Audio Amplifier Design

Discussion in 'Homework Help' started by Electrical09, Dec 5, 2009.

  1. Electrical09

    Thread Starter New Member

    Dec 5, 2009
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    I am working on a problem to design an audio amplifier. It must have a voltage gain Av = 952, an input impedance of Z = 50Ω, and a load impedance of RL = 50Ω. To complete this design I am able to use resistors of any value, capacitors of any value, 9 volt batteries, and npn transistors with β =100 . It is assumed that the temperature is 300K.

    In my calculations so far I have been unable to get a voltage gain anywhere near 900. I am having trouble combining the transistors together which I believe is the way to go about solving it.

    Any help would really be appreciated.

    Thanks
     
  2. giovanni645

    New Member

    Dec 5, 2009
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    I am working on the exact same problem, let me guess Jazi's class for Industrial Electronics?

    What do u think the load impedance is? The load resistance or RL || RC ?

    I need help with this too, just can't figure out the resistance values to get the right Voltage gain.
     
  3. TrevorP

    Active Member

    Dec 8, 2006
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    You could do something like two or three biased and cascaded NPNs with by-pass capacitors to get really high gain. Then use an emitter-follower to drive the low 50ohm resistance, it will reduce your gain a bit but not as bad as without it.
     
  4. giovanni645

    New Member

    Dec 5, 2009
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    How much max gain do u think can be achieved using a normal common emitter amplifier? With a voltage divider bias?
     
  5. TrevorP

    Active Member

    Dec 8, 2006
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    I have no idea...you could consider doing it more like a darlington pair.
     
  6. giovanni645

    New Member

    Dec 5, 2009
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    just confirming, if we use a darlington pair, only the voltage drop doubles and the DC gain (beta) increases to beta squared?

    does it affect anything else?
     
  7. ftsolutions

    Active Member

    Nov 21, 2009
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    Just be aware of the total power and current you are passing through the pair configuration - the power dissipation of either transistor must not be violated. It may be easier to just use (2) similar common emitter stages each with a gain of ~ 31 or so, followed with an emitter follower as suggested.
     
  8. giovanni645

    New Member

    Dec 5, 2009
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    we can only use npn transistors.

    If I use a gain of say 30 (sqrt of 900 Voltage Gain) for each, how do i go about calculating values for the resistors? we aren't given anything for the values of resitors

    Av = (RL' * beta)/rpi

    How do i get Icq if i dont have any resistor values? is it best if i just use trial and error?
     
    Last edited: Dec 5, 2009
  9. giovanni645

    New Member

    Dec 5, 2009
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    Also, can anyone tell me how to find the capacitor values for common emitter amplifiers?
     
  10. Jony130

    AAC Fanatic!

    Feb 17, 2009
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  11. giovanni645

    New Member

    Dec 5, 2009
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    so i did a few calculations. we are allowed only 9 volt batteries.

    Vcc = 9V

    Vb = 4.5V also implies R1 = R2

    Rl' = RL || RC where RL = 50 according to question (is load impedance RL?)

    i'm just really confused, I can't even get the PSpice simulation to work with a gain of 100 for which already have R values.

    what am i doing wrong?
     
  12. Audioguru

    New Member

    Dec 20, 2007
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    Didn't your teacher tell you that the voltage gain is about Rc/Re?
    So your gain is only about 5.5 without the 50 ohms load. If the external load is 50 ohms then the gain is almost nothing.

    You don't want the base of the transistor to be biased at half the supply voltage, you want the collector at half the supply voltage so it can have the maximum amount of voltage swing.
     
  13. giovanni645

    New Member

    Dec 5, 2009
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    So the load impedance is RL || RC ? since RL can't be 50 ohms?

    the question kinda confuses me.

    according to my professor Av = ((RL||RC) * beta) / Rpi, also in one of the example he solved RC = RE and voltage gain is -106.
     
    Last edited: Dec 7, 2009
  14. giovanni645

    New Member

    Dec 5, 2009
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    no idea what i'm doing wrong now. Pspice just won't give me the right result.

    nevamind the attachments ...just realized the bias voltage wasnt reaching the transistor since it wasnt making a freaking node in Pspice, can some1 answer the questions above that?
     
    Last edited: Dec 7, 2009
  15. Jony130

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    Feb 17, 2009
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    It looks like is something wrong in you simulation, because circuit is ok.
     
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  16. JoeJester

    AAC Fanatic!

    Apr 26, 2005
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    Jony,

    He has corrected it in his posting. His diagram didn't have the junction of R1/R2 connected to the base of the transistor, so the transistor only had the input signal for bias.
     
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