Astable Multivibrator Generates Sub-Ground Voltages

Hi and thanks. I've just double-checked the simulation and it's actually the *bases* of both devices that go negative to -4.3V rather than the collectors, so my apologies. But the question still remains about how *any* node in such a circuit can go below GND. I will post the schematic if the above correction still doesn't account for what I'm seeing.

-J.

Hi and thanks. Yes, it's the bases that are going negative. I'm not sure why you mention "breakdown voltages" since we're only talking about a Vcc of 5V and I know of no transistor that breaks down at such a low reverse Vbe., nor why a spice model should fail to take account of it if such should be the case. It seems like a pretty significant flaw in the model if so. BTW, in answer to Mr. Thompson's question, I'm using LT Spice.

Which "Spice"? Can you post a schematic, either ASCII, or a real schematic (preferred) to alt.binaries.schematics.electronic ?

...Jim Thompson

Okay, thanks for that. So it appears it's either a model deficiency or real-world capacitive effect. I need to know which, so here's the netlist of the circuit concerned and I invite comments on it:

"ExpressPCB Netlist" "SwCAD III Version 2.16i"

"Net Names Table" "N005" 1 "N003" 5 "0" 8 "N002" 12 "N004" 15 "N001" 18

"Net Connections Table"

When you capacitively couple two stages, the coupling capacitor will charge up to the power supply minus the base voltage. When the transistor collector goes to ground, you now have a charged capacitor whose positive end is near zero volts. Thus the other end will be nearly the supply voltage below ground. The capacitor will then discharge and charge up in the other direction to the difference of the collector saturation voltage and the base voltage.

Which reminds me, I've ran such a circuit at 18V before with no problems. .

Tim

-- Deep Fryer: a very philosophical monk. Website:

Capacitive coupling?

...Jim Thompson

i guess that would depend on where the test points are? you maybe getting a - reading but maybe your testpoints are between to area's that are reverting the polarity ? the only other thing i could think of is that you are not testing with DC but maybe via a cap that could generate - voltages.

sounds like the Cap Discharge effect.

Jeff,

I wouldn't expect the collectors to go beyond the supply rails, but the bases will go negative. Since the B-E breakdown isn't usually modeled in SPICE, this can give some un-realistic waveforms in simulations.

--Mike

Jeff,

It probably won't breakdown at 5V. But it might around 6. Usually transistor breakdown isn't modeled because the normal design flow is to simulate to find what the peak voltages are and then buy/fab a transistor that won't breakdown in the circuit. If the transistor breaks down in simulation, then you don't know what kind of transistor you need to buy/fab. Transistors usually aren't used in a manner that includes breakdown, because that has a permanent effect on the device (though the effect from B-E breakdown is not too dramatic). But basically, if you simulate with transistors that breakdown in simulation, you might want to think for a second and make sure your not designing the circuit to fail.

LTspice comes with an example astable circuit typically installed as C:\\Program Files\\LTC\\SwCADIII\\examples\\Educational\\astable.asc

--Mike

It's real. Stop thinking about your simulator for a minute. If you look at an astable multivibrator and analyze it, you will see. You have a charged capacitor and suddenly you pull the high end of the capacitor to ground. Well, obviously the low end of the capacitor will go to a potential that's negative with respect to what you are considering "ground," which by the way is only a reference, not a barrier. You could analyze the circuit with a positive "ground" at Vcc, all the potentials would be negative with respect to that. I suspect that it wouldn't bother you that the negative terminal of the cap flies up and down, since it never crosses Vcc "ground" in doing so.

Okay, Fred and Kell. Some really eye-opening and helpful ways of re-examining the 'problem' (such as it was) for which I'm most appreciative. Thanks!

Consider the circuit below:

o------o------. +V | | | | o S1 |=|>

| o | ----------o------------ | | | | | | | .-. | | | | | | | |R1 | | '-' | | | | | | | | | o | .-. | |=| S2 | | | o | | |R2 --- | '-' --- C | | | | | | | '---------o | | | | | V D | - | | | o--------------o-----------o--. 0V

R2 >> R1

Close S1 until C has charged to +V, minus the forward drop of D

Open S1 and close S2

C cannot discharge instantly, only at a rate set by C(R1+R2). Its upper terminal is now connected to 0V, hence its lower terminal must be negative wrt 0V. D is now reverse biased.

We now have a decaying negative voltage at the junction of C and D anode. The rate of decay is set by C(R1+R2)

That's how you get nodes going below 0V

That's normal and does happen in the real world.

The culpret is the capacitor attached to the base, when Q1 switches on the other end of the capacitor does from somewhere close to the supply voltage down to near gound, this voltage change is mirrored at the other end of the capacitor pushing the Q2 base below ground.

Bye. Jasen