I'm developing a circuit simulator and tried to create an ideal astable multivibrator (I set some values not to be completely symmetric for the correct startup). The simulation usually stuck in equilibrium states. Then I put capacitors between the emitter and base on both BJTs. This fixed the simulation , everything worked fine. Is it possible that EB capacitors/delay effects are necessary in the BJT model to create a multivibrator?
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\$\begingroup\$ The textbook example of astable multivibrators. Collector resistors: 1k, base ones: 10-15k, capacitors: 1mF, ideal Ebers-Moll transistors with beta~100, Uebon~0.7V, Ucesat~0.15V. The EB capacitor I added is 0.1mF. I know its unnatural, but only the capacitance ratios count here. I made slight asymmetries in the values for the peaceful startup. \$\endgroup\$– RobertSziliCommented Nov 23, 2020 at 14:46
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\$\begingroup\$ Show your schematic. \$\endgroup\$– Andy akaCommented Nov 23, 2020 at 15:00
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\$\begingroup\$ google.com/… \$\endgroup\$– RobertSziliCommented Nov 23, 2020 at 15:05
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\$\begingroup\$ Listen, I'm not going to waste any more time on this because you just don't seem to appreciate things; you simulated it and that simulation circuit is the only relevant schematic. \$\endgroup\$– Andy akaCommented Nov 23, 2020 at 15:08
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2\$\begingroup\$ Why didn't you compare results with a regular sim package because then you'd see that a regular sim would start OK and, as pointed out in the answer, you should use a different (more applicable) model. \$\endgroup\$– Andy akaCommented Nov 23, 2020 at 15:52
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1 Answer
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Ebers-Moll only contains the information to model the currents if a bjt. Add capacitance of the transistor to model AC effects (and memory over time).
Try the Gummel poon model:
Source: Gummel Poon Model
answered Nov 22, 2020 at 17:02
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\$\begingroup\$ Yes, this is why I added an E-B capacitor to the circuit. And it made the simulation work. I think the memory (or delay) of the BJT is necessary for the multivibrators. \$\endgroup\$ Commented Nov 23, 2020 at 14:44