small-signal analysis

Discussion in 'Homework Help' started by zdzislavv, Jun 8, 2009.

  1. zdzislavv

    Thread Starter Member

    Jun 5, 2009
    22
    0
    Hello!
    Here there is what I want to solve: http://images38.fotosik.pl/135/35e73fda57d25875.jpg
    I found some informations on Q-point stabilization: http://images43.fotosik.pl/139/09ebd31e0c12d268.jpg
    And about small-signal analysis: http://images45.fotosik.pl/139/9a75b7c041376cf6.jpg
    And finally I tried to draw scheme with substituting transistors to models (but some of wires are connected to both transistors - and I don't know what to do with them, e.g. Rc and Rb): http://images49.fotosik.pl/139/c2964a1dbbc28db2.jpg
    I want to solve the whole exercise but the most important part for me is small-signal analysis. May you help me with drawing the proper circuit and give some advices so that I can try to finish this exercise?
    Thank you very much in advance
    Greetings!
     
  2. Jony130

    AAC Fanatic!

    Feb 17, 2009
    3,957
    1,097
    If I am not-mistaken the correct small-signal schematics should look like this.
    And good luck with the Q-point calculation, And remember that current flow through RB must be equal I=0.6V/Rb

    I nie za trudne te zadanie dla ciebie?
     
  3. zdzislavv

    Thread Starter Member

    Jun 5, 2009
    22
    0
    Thanks very much, Jony130 :).
    I see that capacitors are shorted - that's right. But shouldn't Rg be connected directly to G (gate) of JFET, not through Rb? In Your schematics it looks like there is connection with no resistors between D (drain) of JFET and emitter of BJT. Shouldn't it be rather connection between D and collector through Rc? And what about diode, Rz and Re?
    Greetings!
     
  4. Jony130

    AAC Fanatic!

    Feb 17, 2009
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    Rg is directly connected to gate. Look at Uge voltage arrow.
    For AC signal, Drain, Rc are shorted by rz=0Ω (nie mylić z "rz" :) ) of Zener diode, and emitter is short by Ce.
     
  5. zdzislavv

    Thread Starter Member

    Jun 5, 2009
    22
    0
    http://images39.fotosik.pl/147/266ced692a1e4832.jpg

    Thanks! I've got two questions. First one - why is this Rc placed here? In your schematic it is between C and E, ok. And there is just wire between D and E (they are the same point). In other words D-C is the same as E-C so Rc is placed in proper way between drain and collector. So I know why it is placed in this way - but what for? Wouldn't it be more natural to draw it in this way: http://images44.fotosik.pl/151/de2e3e90b40662a9.jpg?

    The other question is connected with rather basic knowledge about circuit theory. Why cannot I replace diode with this voltage source as shown in my schematic (second photo)?

    Greetings!
     
  6. Jony130

    AAC Fanatic!

    Feb 17, 2009
    3,957
    1,097
    Well maybe, but Zener diode has rz=0Ω and that means that Drain and upper side of Rc is short to the GND.

    The same question you can ask about the BJT and base to emitter junction and Vbe=0.7V.
    Take look on this link
    http://www.edw.com.pl/pdf/k01/35_05.pdf
    http://www.edw.com.pl/pdf/k01/37_02.pdf
    http://www.edw.com.pl/pdf/k01/18_12.pdf
     
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