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why resistor

Discussion in 'Electronic Basics' started by novice, Dec 17, 2005.

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  1. novice

    novice Guest

    Why the resistors are connected at the input of any transistor
    configurations(CB,CE,CC).What happens if the resistors are not
    connected at the signal input? how do you decide the value of resistors
    at i/p and o/p? Thanks a lot in advance.
  2. Tim Williams

    Tim Williams Guest


    Do you have an electronics book? This is explained in detail in the most
    basic amplifier textbooks.

  3. Bob Monsen

    Bob Monsen Guest

    The bipolar NPN transistor is basically a couple of diodes wedged
    together on a piece of silicon. These diodes point from base to emitter
    and base to collector. So, if you put more than the diode forward voltage
    (usually about 0.7 volts) across the base to emitter (or base
    to collector), it will pass lots of current. Thus, people often to put
    resistors in series with the base, in order to prevent the diode from
    overheating and failing.

    Deciding the values of the resistors is a matter of deciding how much
    current you want, and using the resistors to limit that current. If you
    use the simple 'beta' model of a transistor, in which a transistor is a
    current amplifier with a gain of beta, it is obvious that you want to
    limit base current to the desired collector current divided by this gain,
    which is often near 100 (sadly, sometimes much more, sometimes much
    less...;) This simple model really breaks down for precision analog
    amplifiers, but you'll find out about that later.

    The gain of the various amplifiers (your CB,CE,CC) is related to the
    values of the attached resistors (and other circuit elements), so picking
    them will determine the output.

    Bob Monsen

    He was not in a hurry, "hurry" being one human concept he had failed
    to grok at all. He was sensitively aware of the key importance of
    correct timing in all acts but with the Martian approach: correct
    timing was accomplished by waiting. He had noticed, of course, that
    his human brothers lacked his own fine discrimination of time and
    often were forced to wait a little faster than a Martian would but he
    did not hold their innocent awkwardness against them; he simply
    learned to wait faster himself to cover their lack.

  4. Do yourself a favor and get a good book: Malvino's Electronic Principles is
    a particularly good one.
  5. Bill Bowden

    Bill Bowden Guest

    Why the resistors are connected at the input of any transistor
    You don't need a resistor for a CC configuration. The load is in the
    emitter side so that the load voltage is always 0.7 volts below the
    base. No resistor needed.

  6. Pooh Bear

    Pooh Bear Guest

    It needs to have a 'bias point' though. That requires a couple of resistors
    connected to the base.

  7. PeteS

    PeteS Guest

    As Tim said - Biasing, for one.

    Try this page and navigate your way



  8. redbelly

    redbelly Guest

    An engineer I used to work with didn't use resistors at all. He used
    conductors instead. Wherever a circuit schematic had a resistor he
    would replace it with a conductor in the actual circuit, using a
    conducance equal to the reciprical of the resistance indicated by the

    He claimed that any circuit built this way would work just fine.


  9. He was a liar.
  10. Bill Bowden

    Bill Bowden Guest

    It needs to have a 'bias point' though. That requires a couple of resistors
    Yes, if the input signal is not at the desired DC level. The bias
    resistors would shift the input level up or down as required, but would
    also attenuate the input signal.

    But if the input signal is already at the desired DC level, no
    resistors are needed.

  11. Pooh Bear

    Pooh Bear Guest

    Usually not a big deal - probably by less than a dB or better.

    True enough but rarely the case IME.

  12. Don Bowey

    Don Bowey Guest

    No, he was absolutely correct. Read it again carefully.


  13. How about a single bleeder resistor across the output of the power
  14. Ralph Mowery

    Ralph Mowery Guest

    If most would use a 50,000 ohm resistor , he would use a 0.00002 mho
  15. Don Bowey

    Don Bowey Guest

    Right on.
  16. Jasen Betts

    Jasen Betts Guest

    I wouldn't go that far. He was only calling resistors conductors.

    Resistance and conductance are merely two different ways of expressing
    the same quantity.
  17. redbelly

    redbelly Guest

    Yep. Besides, I was joking.

  18. Rob B

    Rob B Guest

    so is that the same as a wire wound resistor ?

    how would one go about determining the tolerance and power rating of a
    wire-sister ?

    by "work just fine" did the Engy mean work and behave exactly like the
    circuit built with other types of resistors ?

    are there any tyoes of circuits one would not want to use the wire-sister?

    curious robb
  19. Jasen Betts

    Jasen Betts Guest

    metal is not the only conductive material. carbon, for instance has a low
    conductance that can make it handy in certain aplications.
    also a thin layer (or film) of metal caoul be used ... :)

    If you missed the earlier posts conductance is the reciprocal of resistance.

    All conductors are resistors.
  20. Rob B

    Rob B Guest

    I did't miss earlier posts i am trying to learn more, i have always found
    electronics fascinating, want to learn, and was trying to relate my obvious
    limited knowledge of standard resistor attributes/properties to the concept
    of wires (i.e. conductors) as resistors.

    i read about resistance of wires , that most all wires ( conductors and not
    super-conductor) have some resistance depending on several factors,
    composition , size ( length , cross sectional area) , shape ( for inductance

    i read about resistors being voltage <-> current convertors via definition

    but i could not relate tolerance and power rating to resistor wires ?
    because the example given of the (0.00002 mho conductor ) as 50k ohm
    resistor made me wonder where does one figure the power rating as it seems
    like such a tiny wire that would easily melt at least i have have melted
    bigger wires in what i thought were prett mild simple circuit

    sorry to sound like giving an amateur lesson but i wanted to show that i did
    put some time in to researching before wasting your time with a question :)

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