High Current on pHEMT Microwave Amplifiers

Discussion in 'The Projects Forum' started by rounin, Jul 11, 2008.

  1. rounin

    Thread Starter New Member

    Jul 11, 2008
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    What causes high currents on commercialized MMIC pHEMT amps?
    Typically, the Vd is below the rated specs. Why is this so?
     
  2. theamber

    Active Member

    Jun 13, 2008
    318
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    I think is because when you apply voltage to the gates it alters the conductivity to the GaAs layer which is undoped. Then the electrons can move quickly without colliding with any impurities in this layer with almost no resistivity. That is why you use them in applications requiring high gain and low noise and at high frequencies.
     
  3. rounin

    Thread Starter New Member

    Jul 11, 2008
    2
    0
    thank u for the answer but i guess i have not made my question clearer..

    the situation is this: i have a phemt amp with typical bias points at
    Vdd = 5.5V
    Vdrain = 3.5 to 4.5 V
    Vgate = -1 to 0.3
    say Id typ=400mA through a drain resitor..

    why is it that Vdd after usage goes way below the minimum specs say 1.5V. this is a high Id, right? why does this happen when the initial bias is set at right values?could it be the loading at different power level?
     
  4. theamber

    Active Member

    Jun 13, 2008
    318
    0
    What are you designing like a 50:-30dBm low noise amp?
    Yes the Id is too high. I think you are using depletion mode phempts If you were using the Enhancement mode phempt which does pulls nearly zero drain current when Vgs=0 which function oposite to the depletion mode ones. Actually the gate must be made positive with respect to the source for the enhancement mode phempt to begin pulling drain current. I think you are using the wrong transistor for your design.
    If you want to know why you will need to look at the manufacturing process of the hetereostructure.
     
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