BJT saturation

Discussion in 'General Electronics Chat' started by KCHARROIS, Nov 3, 2013.

  1. KCHARROIS

    Thread Starter Member

    Jun 29, 2012
    292
    1
    Hello,

    I'm analyzing a sawtooth waveform generator below made with discrete components. The circuit does work when the Re resistor is above 5K but doesnt generate a sawtooth when Re is say 2K. My question is why? I noticed that the transistor is in saturation with the 5K and not with the 2K im not sure if that helps?

    Thanks
     
  2. mbohuntr

    Active Member

    Apr 6, 2009
    413
    32
    Certainly not any kind of expert, but look at the voltage at the base of Q2, it appears to drop below saturation down into the mV range when you change the resistor... :confused: Q1 and Q2 appear to function as a latch, providing a path to ground. It looks like your switching it off, hence no ground.
     
    Last edited: Nov 3, 2013
  3. Alec_t

    AAC Fanatic!

    Sep 17, 2013
    5,776
    1,103
    Low values of R5 allow sufficient current through Q1,Q2 to keep the latch conducting, so the cap can't charge.
     
  4. #12

    Expert

    Nov 30, 2010
    16,257
    6,757
    Another way to say this is, your scr has enough hold current to stay latched on. You can keep the current low with R5 or lower the quality of the latch with a resistor from base to emitter of Q2.
     
  5. KCHARROIS

    Thread Starter Member

    Jun 29, 2012
    292
    1
    Ok I see it now... I did not suspect the scr made with the 2 transistor be running with such low current.

    Thanks!
     
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