BJT based mutivibrator pspice simulation in CADENCE ORCAD


BJT based multivibrator

I am trying to simulate a basic BJT based multivibrator circuit in cadence orcad pspice. However, I am not getting the desired output.

If I simulate the same circuit in NI multisim, I am getting the desired output.

Do we need any specific simulation settings in ORCAD in order to view the output?


I have tried the recommendation. However, unfortunately, there is no oscillation at the output. Please find below the schematic and output results.enter image description here

enter image description here

Best Answer

I'm not familiar with your particular taste of SPICE (pun intended), but this is actually a very fun and well-known example about simulated vs. real circuits.

Solution: Your circuit is in perfect symmetry, and thus, it won't start. Try 150.001 nF or 82.001 kΩ for one capacitor or resistor, and chances are it will kick into oscillation.

As long as you keep the symmetry, both transistors will "come to action" in exactly the same way, and the circuit won't start to oscillate.

(It won't oscillate in LTspice either, for example - and maybe there is some rounding error in multisim that will start the oscillation. LTspice even has this circuit in the demos that are provided with the download, and it has one 100k resistor and one with 101k, IIRC.)

A real circuit will never have exactly the same components twice, so it will start to oscillate with no problem. It's very educating that imperfections are an absolute requirement for one of the most well-known basic circuits in electronics to work at all.