RFC at the collector leg of transistor

Discussion in 'Wireless & RF Design' started by xylon89del, Feb 3, 2012.

  1. xylon89del

    Thread Starter New Member

    Dec 28, 2011
    17
    0
    Hi,

    When designing an amplifier or oscillator in common-emitter configuration,
    at the collector leg, sometimes the resistor is replaced with inductor?

    What is the purpose of replacing the resistor with inductor?
     
  2. crutschow

    Expert

    Mar 14, 2008
    13,006
    3,232
    Higher AC gain. The inductor can be selected to have a higher impedance at the signal frequency than a resistor which gives higher AC gain. The value of the resistor is limited by the required transistor bias current and supply voltage.
     
  3. radiohead

    Active Member

    May 28, 2009
    474
    31
    Also, when using the (NPN) transistor in an RF application, the inductor (RF choke) will assist in keeping RF from straying off an intended path.
     
  4. xylon89del

    Thread Starter New Member

    Dec 28, 2011
    17
    0
    Why cant we just put a resistor which its value is the same as the impedance of the inductor at the specific frequency?
     
  5. crutschow

    Expert

    Mar 14, 2008
    13,006
    3,232
    Because the impedance of the resistor would be too high to carry the desired bias current for the transistor using a normal low amplifier supply voltage.
     
  6. xylon89del

    Thread Starter New Member

    Dec 28, 2011
    17
    0
    Can you explain in more detail?
     
  7. bertus

    Administrator

    Apr 5, 2008
    15,648
    2,346
    Hello,

    Have a look at this PDF:
    RF-prototyping.pdf

    From page 8 and on there is an explanation on the RF chokes.

    Bertus
     
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