Timeline for Why does this two NOT circuit oscillate?
Current License: CC BY-SA 3.0
23 events
| when toggle format | what | by | license | comment | |
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| Jun 26, 2017 at 7:44 | history | tweeted | twitter.com/StackElectronix/status/879243984408907777 | ||
| Jun 25, 2017 at 22:32 | comment | added | user84299 | analogsystemsrf. Thanks for the reply, I did know that the circuit was an astable oscillator (I should have been clearer in the question). However the Wikipedia article on the multivibrator, which I am aware or, doesn't help in this case. It gives a bird's eye view of the concept with the classic transistor mutlivibrator as a specific case. An explanation in terms of the actual implementation I posted is what I was after. This is why I asked the question, but no one seems to know. However I figured it out in the end. | |
| Jun 25, 2017 at 22:20 | comment | added | user84299 | In summary, the best answer I got was do my own research and that's the advice I will give in response to my own question. I could give the answer in all its gory detail, but in the long run it’s better to figure it out yourself although it might take some time. The comparison with a transistor multivibrator turns out to a red herring. True in the big picture the feedback pattern is similar but the implementation is different. Note there are no explicit resistors in the circuit diagram. | |
| Jun 25, 2017 at 22:13 | comment | added | user84299 | Neil_UK: Yes I learned lots more by figuring it out myself, In total I probably spent 6 to 12 hours studying, 6 before I asked the question and another 6 afterwards. I didn't find any explanation on the internet (and I searched....) but instead I found a detailed explanation in an obscure 60s article at the library. I am familiar with astable oscillators but the details of the one I posted had some subtle implemenation issues which I struggled with. I knew it was a long shot to ask stackoverflow as it's is a bit of an obscure implementation. | |
| Jun 24, 2017 at 14:51 | answer | added | Ale..chenski | timeline score: 3 | |
| Jun 24, 2017 at 4:36 | comment | added | Neil_UK | Did you learn more by nutting it out yourself, or being spoon-fed the answer? The question needs an answer. You're now in a position to post your own reply, that's permitted and indeed encouraged on this site. Don't forget to draw the parallel with multivibrator above. Will this circuit work with other logic gate families? I am an engineer with 40y experience, and while I know the multivibrator circuit well, it had not occurred to me to use TTL NOTs for the transistors, so I've learnt something from your question as well, +1. | |
| Jun 24, 2017 at 4:33 | comment | added | analogsystemsrf | @ rody Using bipolars, with base resistors, this is the classic astable oscillator. See Figure 1 on this link en.wikipedia.org/wiki/Multivibrator | |
| Jun 24, 2017 at 3:44 | comment | added | user84299 | I think I figured it out, its very subtle, not at all obvious. Thanks for the advice, more research did infact succeed. It's not a circuit we commonly see today so the books don't tend to mention it. | |
| Jun 23, 2017 at 23:40 | history | edited | user84299 | CC BY-SA 3.0 |
Added circuitlab simulation
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| Jun 23, 2017 at 23:37 | comment | added | user84299 | Change again, turns out the resistors weren't important, I took the resistors out and it still oscillated. Perhaps I had an invisible break somwhere than prevented a time course simulation. The original problem remains however, why does it oscillate. And no comments like 'because it has a positivefeedback. Let's be cosntructive, please provide more information, eg where is the positive feedback or better still a walk through of the events that result in an oscillation. | |
| Jun 23, 2017 at 23:03 | comment | added | user84299 | Eugene: That didn't help but when I put a 10/100K resistor around each gate then it did oscillate. I see if I can figure out why it oscillates. | |
| Jun 23, 2017 at 22:59 | comment | added | user84299 | ok I'll try it. | |
| Jun 23, 2017 at 22:51 | comment | added | Eugene Sh. | Between the input and the ground. | |
| Jun 23, 2017 at 22:37 | comment | added | user84299 | Ali Chen: Perhaps you could enlighten us by identifying where the positive feedback is. Note that two NOT gates connected like this without capacitors will not oscillate. My take is this. Assume all capacitors are initially discharged. Assume the output of A is high, this charges the capacitor next to it which slowly raises the voltage to the point where gate B flips from high to low, this does not cause the second capacitor to charge which keeps the input at gate A low. In order words we've returned to exactly the same state, no oscillation. Have I got the capacitor charging wrong? | |
| Jun 23, 2017 at 19:28 | comment | added | Ale..chenski | The circuit oscillates because it has positive feedback. What is the problem? | |
| Jun 23, 2017 at 19:15 | comment | added | Eugene Sh. | It won't oscillate for ideal components. Try redrawing the gates with some finite input impedance. | |
| Jun 23, 2017 at 19:05 | comment | added | user84299 | I tried again with circuitlab, reduced the time to a much smaller end point, this time it crashed. I did a dc static simulation, all nodes are at 5 volts. Don't think this helps. | |
| Jun 23, 2017 at 18:57 | comment | added | user84299 | I'd like to add that I've done a lot of reasearch on this problem. Most of the gate oscillators I understand including the negative feedback and relaxation oscillators. This one has stumped me. If stackoverflow can't help I'll talk to an electrical engineer at the local university. | |
| Jun 23, 2017 at 18:56 | comment | added | user84299 | I tried to simulate it on circuitlab but it would take almost three hours to simulate 1 second of model time. See if I can do something about that. | |
| Jun 23, 2017 at 18:48 | comment | added | Eugene Sh. | We encourage to exhaust the own research capabilities before asking a question. You haven't done with yours yet. | |
| Jun 23, 2017 at 18:45 | comment | added | user84299 | I will, but from your comment I presume you don't know how it works either? | |
| Jun 23, 2017 at 18:22 | comment | added | Eugene Sh. | I havn't tried simulating it on CircuitLab yet - Try it then. | |
| Jun 23, 2017 at 18:11 | history | asked | user84299 | CC BY-SA 3.0 |