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BJT Amplifiers - Loadline question

  1. Sep 4, 2010 #1

    I had a basic analog electronics course this summer. Now that I'm trying to design my own circuit with a bipolar junction transistor (both for fun and to prepare for the future), I'm finding I don't really understand how and why I have to follow certain procedures to get maximum output voltage swing.

    I don't understand why you have to consider the AC and DC loadlines' intersection to get the quiescent (Q) point of a transistor, for example of a bipolar junction transistor (BJT) amplifier.

    The DC loadline tells me how the collector current will look for a given collector-emitter voltage. It is a negatively linear relationship meaning if I increase the collector-emitter voltage, I get a proportional decrease in collector current. If I want a different result, I have to change the collector and/or emitter resistors.

    The AC loadline tells me my maximum possible output swing. Similarly, if I want a different characteristic, I need a different set of impedances or perhaps my input signal could be changed.

    Question: Why do I need the intersection to determine the Q point? Why can't I just use the DC loadline's middle to give me the quiescent (Q) point?
  2. jcsd
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