Why is this astable multivibrator circuit getting balanced in a flash and not oscillating? It works on a breadboard. It once worked fine in simulation, but one day it stopped oscillating in simulation. Very weird. I am wondering what clamps both transistors at the same base voltage simultaneously.

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I happened to have another circuit which oscillates on the left and I mimicked one circuit and it alone does not oscillate, for well-matched factor, I changed transistor to different NPNs or even tried replacing resistors with different values prior to both transistors and also the base voltages are stuck.

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  • \$\begingroup\$ as usual: 1. your models, especially for things like transistors, not necessarily simulate the same aspects of the device that are important for your specific from of application. For example, your real-world components will have variances, whereas Q1 and Q5 are perfectly identical in your simulation. \$\endgroup\$ Nov 24 at 10:57
  • \$\begingroup\$ 2. breadboard (if we're talking the one where you can manually plug in component leads to connect them without soldering) has considerable parasitic effects – both inductive effects for current travelling along the "bars" and jumper cables, as well as capacitive coupling between adjacent rows. Wouldn't be suprised if especially the capacitive coupling adds up to be in the same order of magnitude as your capacitors! \$\endgroup\$ Nov 24 at 10:58
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    \$\begingroup\$ 3. – 238. Real world has noise that will happily bootstrap an oscillator that your simulation does not have. It also has noise that makes everything else behave non-deterministically. \$\endgroup\$ Nov 24 at 10:59
  • \$\begingroup\$ The trick to getting oscillator simulations to start is to make them not quite exactly symmetrical. Try changing one resistor value by a percent or so and leaving the counterpart on the other transistor the same. \$\endgroup\$
    – Hearth
    Nov 26 at 18:20


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