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Collector to Base Resistor - Why?

Discussion in 'Electronic Basics' started by Lumpy, Aug 11, 2006.

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  1. Lumpy

    Lumpy Guest

    Help me to understand the purpose of this
    resistance, please.

    Typical single Q NPN amplifier (guitar boost).

    o--/\/\/--o +v
    |
    R? |
    o--\/\/\--o--||-----o OUT
    | /
    | /C
    | |/
    IN o----||--o-----|B
    |\
    \E
    \o--/\/\/--o
    |
    |
    # GROUND

    What is the R? doing in this circuit?

    Taking it further, if I want to install back to
    back clipping diodes at that C to B point, how
    will that R? resistance interact?

    Thanks -


    Lumpy
     
  2. Eeyore

    Eeyore Guest

    It's providing bias current for the transistor. Without it, it wouldn't be an
    amplifier ! It's a failry crappy way of providing bias btw since the operating
    point wil be heavily affected by beta/hfe.

    The resistor will be the last of your problems. That will also stop it being an
    amplifier too. Nor do you need 'back to back' diodes either. Since when will the
    collector ever be negative wrt to the base ?

    I think you need to go learn some circuit basics.

    Graham
     
  3. Lumpy

    Lumpy Guest

    That's why I came here. To gain knowledge from
    those willing to help a novice.

    I'd be happy to exclude you from that pool of
    knowledge if you insist on being such an asshole.


    Lumpy
     
  4. Bob Eld

    Bob Eld Guest


    The resistor, R provides base current as has been mentioned but it also
    provides negative feedback and helps stabilize the operating point of the
    transistor. The operating point is establised by the Beta of the transistor
    and the voltage drop across R. If the collector voltage gets too high, the
    transistor turns on harder reducing the base drive which lowers the
    collector voltage. Likewise, if the collector voltage is too low, the
    collector rises turning the transistor on harder. This is a stabilizing
    action. The local negative feedback also reduces distortion. Think of "R" as
    the feedback resistor in an op-amp circuit where the transistor is a "poor
    man's" op-amp. This circuit was very common years ago in simple transistor
    amplifiers like in radios, etc. Bob
     
  5. Eeyore

    Eeyore Guest

    I think you ought to read a decent book on the subject first. A half-decent
    working knowledge of simple circuitry is a reasonable prerequisite before asking
    question as basic as this one !

    Graham
     
  6. James Beck

    James Beck Guest

    This was posted to SEB.
    The B is for BASIC.
     
  7. "Lumpy"....

    When someone advices you to go out and learn something about basics, it
    isn't meant to piss you off. Your reaction is absolutely out of proportion.
    The only thing you have assured is no help in the future and that is a bad
    way to start your knowledge build-up. The question you asked is part of one
    of the first lessons in Transistor technology.

    Go out, buy a book about basic electronics (Any), read it, try it, read it
    again and design your own. After that.. Maybe, someone is willing to help
    you again. (Ever heard of kill-files? They work like a black hole).

    One pointer for your next question: It is bad practice to build your
    question like : "I have a problem, someone has the answer, Gimme, gimme,
    gimme and if you don't you're an asshole."

    Try it in the following format the next time:
    - Tell us what you want to achieve.
    - What you read already about the subject.
    - Your approach so far. << Very, very important >>
    - Where are you stuck..

    Don't forget that people on Usenet (Some excluded) are here to share
    knowledge to those who are willing to put some effort in the subject
    themselves. And they are all volunteers. Putting a question on the internet
    doesn't give you the right to demand an answer. You'll just have to wait if
    someone is willing to spend some of his or hers *spare* time in your
    problem. If you're here expecting direct answers to your problem, you're in
    the wrong place. You'll only find pointers here to (maybe) a solution.
    There's always an effort required from your part.

    For now your only achievement was insulting someone who was willing to help
    you point you in the right direction and the only problem you have at the
    moment is your attitude.

    Kind regards,

    Peter
     
  8. That sure doesn't mean this is an educational institute, right? People here
    are allowed to expect some effort from the one who's asking questions.
    After all, I went to 8 years of study and almost 25 years of experimenting
    to earn my Electronics Engineering Degree.

