1 transistor RESET oscillates at 100s MHz. Why? (Source: Bob Pease). - Page 1
Forum section: electronics / projects

author: 741
author_rank: Frequent Contributor
timestamp: January 06, 2025, 09:36:07 pm
content: A "microprocessor RESET" circuit that actually oscillated in practice. Shown in"Troubleshooting Analog Circuits" by Bob Pease, p109"The little transistor would run at over 10mA and, with a bypass capacitor at its base, the transistor would oscillate" [at a "couple hundred megahertz"](The simulation does not oscillate).What is the mechanism of the oscillation (maybe akin to a phase-shift oscillator?) - do I need to add any real-world parasitics to the simulation? Maybe just a current kick / impulse?

author: tggzzz
author_rank: Super Contributor
timestamp: January 06, 2025, 10:36:53 pm
content: Quote from: 741 on January 06, 2025, 09:36:07 pmSchematic in form viewable in a browser?Photo of construction technique?Without knowing those, one guess is shown athttps://entertaininghacks.wordpress.com/2024/03/16/practical-traps-with-a-one-transistor-audio-amplifier-solderless-breadboards-and-oscilloscopes/which shows-a one transistor audio amplifier using jellybean components, an audio function generator, a voltmeter and an oscilloscope-how a gross problem is easily missed, not only with simulation but also experimentally-what’s necessary to reveal the problem-a subtle variation in the way the problem manifests itself-common tools showing differing measurement results-the problem’s cause, and how it can be verified with adequate simulation-professional products which rely on such “problems” for their basic operation

author: Xena E
author_rank: Frequent Contributor
timestamp: January 06, 2025, 10:46:21 pm
content: Heres the circuit the op was referring to.

author: moffy
author_rank: Super Contributor
timestamp: January 06, 2025, 10:55:43 pm
content: I managed to get it to oscillate at about 350MHz with some minor tweaks, one of the important things for SPICE is to reduce the timestep, otherwise it just steps over any oscillation.

author: tggzzz
author_rank: Super Contributor
timestamp: January 06, 2025, 11:07:12 pm
content: Quote from: Xena E on January 06, 2025, 10:46:21 pmI suspected as muchWe don't know the physical implementation, but I'll bet the cause and resolution is given in the article I referenced.

author: Benta
author_rank: Super Contributor
timestamp: January 06, 2025, 11:25:01 pm
content: Yeah, emitter-followers aren't as harmless as they look. I've experienced this enough when making constant-current sources for LEDs.The trick is to include a base resistor, that normally stops the behaviour.

author: Analog Kid
author_rank: Super Contributor
timestamp: January 06, 2025, 11:32:04 pm
content: Quote from: Benta on January 06, 2025, 11:25:01 pmSo in general, what value of "stopper" resistor ought to be used? (Assuming a small non-power transistor.)Something in the ballpark of 100Ω or less?

author: moffy
author_rank: Super Contributor
timestamp: January 06, 2025, 11:43:26 pm
content: Quote from: Analog Kid on January 06, 2025, 11:32:04 pmIt's like the gate resistor applied to MOSFETS, it should be as near to the base as possible and 100R would be fine in this case maybe even as low as 10R could work but it is a practical decision based on the circuit parasitics, in the case of the sim I posted 10R works.

author: tggzzz
author_rank: Super Contributor
timestamp: January 06, 2025, 11:52:58 pm
content: Quote from: Benta on January 06, 2025, 11:25:01 pmOr a ferrite.Or have decent layout/construction techniques, based around an understanding of what is happening and why.IMHO understanding is preferable to suck-it-and-see coupled with "it worked with last year's batch of transistors". Replacing original 1980s 2N3055s with modern 2N3055s anecdotally enables them to oscillate in some cases. Presumed cause: a much higher fTthan the specified 2.5HMz min.

author: TimFox
author_rank: Super Contributor
timestamp: January 07, 2025, 12:00:53 am
content: Quote from: Benta on January 06, 2025, 11:25:01 pmA fundamental issue with emitter followers is their operation into a capacitive load.(This is also true with cathode followers and source followers.)The base (grid, gate) stopper spoils the negative resistance component that appears at its input causing oscillation: the full diagram including parasitic capacitances resembles a Colpitts oscillator.

