Two Transistor Question

Discussion in 'General Electronics Chat' started by raghavanunm, May 24, 2013.

  1. raghavanunm

    Thread Starter Member

    Mar 21, 2007
    14
    0
    Guys,
    Please find attached, a copy of circuit for which, i have few questions:
    - Can you tell me which one, turn ON first Q1 or Q2.

    Also, there are few more questions in the picture..could you please help me out ?

    Thanks,
     
  2. Dodgydave

    Distinguished Member

    Jun 22, 2012
    4,963
    743
    Q1 turns on first,that turns Q2 on.
     
  3. c.marsh

    Active Member

    May 16, 2009
    100
    0
    isnt that a darlington pair?

    no because the emitters are in different configurations?
     
  4. raghavanunm

    Thread Starter Member

    Mar 21, 2007
    14
    0
    Does Q1 Turn ON first because , Base of Q1 is connected to ground ? ( or the PNP emitter is more positve than Base ?)

    Also, how does the capacitor charge? ( the Path..)
     
  5. crutschow

    Expert

    Mar 14, 2008
    12,973
    3,219
    That's a poorly designed circuit since it has DC going through the speaker voice coil.
     
  6. studiot

    AAC Fanatic!

    Nov 9, 2007
    5,005
    513
    Is this a homework question?

    You need to tell us your thoughts to receive help. I see that the notes believe you have already studied how a simpler similar circuit works. Did you understand that?
     
  7. raghavanunm

    Thread Starter Member

    Mar 21, 2007
    14
    0
    I did understand a similar circuit..but i don't understand the capacitor's charging path.
     
  8. raghavanunm

    Thread Starter Member

    Mar 21, 2007
    14
    0
    If q1 turns ON first, what can ever turn it off, isn't it connected to the ground visa the resistors ?
     
  9. Jony130

    AAC Fanatic!

    Feb 17, 2009
    3,957
    1,097
    At the beginning Q1 turns on first, because the base current finds a path to ground through resistors R1 and R2. Turns on Q1 opens the path for Q2 base current.
    So Q2 is also ON. And C1 capacitor start charging. The C1 is quickly charge in this current path.
    +4.5V ---> Q1 emitter-base junction ---> C1 ---> Q2 collector-emitter--->GND.

    [​IMG]



    After C1 is charge to 4.5V - 0.7V = 3.8V capacitor end the charging phase.
    This causes that the base current of Q1 is decreases. This also decreases IcQ1 and IbQ2, IcQ2 currents, this rise the Q2 collector-emitter voltages.
    This small change in VceQ2 voltage is "feedback" by C1 capacitor to Q1 base. This will increase the voltage at Q1 base. So Q1 and Q2 will decrease his currents. This will increase VceQ2 and Q1 base voltage even faster.

    [​IMG]


    So we have a very strong positive feedback in this circuit.
    This cycle ends when Q1 and Q2 turns off. And capacitor start slowly discharge.
    bottom C1 plate ---> R2 --> R1 --->Batter---> speaker--->R3 ---> upper C1 plate.
    And after C1 is full discharge ( voltage at Q1 base drop form 8.3V to 3.8V).
    Q1 and Q2 turns ON and C1 is quickly charge and speaker "play the sound".
    And the whole cycle repeats itself.
     
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