    Kind regards,

    Peter
     
  9. Eeyore

    Eeyore Guest

    Sure but not not s.e.k ( sci.electronics.kiddies )

    Graham
     
  10. It provides a bias current to get the transistor conducting a little,
    so that it can respond to the AC signal arriving through the base
    capacitor. Since it delivers a current roughly proportional to the
    collector voltage, the bias point is somewhat stabilized for different
    transistor gains (if a high gain transistor over reacts to the bias
    current by turning on too much, the low output collector voltage
    lowers the bias, reducing the effect of higher current gain).

    Unfortunately for high impedance guitar source, the negative feedback
    does not only apply to DC bias current, but to the signal frequencies
    as well. The net effect is that the feedback tries to hold the input
    node at a more fixed voltage, lowering the input impedance of the
    amplifier. Guitar signals work best into high (relative to the guitar
    pickup) impedance amplifiers.
    The feedback resistor will limit the gain to some maximum value when
    neither diode is conducting. You also need to think about the DC
    voltage across this resistor, and what that will do to the diodes.
    For instance, you may need to add a capacitor in series with the diode
    pair, so that only AC signals are altered by those diodes, not the DC
    bias point.
     

  11. Ignore him. He's an asshole.


    --
    Service to my country? Been there, Done that, and I've got my DD214 to
    prove it.
    Member of DAV #85.

    Michael A. Terrell
    Central Florida
     

  12. You're right. We didn't see any need to create a newsgroup just for
    you and the other ignorant trolls.


    --
    Service to my country? Been there, Done that, and I've got my DD214 to
    prove it.
    Member of DAV #85.

    Michael A. Terrell
    Central Florida
     
  13. James Beck

    James Beck Guest

    I guess maybe I don't understand what USENET is for then.

    Jim
     
  14. Darn, that must have been one heck of a tough school. That is 29 years
    longer than most four-year degrees take to complete ;)

    Richard
     
  15. Lumpy

    Lumpy Guest

    Thanks very much, Bob. The op-amp FB analogy
    helps very much.


    Lumpy
     
  16. Eeyore

    Eeyore Guest

    Pot, kettle, black.

    Graham
     
  17. Lumpy

    Lumpy Guest

    Whatever your personal fantasy is about what
    this NG is, it's a public, unmoderated forum
    with the words electronics and basics in the
    title. So if your nose is so far up your own
    ass the the purpose of YOUR being here is to
    look down on people asking basic electronic
    questions, that's your own problem.

    Congrats on that 8 year electronic degree.
    Isn't there a newsgroup for people of
    your self perceived importance that
    doesn't lower you to basic status?


    Lumpy
     
  18. Lumpy

    Lumpy Guest

    Thanks John.

    Here's a real world example -
    http://digitalcartography.com/eir/onetransSoftAndHardClip.gif

    Disregarding the diodes at the output for now,
    I'm concentrading on the D1 D2 pair. No cap.
    It clips the audio signal, but way too hard.
    So I'm assuming, perhaps incorrectly, that
    what I want to do is put some R in series
    with the diode pair. I don't have a scope,
    but the sound I hear at the output is that
    of a really hard clipped square wave.

    Thanks -


    Lumpy
     
  19. Without a series DC blocking capacitor, one of the diodes provides
    enough average bias current to pull the collector all the way down to
    1 diode drop above the base voltage at no signal. and the other one
    is not used at all, because the collector to base junction would do
    its job.

    Try it with a fairly large blocking capacitor (a few microfarads, with
    the positive end toward the collector, if electrolytic) and also a
    resistor in series with the diodes, to soften the clipping. Something
    around R/4 to R/10 might work better. Then adjust the value of R to
    get an average collector voltage a little above half of the supply
    voltage, so the diodes control the clipping, not the transistor
    saturating.
     
  20. Hi Sr.!
    Your primary school math is perfect;^).
    But how many years it took, after the degree, to admit that on some
    subject you know nothing, and approach someone more experienced to ask?
    "Engineer knows everything about nothing, merchant knows nothing about
    everything."

    Have fun

    Stanislaw
    Slack user from Ulladulla
     
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