Analysing resistances of transistors

Discussion in 'Homework Help' started by wind_blast942, Oct 17, 2010.

  1. wind_blast942

    Thread Starter New Member

    Sep 7, 2010

    Generally i do not understand how to determine the resistances of transistors. Please help me check if my reasoning is correct.

    For the following example, to find Rin, i look into the wire and see Rb to the left and MN1 to the right. Since gate of MN1 has infinite resistance, i follow the Rb to see MN1A.

    MN1A is parallel to MN2A as there is an additional wire connected from its gate to MN2A's source. However when i look into MN2A, my lecturer tells me it is MN2A in series with Rbias2.

    This is the part i am not sure. Why is it in series when the connection is similiar to MN1A's, and since MN1A is parallel to MN2A, shouldn't MN2A be the parallel to Rbias2 too?

    If not what would be a good way to analyse this, as the gate connecting back to the drain part is making me confused.

    Thanks for the help.

  2. Kermit2

    AAC Fanatic!

    Feb 5, 2010
    Transistors not only amplify, they can be low ohm value switches as well, remember? hint hint. When that switch is closed base current can flow(a little) look at the input impedance chart/graph on some datasheets to get the idea of how resistance will change with the conduction or non-conduction of the semiconductor device.
  3. DonQ

    Active Member

    May 6, 2009
    You need to recheck the definition of series and parallel. This is largely where your problem lies.