RF Amplifiers

Discussion in 'Wireless & RF Design' started by Mazaag, Apr 30, 2008.

  1. Mazaag

    Thread Starter Senior Member

    Oct 23, 2004
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    0
    Hey guys...

    I had a question..

    What makes an RF amplifier different a regular amplifier? I understand that RF refers to the Radio Frequency ( frequency of the input) , but what about the circuits? how are they different?
     
  2. Papabravo

    Expert

    Feb 24, 2006
    10,144
    1,791
    In contrast to baseband or audio amplifiers, RF amplifiers make use of tuned circuits. This means the amplifier has both a high frequency cutoff, and a low frequency cutoff.

    The performance of an RF amplifier is more dependent on the source impedance and the load impedance. They are often designed to have a complex conjugate match at the frequnecy of interest.

    Other than that -- gain is gain
     
  3. Mazaag

    Thread Starter Senior Member

    Oct 23, 2004
    255
    0
    Is there any specific reason why they have both a low and high freq cut-offs?

    Why is impedance matching particularly important for RF circuits?

    Could we use regular transistors and such elements?

    Thanks
     
  4. Papabravo

    Expert

    Feb 24, 2006
    10,144
    1,791
    Is there any specific reason why they have both a low and high freq cut-offs?
    A tuned circuit only operates over a range of frequencies.

    Why is impedance matching particularly important for RF circuits?
    Because an impedance mismatch will cause the signal to be attenuated instead of amplified.

    Could we use regular transistors and such elements?
    Yes, but not for frequencies above about 30 MHz. As you move into the VHF range, transistor parameters and circuit layout become increasingly important.
     
  5. bertus

    Administrator

    Apr 5, 2008
    15,648
    2,347
    Hello,

    There are differen types of RF amplifiers.
    There are special amplfiers for recieving called LNA (low noise amplifier).
    They are specialy made for recieving small signals, like in satelite reception.
    There are special amplfiers for amplifing the signal of a transmitter.
    They are special tuned for the frequency of the transmitter.
    The matching for this amplfier is importend to get the most of the power into the antenna.
    RF amplfiers are often made with special transistors.
    For recieving and in IF stages IC's like MAR MAV or ERA types are used.

    Greetings,
    Bertus
     
  6. Wendy

    Moderator

    Mar 24, 2008
    20,766
    2,536
    What is a negligible factor in audio can be a major component in radio, a 1nF capacitance, for example, is under 2 ohms reactance in radio, and practically invisible in audio.

    Mistakes in RF circuit design tend to oscillate.
     
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