Cursed circuits #6: reverse avalanche oscillator

ssurprisetalk 5 days ago 20 commentsRead Article on lcamtuf.substack.com

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voidUpdate•about 9 hours ago

You can even use it to make a simple audio synth: https://www.lookmumnocomputer.com/simplest-oscillator

Agentlien•about 3 hours ago

This is amazing. Now I'm even more upset he scored so low on Eurovision.

mock-possum•about 8 hours ago

The man is a treasure

cryo32•about 9 hours ago

This isn't particularly cursed. I mean we've had avalanche pulse generators for well over half a century easily. It's just exploiting semiconductor characteristics they don't teach you in the usual garbage undergrad textbooks.

marshray•about 5 hours ago

Well, if it's not an advertised spec on the datasheet, and constrained in the direction you're using it, then all guarantees are off and it's on you to qualify the parts on a batch-by-batch basis.

I think that counts as "cursed" from a design-for-manufacturing perspective.

"After a production run of 12,000 units the TR-808 was no more." https://secretlifeofsynthesizers.com/the-strange-heart-of-th...

kazinator•about 4 hours ago

Behringer sells a clone of the TR-808 called RD-8. They must have solved that problem somehow. They went as far as cloning some vintage Roland silicon that cannot be sourced, so the transistor issue seems minor by comparison.

denotational•about 8 hours ago

> the usual garbage undergrad textbooks

Out of interest, please could you give some examples of textbooks you consider garbage, and some you consider not to be (undergrad or otherwise)?

cryo32•about 8 hours ago

Garbage -> Sedra and Smith particularly - hate it, anything which kicks you in the nuts right up front with Laplace and networks abstractions, anything from Pearson - have never seen a good one.

Good -> Razavi (Fundamentals of Microelectronics), Art of Electronics, most Jim Williams stuff (AN's and articles), Bowick RF Circuit design. They're actually useful.

SAI_Peregrinus•about 8 hours ago

I find Sedra/Smith a terrible introduction, but a good reference. It's nice once you've already built an intuition for how things work to be able to go back & build up the mathematical models, but trying to understand the behavior of circuits from the math first is a bad order.

xqb64•about 3 hours ago

What do you think about the author's upcoming book ("The Secret Life of Circuits")[0], for someone who is just starting out with electronics?

[0]: https://nostarch.com/secret-life-of-circuits

variadix•about 5 hours ago

Agreed on Sedra and Smith vs Razavi. Razavi uploaded lectures to YouTube that helped me a ton at the time.

jacquesm•about 3 hours ago

https://en.wikipedia.org/wiki/Pearson%E2%80%93Anson_effect

laughing_man•about 2 hours ago

I would never have thought to try this circuit for fear of letting out the transistor's magic smoke.

MarkusQ•about 6 hours ago

What would happen if you added a capacitor (to ground) on the base? Could you adjust the frequency?

adrian_b•about 4 hours ago

No. That capacitor would charge at a few volts, i.e. the sum between the voltage on a conducting LED and the voltage on a conducting silicon diode (the base-collector junction).

So the capacitor would initially divert a part of the discharge current from the LED, but later it would discharge itself through the LED. So it might make the current impulse through the LED smaller and wider. Such a capacitor would increase the probability that the transistor would be damaged by the periodic breakdowns, by extracting a big current pulse through the base. Depending on the transistor structure, a capacitor on the base could also stop the oscillations, because the current extracted through the base could eliminate the negative resistance that appears on the voltage-current characteristic in the breakdown region when the base is not connected or it is connected somewhere only through a big resistor.

The frequency is very easily adjustable without changing the schematics, by changing the value of the resistor that charges the capacitor, before the breakdown voltage is reached, or by changing the value of the capacitor.

The RC product determines the charging time, which constitutes most of the period of the pulses.

MarkusQ•about 4 hours ago

Thank you!

fnands•about 8 hours ago

Delightfully cursed.

I wonder how consistent the breakdown voltages are between manufacturers? I mean, I am sure there is some spec, but is it not just a minimum in this case?

adrian_b•about 4 hours ago

While the breakdown voltage of the base-collector junction varies wildly even between transistors of the same type, the breakdown voltage of the base-emitter junction varies very little between planar-epitaxial transistors, because in all such transistors the emitter has the maximum possible doping, which is limited by the solubility in silicon, and the base must also be much more strongly doped than the collector, but much less doped than the emitter, which limits the range of possible dopings. Because in this case the breakdown voltage of the base-emitter junction is determined mainly by the doping of the base, there is little variance between transistor types and between manufacturers.

Typical breakdown voltages for base-emitter junctions are e.g. around 10 volt, while the manufacturers specify breakdown voltages like 7 volt, to have a safety margin.

cryo32•about 6 hours ago

It's one of those characteristics you can't rely on unless you select / bin parts. Also depends on temperature. Which is why no one does this in a production design - it's expensive and unreliable.