author: tggzzz
author_rank: Super Contributor
timestamp: January 07, 2025, 12:04:31 am
content: Quote from: TimFox on January 07, 2025, 12:00:53 amI.e. operating as a common-base amplifier with lots of voltage gain (cf the presumed common emitter amplifier with voltage gain <1)

author: vk6zgo
author_rank: Super Contributor
timestamp: January 07, 2025, 12:26:43 am
content: Quote from: 741 on January 06, 2025, 09:36:07 pmSimulators "lie in their teeth".Unless they are provided with typical values of stray inductance & capacitance, they assume both Ls & Cs are zero. The real world is very different, & common values of both may well cause VHF oscillation.

author: Geoff-AU
author_rank: Frequent Contributor
timestamp: January 07, 2025, 12:54:05 am
content: Quote from: vk6zgo on January 07, 2025, 12:26:43 amI disagree. A model is an approximation of reality, if it doesn't match reality then your approximation is too coarse and you need to do as you say (consider what else in your circuit might matter, eg parasitics). See also: GIGO theory (garbage in, garbage out). A simulator is just a tool, it's up to the user to use it appropriately.

author: Benta
author_rank: Super Contributor
timestamp: January 07, 2025, 01:13:28 am
content: Quote from: vk6zgo on January 07, 2025, 12:26:43 amYou underestimate simulators.Fed with the right models (including parasitics), they provide excellent results, but an engineering brain has to accompany them.Ngspice was actually the reason I found out why a constant-current source was misbehaving, and led me to the dangers of the common-emitter circuit.

author: moffy
author_rank: Super Contributor
timestamp: January 07, 2025, 01:43:34 am
content: Quote from: vk6zgo on January 07, 2025, 12:26:43 amBob Pease certainly agreed with your sentiment as he threw his computer from the roof of the building, but they have very practical uses if you accept the limitations.

author: amyk
author_rank: Super Contributor
timestamp: January 07, 2025, 01:57:27 am
content: If you add in the parasitics, there can be enough feedback to cause oscillation.Also reminds me of this thread:https://www.eevblog.com/forum/projects/challenge-thread-the-fastest-breadboard-oscillator-on-the-mudball/

author: temperance
author_rank: Super Contributor
timestamp: January 07, 2025, 02:35:16 am
content: A class of transistors also known as Low VCE-SAT transistors meant for switching circuits are excellent oscillators when used in emitter followers.An example here by Dave. At around 6 minutes into this video the output stage transistor is replaced by a FZT849 and Dave wrongly assumes that the compensation around the op amp needs adjustment.Most of the comments state the same and trow wild values around for the compensation circuit while it is not the op amp which is oscillating but the unfortunate choice of transistor which makes the output stage a good oscillator and the op amp is trying to "compensate" the oscillating output stage.What is needed is a resistor in series with the FZT849 base to compensate for the negative input resistance caused by a more often than not invisible capacitive load such that input resistance stays positive. Finding out how large this resistor must be is the difficult part.

author: 741
author_rank: Frequent Contributor
timestamp: January 07, 2025, 07:48:19 am
content: Here is an image of the LTSpice circuit as requested (I thought maybe it would be disallowed to post a photo of the page in the book).Also shown is the circuit of Reply #3, which does oscillate.

author: tggzzz
author_rank: Super Contributor
timestamp: January 07, 2025, 12:10:36 pm
content: Quote from: vk6zgo on January 07, 2025, 12:26:43 amEverybody should read learn and inwardly digest the aphorism "all models are wrong, but some are useful".Too many think "X true because the computer says so". (I use the word "think" loosely, of course).

author: harerod
author_rank: Frequent Contributor
timestamp: January 07, 2025, 01:04:06 pm
content: Quote from: tggzzz on January 07, 2025, 12:10:36 pmI'd suggest: Too many believe "X true because the computer says so".A professional needs the ability to make rough estimates and checking the plausibility of a calculator result. The extend of a "rough estimate" varies. When I design a circuit, this may well be equations and transforms that span several pages. For stuff like that I combine a pencil, sheets of paper and the computer (LTspice and Excel work surprisingly well for me).Incidentally: Bob Pease suggests a remedy (series base resitor) on the very same page, above the diagram. However, he leaves the detailed analysis to the reader.

