Transistors (BJT, J & MOS -fet's) and their Circles in schematics. - Page 1
Forum section: electronics / general

author: Doctorandus_P
author_rank: Super Contributor
timestamp: February 03, 2019, 06:17:28 pm
content: In the schematics progam I use (KiCad) all transistors seem to become standarized with circles around them, and I don't care much for those. It remind's me of conventions which I thought were abandoned 40+ years ago and they look archaic to me.So now I'm wondering:1). What Schematics / PCB packages do you know / use.2). Is there a convention for these circles and how are the transistors drawn.3). What is your personal preference about these circles.

author: glarsson
author_rank: Frequent Contributor
timestamp: February 03, 2019, 06:42:44 pm
content: The circles are there so we can tell the difference between a dual transistor and two individual transistors, between a darlington transistor and two individual transistors in a darlington configuration, a monolithic transistor or a bunch of individual transistors, e.tc.

author: SiliconWizard
author_rank: Super Contributor
timestamp: February 03, 2019, 06:55:46 pm
content: Yes, additionally, at least IMO, the circles surrounding transistors are there to symbolize discrete transistors as opposed to functional transistors (on theoretical schematics) or integrated transistors (in IC design). I've worked on both discrete electronics and microelectronics, and seeing non-circled transistors on schematics will always tend to not look right to me if they are meant to be discrete. Just my opinion of course.

author: boB
author_rank: Frequent Contributor
timestamp: February 03, 2019, 06:58:35 pm
content: Maybe you could start a trend by surrounding them by squares instead of circles !

author: free_electron
author_rank: Super Contributor
timestamp: February 07, 2019, 01:37:40 am
content: Circles ? how about big fat oblongs ?On mosfets is put the gate closer to the source , so it is immediately obvious where that thing is.Many mosfets have multiple pins.Even simple bipolars can have 4 pins like for a sot89 or sot223 package.Even though many cad packages allow duplicate pin numbers ( and pad numbers) : NEVER EVER do that ! it causes big problems in the derived netlist when you want to test such a board . The flying probe testers and finished board testers throw a fit as they can't resolve the pads . Same if you ever have to send such a board through an external router like SPECCTRA.Same for interacting with field solvers like CST or AnsysSame for manufacturing shop floor and reliability tools like Valor and Sherlock.Many external programs can't cope with that because they read information based on pin number.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: February 07, 2019, 05:02:34 am
content: Quote from: free_electron on February 07, 2019, 01:37:40 amFYI, I've never seen a problem related to pin naming from Valor. Test Expert can give odd results though.1. Regarding symbols, the use of a bounding shape is ancient, originating with the physical glass envelope of a vacuum tube. Multi-part tubes would be drawn wide (all parts together), or with an open side (that side pointing in the cardinal direction of its companion).This carried over to multi-part semiconductors, in a common package. Single transistors, duals and such went inside a single circle, or were drawn separately when necessary; complex ICs went inside more abstract, meaningful shapes like amplifiers and logic gates, or defaulted to generic rectangles, probably reflecting the often rectangular form factor of the part itself.IC designers continued with this design style. Many internal/equivalent schematics show transistors without circles, because indeed, they're all common to one package! This also serves as a shorthand to limit visual clutter -- a big deal with designs regularly pushing 50+ transistors. Likewise the use of multi-(collector/emitter) transistors, or showing diode-strapped transistors as mere diodes. These symbols, typically used in familiar structures such as current mirrors and bandgap references, help indicate meaning to the reader.Likewise for CMOS, the substrate connection is default VSS/VDD, so it's often omitted entirely; this would otherwise remove the channel polarity indication, so the gate is modified to show a straight electrode (N-channel) or a complemented one (inversion circle, P-channel).So as you can see, there is a language behind these representations. It's regrettably not well known, but to those that understand it, it's clear and meaningful.And so, the less said about the "MOSFET with an emitter" symbol, the better...2. My preference -- I use a yellow-tinted enclosure for integrated circuits (adopting Altium's tradition of yellow boxes for things), and a blue enclosure for discrete components. Simple example:https://www.seventransistorlabs.com/Images/AltiumDiscreteFlybackSch.pngNote that TL431 is an IC, so it's yellow.It's a bit of an interesting case, really, because it's very transistor-like in operation, and most definitely not zener-like. I begrudgingly use that symbol, only because it's been used for the last generation+, and therefore has adopted its meaning by force rather than by description.Another example:https://www.seventransistorlabs.com/Images/555%20Boost.pdfIncidentally, I don't encircle diodes; that would be the more correct step, but I think it adds more clutter than it's worth. LEDs maybe -- they have more function than a regular diode and may be worth drawing the visual attention (they are, after all, literallyshiny... ooh shiny!). Historically, diodes have been in and out of circles; the vacuum tube variety of course always in one, but the semiconductor kind has had a rich history. Consider cats-whisker diodes -- no enclosure there. Copper-oxide and selenium rectifiers, maybe, but they're still pretty open, so.. meh?I could go either way on the solid-filled versus hollow diode; I've went with solid. You most definitely wouldn't draw an open triangle with a straight line through it! But I've seen that before...Tim

author: Chris56000
author_rank: Frequent Contributor
timestamp: December 30, 2020, 10:04:42 pm
content: Hi!I'll add that in discrete multi–transistor packages, e.g., CA3046 etc., I find it preferred practice to show them as individual discrete devices with an envelope representation, as this is the most convenient way of drawing the circuit diagrams – and annotate e, b and c with the package pin number, but instead of using "TR" or "Q" as the prefix, I number them UCxA to ICxF etc., then add a key to the device at the bottom of the circuit diagrams.When it comes to analogue and linear i.c. devices where a block diagram is provided on the datasheet, I redraw the block diagram to match the schematic grid and leave space just inside the the rectangular outline for the pin designations.None of Altium's yukky yellow boxes for me!As to discrete diodes, the horrible i.e.c. forced their grotty single–line representation of everything on us, including diodes with the line through them, a counter–intuitive symbol of ever I saw one!As a rule I draft my diagram symbols to BS3939: 1966 (or 1975 for newer ones) but have retained a lot of much older ones as I've always thought the original BS530 symbols gave a far better idea of what the component does than the modern symbols do - e.g., a thermistor is depicted as a rectangular resistor symbol with a black or white spot in it – Rank/Toshiba used this technique on their TV circuit diagrams for years!Chris Williams