Common Emitter Amplifier

Discussion in 'Homework Help' started by Madgesture, May 2, 2010.

  1. Madgesture

    Thread Starter New Member

    May 2, 2010
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    A technician installs a transistor, in a common emitter layou, with the Collector and Emitter terminal reversed. What would be seen a cross the load resistor?

    Any help would be greatly appreciated. On the last page of the homework and my brain is already fried.

    Mike
     
  2. hobbyist

    Distinguished Member

    Aug 10, 2008
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    NPN transistor to conduct:
    Collector more positive than base reversed biased
    base more positive than emitter forward biased

    PNP transistor to conduct:
    collector more negative than base reversed biased
    base more negative than emitter forward biased.
     
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  3. studiot

    AAC Fanatic!

    Nov 9, 2007
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    Well this is still homework, with or without fries, so we shouldn't tell you directly without you say what you think first.

    So here are some hints.

    Firstly the question asks what is the voltage across the load resistor, not across the transistor.

    So where is the load resistor in a CE amp?

    OK so now is the transistor on or off?

    So is it passing current?

    So what does that tell you about the current in the load resistor?

    So what does that tell you about the voltage across the load resistor?

    Does it make any differernce to the answer if the transistor is NPN or PNP?
     
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  4. Madgesture

    Thread Starter New Member

    May 2, 2010
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    What I see with the transistor installed backwards is an NPN, Common Collector Amplifier. The Load resistor will now be on the emitter. The Output on Load Resistor will basically be the input voltage, as there is very little or no voltage gain in in a common collector amplifier, just power and current gain.
     
  5. Madgesture

    Thread Starter New Member

    May 2, 2010
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    The load resistor on an NPN CE Amp is on the collector. The transistor installed backwards it will be on the Emitter making the cicuit a NPN CC Amp

    There is 1.08V at the base so the Base Emitter junction will be forward biased.

    With the BE forward biased there will be current passing and also current and voltage on the load resistor.

    Since the this is now a CC Amp the Output Voltage will be the roughly the same as the input as there is no voltage gain and the Output will be in phase with the input.
     
  6. studiot

    AAC Fanatic!

    Nov 9, 2007
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    Just try answering my questions is sequence.
     
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  7. Madgesture

    Thread Starter New Member

    May 2, 2010
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    So where is the load resistor in a CE amp?
    On the Collector

    OK so now is the transistor on or off?
    On 1.08V at Base before reversal

    So is it passing current?
    Yes

    So what does that tell you about the current in the load resistor?
    It will be higher than input current

    So what does that tell you about the voltage across the load resistor?
    There will be some voltage

    Does it make any differernce to the answer if the transistor is NPN or PNP? Yes
     
  8. studiot

    AAC Fanatic!

    Nov 9, 2007
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    Look at my sketch, (you could have done one of these).
     
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  9. Madgesture

    Thread Starter New Member

    May 2, 2010
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    So, basically with reversing the Transistor it is going from a CE Amp to an CC Amp.

    The major differences would be:

    No Voltage Gain in CC Amp
    No Phase Shift in CC Amp

    Thanks
     
  10. hobbyist

    Distinguished Member

    Aug 10, 2008
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    Here is a hint:

    draw 2 circuits using a CE layout properly biased for a NPN and a PNP.

    Then redraw them with the transistors backwards with there respective circuits.

    Then using the info in above posts analyse what you see.

    The key is don't change battery polarities when you change the transistors to be backwards.

    To change from CE to CC with backwards transistors the polarities of the battery needs to be reversed as well.
     
    Last edited: May 2, 2010
  11. Audioguru

    New Member

    Dec 20, 2007
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    When the emitter-base junction of a silicon transistor is reverse-biased then it has avalanche breakdown (like a zener diode) at about 7V.
     
  12. Ron H

    AAC Fanatic!

    Apr 14, 2005
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    An inverted transistor (collector and emitter swapped) still acts like a transistor, except the beta is low, and generally unspecified. Also, as AG stated, Vbe is now Vce, so the breakdown voltage will be low.
    There is insufficient information in the statement of the problem to give a quantitative answer.

    The circuit is NOT now a CC amp. It is still CE, but with the characteristics mentioned above.

    EDIT: From Wikipedia:
     
    Last edited: May 3, 2010
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