author: temperance
author_rank: Super Contributor
timestamp: January 07, 2025, 02:34:46 pm
content: Quote from: harerod on January 07, 2025, 01:04:06 pmExcept that this case is not a matter of simple calculations and rough estimates. It requires good insight into analog circuit design to identify such unintentional invisible oscillators as I stated before in my earlier post with a nice example.

author: tggzzz
author_rank: Super Contributor
timestamp: January 07, 2025, 03:14:27 pm
content: Quote from: temperance on January 07, 2025, 02:34:46 pmAs I stated (but was snipped before your response) "Everybody should read learn and inwardly digest the aphorism "all models are wrong, but some are useful"."Design requirestheoretical understanding. Without that it is blind fumbling and/or cargo-cult engineeringmodels of the system being consideredunderstanding what the modelsdon'taddress. Modelscannotaddress everything, because by definition they are a simplification of realitypractical and theoretical understanding of the extent to which the system being considered is not validexperiment to validate all the aboverepeat all the above when a discrepancy is foundIn the case of a circuit that is presumed to be common collector amplifiers (i.e. emitter followers) with voltage gain <1, parasitic components can cause it to act as a common-base amplifier with voltage gain >1.

author: iMo
author_rank: Super Contributor
timestamp: January 07, 2025, 04:21:58 pm
content: Still missing perhaps a dozen of parasitics there..159MHz..PS: 100mm long straight wire is aprox 100nH..

author: temperance
author_rank: Super Contributor
timestamp: January 07, 2025, 04:29:16 pm
content: Quote

author: moffy
author_rank: Super Contributor
timestamp: January 07, 2025, 11:20:13 pm
content: Quote from: iMo on January 07, 2025, 04:21:58 pmNice.

author: tggzzz
author_rank: Super Contributor
timestamp: January 07, 2025, 11:33:17 pm
content: Quote from: moffy on January 07, 2025, 11:20:13 pmParticularly relevant w.r.t. easy to (mis)use dupont flying leads on solderless breadboards.The 1nH/mm rule of thumb has interesting consequences w.r.t. SMD components at RF, and 6inch scope probe grounds with a tip capacitance of 15pF.

author: Benta
author_rank: Super Contributor
timestamp: January 08, 2025, 12:05:11 am
content: Quote from: tggzzz on January 07, 2025, 11:33:17 pmThat rule-of-thumb can be thrown in the garbage can.It applies to inductive loops that have been sectioned into to 1 mm pieces. It has zero significance to this thread.

author: tggzzz
author_rank: Super Contributor
timestamp: January 08, 2025, 12:33:39 am
content: Quote from: Benta on January 08, 2025, 12:05:11 amSingle straight wire in free space. The formulae are easily found, as at calculators.Loops have a higher inductance.

author: Benta
author_rank: Super Contributor
timestamp: January 08, 2025, 10:42:59 pm
content: Quote from: tggzzz on January 08, 2025, 12:33:39 amI think you've forgotten wire radius. I don't believe a 1 mm thick, 100 mm diameter disc will confirm to your formula, although it will be a 1 mm long piece of wire.

author: tggzzz
author_rank: Super Contributor
timestamp: January 08, 2025, 11:20:32 pm
content: Quote from: Benta on January 08, 2025, 10:42:59 pmThat's changing the subject away from loop vs straight wire.1nH/mm of wire is arule of thumb, i.e. a simple encapsulation of common circumstances useful for indicating whether effects might need to be considered in more detail.Does anybody other than you think that 10cm disk 1mm thick constitutes a wire?It would be difficult to find a 10cm diameter conductor for any high power distribution system, let alone for low power electronics. And especially SMD components and scope probes!

author: David Hess
author_rank: Super Contributor
timestamp: January 09, 2025, 12:17:11 am
content: Here is the paper Jim Williams cited which discusses emitter follower oscillation.

author: Xena E
author_rank: Frequent Contributor
timestamp: January 09, 2025, 02:48:04 am
content: Quote from: tggzzz on January 08, 2025, 11:20:32 pmCould be modeled, but gradients of resistance and inductance across the plane of the disc would dominate, similar to dead bugging a uhf circuit on a ground plane, the return through that has to be taken into account.Common sense should be applied that the wire would have to be Length >> diameter in any "the rule of thumb".

author: Benta
author_rank: Super Contributor
timestamp: January 09, 2025, 11:12:33 pm
content: Quote from: David Hess on January 09, 2025, 12:17:11 am@David Hess, thanks for digging out this paper.