LTspice: triangle wave circuit not producing any waveform

Question

I've implemented this circuit in LTspice from this Triangle Wave Generator tutorial.

Unfortunately, when I run the simulation, it doesn't produce a wave at all, just a flat VDC output. Any help would be much appreciated as to why I'm not seeing any waveform at all.

Even with a higher supply voltage on the 747s this is the result:

Can you try with previous version of LTspice.I am also getting this error with latest version of Ltspice.I simulated couple of circuits always getting VDC at output — Hari, Mar 13 '24 at 17:28
741/747 should have a much higher rail voltage to operate properly - not sure how faithful the model you're using is — Neil_UK, Mar 13 '24 at 17:42
@stretch sorry, I caught the mistake but still the same result. Updated the picture above — Abstract, Mar 13 '24 at 17:47
You can't replace the pot with a single resistor like that if you want it to do anything. And use a more appropriate op-amp model for 5V. The problem isn't the simulator. — Spehro Pefhany, Mar 13 '24 at 17:51
Mandatory read on 741/747: https://electronics.stackexchange.com/questions/304521/reasons-not-to-use-a-741-op-amp — winny, Mar 13 '24 at 18:07

score 1 · Answer 1 · answered Mar 14 '24 at 04:45

This is what you should have built (the relevant parts), according to the site you linked:

schematic

^{simulate this circuit – Schematic created using CircuitLab}

This is what you actually built:

schematic

^{simulate this circuit}

The non-inverting inputs are held at +5V, which means the outputs of U1 and U2 will be permanently high.

I believe this is what you intended to achieve:

schematic

^{simulate this circuit}

You failed to reproduce the potentiometer correctly. This potentiometer is effectively two resistors in series, whose total resistance (in this case) is always 10kΩ, and whose wiper is exactly half way along the resistive track. In other words, two 5kΩ resistors end to end, with the wiper at their junction.

That will produce +2.5V at that junction, or half the supply voltage.

Also, don't use a 5V supply with a 741 (or 747). This model of op-amp's output is severely constrained, and you'll need at least 9V (I recommend 12V) for this to work well.

Tyassin · Answer 2 · 2024-03-13T19:29:55.703

0

You have used 5V on the input of U1(-) and U2(+). Either opamp will saturate. Use a voltage divider to make some 2.5V of the above inputs.

edited Mar 13 '24 at 19:29

answered Mar 13 '24 at 18:24

Tyassin

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1
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score 0 · Answer 3 · answered Mar 13 '24 at 18:49

The problem may be that the op amp is too perfect. LTSpice performs a DC analysis of a circuit first, before starting the transient analysis, and sets up your circuit so that the two (+) and (-) inputs of the op amps are at the same voltage. So the capacitor voltage across C2 never changes, and the output of U2 never changes.

Try placing an initial condition on capacitor voltage C2 to get the integrator going. In the real world, the integrator output will always move because of op amp errors.

score 0 · Answer 4 · answered Mar 16 '24 at 01:45

Not exactly an answer to your question, but comments on what you seem to want:

The output isn't predictable. A purchased 2N2222 has a range of current gains. If it works as tweeked with a pot or substitutions today, it might not tomorrow with the same transistor, and certainly would be less predictable if you tried to duplicate a success.

You're still not carefully following the circuit you found online. That's good somewhat. It's not worth it.

LTspice: triangle wave circuit not producing any waveform

4 Answers4