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Amplitude modulation and amplifier efficiency

  1. Jun 21, 2015 #1
    " A intelligence signal is amplified by a 80% efficient amplifier before being combined with a 12kW carrier to generate the AM signal. if it is desired to operate at 100% modulation, what is the DC input power at the modulator" The choices are a: 12kW b. 7.14kW c.7.5kW d. 15kW

    my solution:

    For 100% modulation the modulating signal power must be one half of the carrier power.
    Pm=0.5Pc=6kW
    Pm is the output of the 80% efficient amplifier.
    efficiency=Pout/Pin=Pm/Pin
    Pin=Pm/efficiency=6kW/0.8=7.5kW
    Is this right?

    How do I solve problems like this? Sometimes i do the same procedure for a similar question but with different values, but I get a wrong answer. Can you suggest a good reference. I am currently using the electronics communications books by frenzel and blake
     
  2. jcsd
  3. Jun 21, 2015 #2

    tech99

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    Gold Member

    This seems to be the correct answer for a sinusoidal modulating signal.
     
  4. Jun 21, 2015 #3
    These are sample problems from Blake's electronic comms book:
    1.) A transmitter has a carrier power output of 10W at an efficiency of 70%. How much power must be supplied by the modulating amplifier for 100% modulation
    Book Solution: efficiency=Po/Ps (I don't know what Po is exactly, but Ps was defined as the DC input power to the final amplifier)
    Ps=Po/efficiency =10W/0.7 = 14.3W
    Pa=0.5Ps=7.125W (Pa is the power required from the modulating amplifier)

    2.)A collector modulated class C amplifier has a carrier output power of 100W and an efficiency of 70%. Calculate the supply power and the transistor power dissipation of 100% modulation.
    Book Solution: For 100% modulation.
    Po=1.5Pc=1.5(100W)=150W
    Ps=P0/efficiency=150W/0.7=214W
    Pdissipated=Ps-Po=64W


    From Problem 1 it seems that Po is the carrier output power. But in problem 2, Pc is the carrier output power? I really don't get. Can you recommend other references regarding this topic

    `
     
  5. Jun 22, 2015 #4

    tech99

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    Gold Member

    In problem 2, I think they are asking for the power supply requirements of just the PA. Its power supply furnishes 100W/0.7 = 143W under no modulation. With 100% modulation, the additional power of 71.5W required by the PA is supplied by the modulator rather than the PA DC supply.
    I am sorry I cannot recommend books because my own are now 70 years old and not easily available!
     
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