conjugate and load line match

Discussion in 'General Electronics Chat' started by anhnha, Nov 6, 2013.

  1. anhnha

    Thread Starter Active Member

    Apr 19, 2012
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    Here is part from RF power amplifier for wireless by Steve Cripps.
    It is about conjugate and load line match for RF power amplifier. I don't know why the load line as Rload = Rgen is above the line Rload = Vmax/Imax.
    Note that the red line is my version and is it possible to have the red line as in the picture?

    [​IMG]

    [​IMG]
     
  2. t_n_k

    AAC Fanatic!

    Mar 6, 2009
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    The transistor of choice will always have a safe operating region in the V-I plane. This region is generally bounded by the device rated voltage, current & power lines. The operating design conditions must be maintained within the SOA to prevent likely failure.
     
  3. anhnha

    Thread Starter Active Member

    Apr 19, 2012
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    I think there is a misunderstanding here. I am confused about this:
    Why the line Rload = Rgen has to be the one above the line Rload = Vmax/Imax NOT the led line?
     
  4. t_n_k

    AAC Fanatic!

    Mar 6, 2009
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    I do see some anomalies in the author's document - as attached.

    Perhaps one needs to read this in the larger context of the entire text.

    The argument for careful attention to device limitations is perfectly reasonable. However, I'm not sure why such a simplistic representation of the RF power amplifier is helpful, but it may prove so to individual readers. I re-iterate that one needs to see the entire picture presented by the author.

    To answer your specific question about where the load lines should lie in relation to one another, I would suggest it all depends on the circumstances. On one hand, the writer presents a very simple ideal model of the device and then proceeds to include a "load line" subject to device limitations - which is confusing in the context of the idealized model. What does a load based on the relationship Vmax/Imax mean in the context of practical RF amplifier design? Perhaps it is intuitive but I fail to grasp the point.

    Successful RF amplifier design is a tricky business with issues such as amplifier class, device frequency dependent characteristics, input & output impedance matching networks and the like (to name a few) all adding to the mix of necessary considerations.

    I'm not sure why you think your line in red is more meaningful or correct than the author's version - I think your question distracts one from the main point of the graph which presumably seeks to show how careless design or poor device selection might lead to problems in amplifier performance.

    As a final comment, the annotations in figure 1.5 are also confusing. The graph shows axes annotated as Vout vs Ig. But Ig is the idealized source current rather than the actual load current, which (the latter) would seem to be more relevant for the purposes of the discussion.
     
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