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Common emitter amplifier question (rc || rl)

Discussion in 'Electronic Design' started by cheese9988, Jan 29, 2005.

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  1. cheese9988

    cheese9988 Guest

    Hi, I have been trying to figure this out and its bugging me. On a
    common emmitter amplifier, you have rl (load resistance) and rc
    (collector resistor). How are these two in parallel with an ac signal?
    It looks more to me like rl is in parellel with the transistor and both
    being in series with rc. Can anyone explain this?
  2. JeffM

    JeffM Guest

    How are these two in parallel with an ac signal?
    Think *Thevenin's impedence of an ideal voltage supply*.
  3. Andrew Holme

    Andrew Holme Guest

    In the AC equivalent circuit (where there is no DC - only AC signals) :
    1. The +ve supply (Vcc) is ground.
    2. The DC-blocking (aka coupling) capacitor between the collector and RL is
    a short circuit.
    3. The transistor is an AC current source

    So, the AC equivalent circuit is a current source, RL and RC - in parallel.

    A current source with parallel resistance can be transformed to a voltage
    source with series resistance; therefore, the AC equivalent circuit can also
    be drawn as a voltage source in series with RL and RC.
  4. Fred Bloggs

    Fred Bloggs Guest

    In order to compute the output voltage, you will want to look at the
    equivalent circuit seen by the transistor at the collector- this is
    clearly rl||rc. Then when it comes time to compute the voltage developed
    across rl, you look back in and see rc || transistor. Does that make sense?
  5. Pooh Bear

    Pooh Bear Guest

    It's only a convenience for modelling using equivalent circuits. rc doesn't
    actually exist as a resistance, it's simply a way of modelling the
    transistor's sensitivity to collector voltage vs collector current when
    inserted into the appropriate equation..

    In comparison Rbb and Ree are real ( bulk resistance of semiconductor
    material ).

  6. I read in that cheese9988>) about 'Common emitter amplifier question (rc ||
    rl)', on Sat, 29 Jan 2005:
    As far as the signal is concerned, the collector DC supply is at ground
    potential (or should be).

    So both Rl and Rc have one end at the collector and one at ground
    potential. That means they are in parallel.

    It's usual to use R for resistors outside the transistor and r for
    resistors in its internal equivalent circuit. Helps understanding.
  7. They are effectively in parallel because at signal frequencies the all
    voltage supply points are ground (there is an 'invisible short' across
    the battery or power supply at signal frequencies so your Vcc and GND
    are one and the same). This only applies at signal frequencies, of
    course, so at DC they're still pretty much isolated from eachother.
  8. lemonjuice

    lemonjuice Guest

    stop wasting bandwidth by repeating his question. The question is
    why is it at Ground. Either give an answer or go and mow your lawn.
    Maybe you're Kevin Alyward under a different name?
  9. Active8

    Active8 Guest

    Maybe he is, but no matter what you call yourself, you're an
  10. lemonjuice

    lemonjuice Guest

    Oh sure I'd love to be called that. Yes I sure remember when I
    untangled you saying that the gain between 2 amplifier stages was
    equal to the sum of the gains of each individual stage! Do you need
    the url on that?
    Now Mr. Brainy can you answer the guys elementary question above?
  11. Active8

    Active8 Guest

    You just were. Want to try for twice?
    F*cking liar. You didn't discuss that with *me*. I knew that over 20
    yrs ago - if you're talking gain in dB, anyway.
    It's already been answered but I have 3 more answers.

    1. If the DC supply is bypassed, it presents a low impedance (short)
    to ground for AC. So connect the Vcc node to ground and you have
    your ac equiv.

    2. Even if it's not bypassed, the source resistance of the DC supply
    is so low that it looks like a short to ground.

    3. If you short the DC source and open the tranny's current source
    to get the Thevinin or Norton resistance, you get a low resistance
    in series with RC (Vcc node shorted to ground) in parallel with RL.
    The output R, Ro, of the tranny is also in parallel with RL and RC.

    Any way you look at it. The DC supply is a short to AC. Short Vcc to
    ground, and it's at ground.
  12. lemonjuice

    lemonjuice Guest

    Ummm I'm not not satisfied with your answers. maybe you can't explain
    yourself . Why should you bypass something that is there? Why is the
    impedance of the DC voltage source low? Why is it a short at AC?
    Try again.
  13. Active8

    Active8 Guest

    Let's put in the part you snipped just to remind the group that
    you're a liar:

    F*cking liar. You didn't discuss that with *me*. I knew that over 20
    yrs ago - if you're talking gain in dB, anyway.

    You didn't even try to worm your way out of that.
    Uhmm... ask me if I care. But if the OP's still around, maybe he'll

    I prefer the 3rd answer - less hand waving. The first 2 answers are
    better suited for someone just learning the basics.
    Filter cap. Filters out ripple and supplies transient current that
    might otherwise change the drop across the supply's Thevinin
    resistance. Keeps the signal off the rail.
    Because if it were high, too much voltage would be dropped across
    its Thevinin resistance, Rth, and it wouldn't make a very good
    voltage source. The voltage at the source's terminals would be more
    dependant on the load it's energizing.
    In #2, I said it "looks like a short" Relative to RC which is in
    series with it, it's a short. In #1, the filter cap is the AC short.
  14. It was a lousy answer of the kind Mike's notorious for. He and Kevin
    are very similar. They only understand half a question and then
    proceed to waffle on about the *other* half. Personally, I killfiled
    'em both several months ago.
    No regrets.... ;-)
  15. lemonjuice

    lemonjuice Guest

    Yeah you're right. Maybe they are twins. I eliminated K.A some time
    back. This one is going now.
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