common-emitter doubts

Discussion in 'Homework Help' started by external, Jun 19, 2010.

  1. external

    Thread Starter New Member

    Jun 19, 2010
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    0
    hi

    i don't have any clue on this graph. Why the line marked with the arrow has this "wave" line? It seems that this is not a function since we see that BVceo has over one value... Why this rupture happens? The "wave" line doesn't make sense!

    what is the meaning of the Q point?? Maybe it means that is a biasing point where we can have the signal amplified without distortion?

    I did not understand the differences between beta for DC and AC beta. Please someone can explain it to me? Thank you.

    Look at the image filename... why there is no signal component between colector and the emitter????

    please, help in this.
     
  2. Audioguru

    New Member

    Dec 20, 2007
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    896
    The current increases at high voltages because the curves are for an old germanium transistor that is very leaky.

    The "wavy line" is the Breakdown Voltage of the collector-emitter.

    The term hFE is the DC current gain when there is no AC signal.
    The term hfe is the AC current gain when the frequency of the signal is specified.
     
  3. external

    Thread Starter New Member

    Jun 19, 2010
    2
    0
    Thanks, Audioguru. But why the "wavy lline" has such behaviour? When we rise the Vce... the line has a backward trajectory! mine doubt was this peculiar behaviour. Or maybe it is a mistake in the graph...

    Hope someone can explain the last question in the first post . And why the reason of calling short-circuit common-emitter current gain (see the image file).. Short-circuit between the collector and emitter, or, collector to the ground...?

    Thanks again. Bye.
     
    Last edited: Jun 19, 2010
  4. Audioguru

    New Member

    Dec 20, 2007
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    896
    I have never abused a transistor to cause its collector to emitter to have avalanche breakdown.
    I have seen the reverse-biased emitter-base have avalanche breakdown and its current has a backwards trajectory that causes it to oscillate if there is a capacitor across it.

    The term hfe is the AC current gain at one current and at one frequency. It is measured with the output short-circuited with a capacitor so that the collector to emitter voltage doesn't change.
     
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