#2 Astable multivibrator

open
nobody
Simulator (6)
5
2009-02-10
2007-05-02
bwana_v
No

If I try to simulate a bjt astable multivibrator made with two identical npn, two identical base resistors, two identical collector resistors and two identical capacitors cross connecting the base and collector node of the two bjts powered with a 15 V battery through a SPST switch, I have no oscillations starting while closing the switch. Putting two oscilloscope probes on C and B nodes shows only noise. Putting voltage probes on the same nodes shows correct saturation voltages. Same way changing hfe of the bjt as they aren't quite identical. Oscillation starts adding and operating a momentary pushbutton between a B and E nodes, but is very erratic and damps off after a while. Adding a couple of diodes before the bjts bases to avoid base junction reverse avalanche does nothing. I suppose there is something in the bjt AC model that behaves uncorrectly in case of positive feedback. Simulating a single transistor
BF amplifier with stabilized polarization (R1,R2,Re)seems to give correct results.

Discussion

  • bwana_v

    bwana_v - 2007-05-02

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    Same way trying to build an inverter RC oscillator. By the way, I think it's mandatory to add a schmitt trigger inverter to the gates list, and possibly choosing various VCC values as for CMOS series, as they are very common and useful control circuit building blocks.

     
  • Jason Lucas

    Jason Lucas - 2007-05-09
    • status: open --> pending
     
  • Jason Lucas

    Jason Lucas - 2007-05-09

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    Could you please sent me your .circuit for this circuit.
    Thanks
    Jason Lucas

     
  • Jason Lucas

    Jason Lucas - 2007-05-09
    • status: pending --> open
     
  • bwana_v

    bwana_v - 2007-05-11

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    Hi Jason, sorry for the delay.
    Here's the file you requested. Let me know if there's something more I can do.
    Best regards
    Alberto
    File Added: astable.circuit

     
  • bwana_v

    bwana_v - 2007-05-11

    astable.circuit

     
  • Julian Bäume

    Julian Bäume - 2009-02-10
    • labels: --> Simulator
     

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