Power Amplifier Query

Discussion in 'Homework Help' started by VaLieN7e, Jan 25, 2013.

  1. VaLieN7e

    Thread Starter New Member

    Jan 25, 2013
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    Hi everyone , this problem has puzzled for quite some time now :confused:
    it's actually about a previous exam in my college , goes as the following :

    for the figure shown , determine the turn ratio of the output transformer , give that ( Vcc = 10V , RL = 8 ohm , maximum current that the transistor can handle is Ic = 2A , maximum peak value Ic' = 1.9A )

    we haven't had many experiences with transformers calculation , but what i know is Ic(max) = Vcc/Rc & Ip(load) = Np/Ns Ip
    i can't seem to find the right formula to connect all these aspects :\
    can you please help me ? thx in advance
     
  2. Audioguru

    New Member

    Dec 20, 2007
    9,411
    896
    You have a very old antique exam.
    Power amplifiers have not used transformers since about 50 years ago.

    Look at Transformer in Google to see that the impedance ratio is the turns ratio squared.
     
  3. VaLieN7e

    Thread Starter New Member

    Jan 25, 2013
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    i'm aware of that :(
    but i thought someone could help me with it :\
     
  4. Wendy

    Moderator

    Mar 24, 2008
    20,766
    2,536
    It has been a while, but with transformers it is the loading that matters, the rest is not as significant. You are matching impedances, which in turn determines the loading. So if you have 8Ω you need to figure out the load requirements on the other side.
     
  5. VaLieN7e

    Thread Starter New Member

    Jan 25, 2013
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    so if the load requirements aren't mentioned ( as in this case )
    the turn ratio can't be calculated , right ? we need at least the current or the voltage of the load , please correct me if i'm wrong ^^
     
  6. #12

    Expert

    Nov 30, 2010
    16,355
    6,852
    Yes. You have a serious lack of data. For instance, those transistors could power that speaker directly, no transformer needed, if the specifications were right.

    The statement 2NP/NS might suggest that the number of turns in each half of the primary is equal to the number of turns in the secondary. That would suggest a 1:1 ratio if you think in terms of a half wave going through 100 turns of primary, and that is coupled to 100 turns of secondary. Then again, I could be entirely wrong. That often results from trying to guess what a cryptic label means.
     
  7. VaLieN7e

    Thread Starter New Member

    Jan 25, 2013
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    totally agree with you #12
    i'm starting to feel that the whole problem is absurd and unrealistic .
     
  8. VaLieN7e

    Thread Starter New Member

    Jan 25, 2013
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    0
    my sincere thanks to everyone ,
    i was finally able to solve it and now i can sleep peacefully :)
    vcc = ic / rc , and the relation between rc and the transformer was the key to solution .
     
  9. crutschow

    Expert

    Mar 14, 2008
    13,056
    3,245
    Assume you want to transfer the maximum power to the load. Then for a 10V supply and 1.9A peak current, the peak available power is 19W. To transfer that to an 8 ohm load requires a peak load current of 1.54A. Thus you need a transformer turns ratio that converts 1.9A in the primary to 1.54A in the secondary.
     
    #12 likes this.
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