Diode circuit model question

Discussion in 'General Electronics Chat' started by Shagas, Jan 10, 2015.

  1. Shagas

    Thread Starter Active Member

    May 13, 2013
    Hello , could someone give me some advice as to what I'm doing wrong in this exam question?

    The picture and my workings are in the attachments.
    Question 1) Graphically determine the diode working point ( q-point , [id, ud] ) .
    Question 2) Analytically determine the exact q-point.
    Question 3) Determine the diode voltage u(t)
    Question 4) How will the q-point change if the DC part of the u1(t) was increased by 1 volt?

    q1 : I created a load line (diode as an open and as a short) and determined that the qpoint is
    qp = [10mA , 0.65V] which looks like a pretty reasonable result.

    q2 : I replaced the diode with a model which is an ideal diode (only conducts forward) + 0.6V voltage source
    + Resistor Rx whos value I determined from the gradient of the graph.
    After doing voltage node analysis I get a Ud of 0.933 which is obviously wrong.

    q3: Pretty straightforward

    q4 : Should I just re-analyse the circuit or is there some other faster way?

    Thanks in advance.
    Last edited: Jan 10, 2015
  2. Shagas

    Thread Starter Active Member

    May 13, 2013
    I have determined the mistake , the RX that I got from the graph is 0.020A/0.1V = 0.2 ohm , not 200 ohm.

    q4 is still on the table though....