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Homework Help: Parasitic Capacitance in Transistor Amp

  1. Nov 10, 2005 #1
    Hi all :)
    I need some help with the concept of parasitic capacitance. I'm currently doing some analysis on a basic tuned transistor circuit. I know the LCR portion of the circuit is there to remove parasitic capacitance. The only problem I'm having is I don't exactly know what parasitic capacitance is. As far as I can tell, it's a problem with the transistor at high frequencies which causes it to cut off. Can anyone please elaborate or correct me?

    I can post a circuit diagram if needed.

  2. jcsd
  3. Nov 10, 2005 #2


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    Parasitic capacitance is exactly what would happen if you soldered a capacitor on top of your transistor. For a MOS transistor that amplifies from the gate and the drain (or source, whichever is used as output), or a bipolar transistor that amplifies from the base to the collector (or emitter if that is used as output), the effect of the parasitic capacitor is to copy whatever is sent to the input over to the output. If the transistor is wired up as an inverter, then this will reduce the output amplitude and therefore reduce the gain.

    Parasitic capacitance comes about from all kinds of ways including, for instance, the natural capacitance between the legs of the transistor.

    But this is probably what you already knew.

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