I had fun today learning the basics of LTSpice using a variation of the "classic multivibrator" (*). I have also built a working version with real components. There is a significant difference between LTSpice and what I see on my cheapo (10Mhz) scope, and that is the voltages on the NPN transistor "protection" diode. As you can see, I put a diode in line with the base to prevent negative current flowing from the significant Vcb the circuit develops. As expected, when I view the NPN bases voltage with my scope, the diode clamps the negative voltage at about -0.6v. But LTSpice shows the full negative Vcb, shifted by 0.6V, at both the anode (node 004) and cathode (node 010). LTSpice shows the diode current at zero, though, except for the (expected?) spike when the voltage goes from forward to backward.

My question is, then: Is this just a side effect of LTSpice getting the voltage wrong when the current is zero? I would not be surprised if the voltages reported by the simulation are essentially undefined when the current is zero between the nodes.

Significant gotchas, maybe: In my real version, the PNP transistor of each Sziklai pair is a germanium PNP power transistor, Hfe probably on the order of 30. No germanium models exist in LTSpice; I've used the "generic" PNP model. (**) Also, the output of the circuit are two 400ma, 6V incandescent lamps, which I have simulated by 10 ohm resistors.

(*) I'm baffled why this circuit is so widely used as an example of how to generate badly formed square waves. It really belongs in a Horowitz and Hill "Bad Circuits" section :-)

(**) My Dad brought home some Ge power transistor samples from his job at TI when I was a wee lad in the early 60s. I thought I would finally do something interesting with them after all these years.

I had fun today learning the basics of LTSpice using a variation of the "classic multivibrator" (*). I have also built a working version with real components. There is a significant difference between LTSpice and what I see on my cheapo (10Mhz) scope, and that is the voltages on the NPN transistor "protection" diode. As you can see, I put a diode in line with the base to prevent negative current flowing from the significant Vcb the circuit develops. As expected, when I view the NPN bases voltage with my scope, the diode clamps the negative voltage at about -0.6v. But LTSpice shows the full negative Vcb, shifted by 0.6V, at both the anode (node 004) and cathode (node 010). LTSpice shows the diode current at zero, though, except for the (expected?) spike when the voltage goes from forward to backward.

My question is, then: I can see why LTSpice is showing the negative voltages on both sides of the diode. If there is no current flow, is the negative base current still potentially harmful even if the current is near zero, except for leakage and the spike that occurs before the diode switches off?

I am inclined to believe the results on my scope, showing the reverse voltage limited to 0.6V, presumably because all of the high pass filtering from the various components' inductances and capacitances, yes?

Significant gotchas, maybe: In my real version, the PNP transistor of each Sziklai pair is a germanium PNP power transistor, Hfe probably on the order of 30. No germanium models exist in LTSpice; I've used the "generic" PNP model. (**) Also, the output of the circuit are two 400ma, 6V incandescent lamps, which I have simulated by 10 ohm resistors.

(*) I'm baffled why this circuit is so widely used as an example of how to generate badly formed square waves. It really belongs in a Horowitz and Hill "Bad Circuits" section :-)

(**) My Dad brought home some Ge power transistor samples from his job at TI when I was a wee lad in the early 60s. I thought I would finally do something interesting with them after all these years.