor an a particular circuit
    is hard to discuss in general, because there are lots of
    possibilities. But a transistor is a device that passes collector
    current and the amount of that current is dependent on both the
    emitter to base situation, and also the emitter to collector
    situation. Bias circuits are intended to set up both these situations
    so that usable gain is available from the transistor, at the same time
    that signal swing is available.

    Another way to say that is that bias circuits are intended to get the
    transistor conducting a middle amount of current, so that when an
    input signal is added to the bias, the the output signal is a larger
    (in some sense) copy of the input signal riding on some bias produced
    average voltage, with acceptable distortion (neither hitting
    completely on or completely off at the peaks of the signal).
     
  3. Jamie

    Jamie Guest

    i'll try to explain in layman's terms.
    think of Biased in english terms.

    in transistors, you determine how much
    current you need flowing between the base
    and emitter to calculate the amount of
    current that will flow between the Collector
    and emitter.
    this is called Beta (Hfe) etc. others call
    it different things. the Hfe is like a gear
    box on a mechanical device. with lets say 1 foot
    pounds of torque on the input with a gear box of
    1:10 for example, will give you 10 pounds of torque
    on the output.
    now transistor operate in similar fashion.
    if you have lets say a beta of 100 and you apply
    lets say 1 ma of current through the base and emitter
    you will have aprox 100 times that flowing from the Collector
    to the emitter. that of course saying there is no Resistor
    from the Vcc..
    so the results would be around 100 ma of current in the collector.
    this is just rough details and are designed to help you better
    understand. other things do swing the behavior of the final results
    of course.

    for the internal workings of this behavior, when current is present
    in the Be (base emitter), electrons are emitted, i guess that is why
    they call it a emitter! :) these electrons kind of help fill the gap
    causing the electrons from the emitter to push along to the other side.
    its like a bridge that helps electrons get to the other side when the
    electrons from the emitter is clouding(some would say boiling off) to
    act as a path..
    etc..
    also remember that These junctions act like diodes, that is they have
    a minimum voltage operating point and is usually starting around 0.6 on
    the average. just one of those extra calculations you need to throw in
    there when doing exact calculations.
    hope that back yard explanation helped you some!
     
  4. Pooh Bear

    Pooh Bear Guest

    Biasing is relevant only to linear operation of transitors. Switching
    circuits don't need bias.

    For an ac amplifer for example, the transistor collecter-emitter curent is
    modulated by the signal both positively and negatively. If the transistor
    started with zero current ( no bias ) only half the signal woud get
    amplified since the current can only increase from zero - not decrease.

    Hence amplifers use bias to set an inital ( no signal ) operating point.
    The best way of doinfgt his varies with application, circuit topology and
    performance issues.

    I can't think of an instance where a collector resistor is required solely
    by bias considerations btw. The collector R usually forms a voltage ouput
    node.

    Graham
     
  5. Brian

    Brian Guest

    Here is a fairly simple approach to it, but it should help. Take a look at
    http://www.fncwired.com/TransistorExample/

    Brian
     
Ask a Question
Want to reply to this thread or ask your own question?
You'll need to choose a username for the site, which only take a couple of moments (here). After that, you can post your question and our members will help you out.
Electronics Point Logo
Continue to site
Quote of the day

-