I was wondering if you can help me to know why my circuit is not working. I have tried with different pre-existing circuit values to make sure the circuit was working properly before I put my calculated values but I am not able to obtain an oscillating output...




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    \$\begingroup\$ Your astable is stable. Perfect components create a perfect symmetry, and there's nothing in the startup condition that would encourage the conduction of either transistor to get ahead of the other and start oscillation. Try introducing a little asymmetry. \$\endgroup\$
    – Phil G
    Jul 9, 2019 at 18:51
  • \$\begingroup\$ Welcome to EE.SE. Tip: crop your screengrabs to optimise the 640 px wide column space that SE uses. Your simulator may be bringing up both transistors at exactly the same rate whereas in reality the components on each side won't be identical. Try changing the value of C1 or C2. \$\endgroup\$
    – Transistor
    Jul 9, 2019 at 18:52
  • \$\begingroup\$ Put an initial value on one of the caps \$\endgroup\$
    – scorpdaddy
    Jul 9, 2019 at 18:53
  • \$\begingroup\$ ....and increase base current (or reduce collector one) to saturate your transistors . The 9V vce you show clearly shows this problem. \$\endgroup\$
    – carloc
    Jul 9, 2019 at 20:16

2 Answers 2


To get an oscillator in spice to work, you need to give the circuit some initial conditions to get it to start. In the real world, noise does this. In a simulation, the bi-stable circuit needs to 'tip' one way or another to start it ringing.

Try these links to help you set initial conditions.


You are thinking hFE=100 in your design, but as it saturates, hFE drops to 10% of its max hFE, so use “different” R ratios than 100.

Simulators may assume Vc,Ic=0 and that starts base current immediately but they may be “perfectly” matched hFE which never happens outside of simulators.

: Include ALL your assumptions next time.

If any design fails , one of these is wrong or missing: (Stewart’s Theorem)


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