How 'floaty' is a floating base, can siginificant C,E conduction occur? - Page 1
Forum section: electronics / projects

author: 741
author_rank: Frequent Contributor
timestamp: May 24, 2021, 08:59:35 pm
content: I know this is a loose kind of question, just interested to see what others think. As an arm-wavey opinion thing:Suppose a small bipolar transistor (BC546 say) has a 10k pull-down that falls off one day. The transistor is used as a switch and the 1k in series to base is high-impedance except when a push-button is pressed.It is on a PCB, no fast signals, nothing special happening, say a normal office envioronment. Let's say the circuit activates a 100mA resistive load from 10V to collector, and the track to the base is 2"/5cm long."What are the chances", as they say, the load sometimes gets significant turn-on (but the push-switch is off): say 10mA in the load, and if it might do, over what kind of time-scales?I am reasoning that there is no source of DC current into the base, and 'pickup' will not be able to drive the base close to 0.6V. I would be surprised if the load ever came on over 10mA in the above scenario. However yes, if I run down a carpet and touch the base something 'might give'.By contrast, it is relatively easy to charge the gate capacitance of a MOSFET with random 'pickup'.A lot hinges here on that word "pickup", whatever that is, wherever it originates.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: May 24, 2021, 09:06:54 pm
content: It'll be impulsive in nature, i.e. capacitive coupling. Also, note that with no DC path, the base rectifies noise, so a steady RF tone won't cause continuous conduction, at least until some return current is drawn by E-B breakdown, and that would be quite strong interference indeed (>7Vpp at the base, and with a wide capacitive divider between base and noise source, who knows... thousands of volts?).So, short of a nearby Tesla coil, it's probably limited to impulses, such as from ESD.How fast does the collector load respond? What happens under partial turn-on, does that register as a detection?Mind that, with no B-E resistor, the transistor basically turns off under its own recombination, which is probably a few to ten microseconds in the worst case. If the load doesn't respond in that time, yeah, who cares?Tim

author: TimFox
author_rank: Super Contributor
timestamp: May 24, 2021, 09:11:59 pm
content: The data sheet will show “Icbo”, which is the leakage current into the base from the collector (reverse biased diode). That current will bo amplified by “hFE”, the DC current gain from base current to collector current. These variables should answer your question for a floating base terminal. Note that the leakage current is exponential in junction temperature, but the current gain is a weak function of temperature. The usual base pull down resistor serves to reroute the leakage current away from the base-emitter junction, so long as the voltage developed across the resistor is below the base-emitter voltage required for substantial current (0.7 V or so).

author: T3sl4co1l
author_rank: Super Contributor
timestamp: May 25, 2021, 12:12:25 am
content: Neat aside: Icbo is so high in germanium transistors, that many times you neednegative base bias currentto stabilize e.g. an amplifier -- it's forward biased as usual (Vbe ~ 0.3V) but you're pulling current out of it just to keep it there.Same is true in silicon at proportionally higher temperatures, though I think this is over 200C? so you'd rarely if ever approach it with commercial devices.So, for silicon at room temperature, compared to Ic ~ 100mA, probably not much, but if you're including at high temp, or counting much of any current flow as a detection, absolutely.Tim

author: 741
author_rank: Frequent Contributor
timestamp: May 25, 2021, 08:04:33 am
content: I looked up Icbo for the BC546. It's 15nA. hFE has massive spread, 110 to 800. Still that is only 10uA Ic at worst.

author: Zero999
author_rank: Super Contributor
timestamp: May 25, 2021, 08:44:08 am
content: I'd say it's highly unlikely the BJT will turn on the load, when the switch is open. Perhaps if the switch is on a long cable, there's some chance it could pick up noise and briefly pulse on, but the cores in the cable run parallel, which will mitigate that to some extent.

author: TimFox
author_rank: Super Contributor
timestamp: May 25, 2021, 01:17:02 pm
content: Quote from: 741 on May 25, 2021, 08:04:33 amYes, that's the process for answering the question, but look at the temperature dependence of Icbo if you have to deal with elevated temperature.

author: bson
author_rank: Unknown
timestamp: May 25, 2021, 05:18:53 pm
content: Quote from: 741 on May 24, 2021, 08:59:35 pmMurphy just told me if this is a bad thing, it will happen. If it's a good thing it won't. Time frame will be the worst possible, definitely when left unattended.

author: Doctorandus_P
author_rank: Super Contributor
timestamp: May 26, 2021, 12:28:37 pm
content: It depends a lot on the wiring between the switch and the base of the transistor. A 5cm long antenna is not so long, so it won't pick up too much noise, but it also depends on shielding around the antenna.It's a fun experiment to build a darlington with two or even with tree transistors on a breadboard, add an antenna (just a piece of wire) and move it around to detect electrical fields. Add a LED with series resistor in the collector so you see it when the transistor turns on.If I touch the probe of my scope probe, I can easily pick up 60Vac of noise, and this same noise can also open a transistor. Small BJT's also have an Hfe varying between about 150 and 900 and this also plays a big role in how much noise is needed to open it "sufficiently".

author: exe
author_rank: Unknown
timestamp: May 26, 2021, 12:38:19 pm
content: Just in case, there are transistors with integrated resistors. May be it will be cheaper and easier to use in the circuit. They are called "prebiased" and "digital" transistor, and available in several configurations.I think I've seen a suggestion to keep the resistor from base to ground because otherwise base leakage current would be multiplied by hfe. Although, I'm not sure what "leakage current" means in this context. Is it current from collector to base? Or, say, thermionic emission of B-E junction? (is this even a thing?)

author: 741
author_rank: Frequent Contributor
timestamp: May 26, 2021, 12:59:06 pm
content: That is a neat idea, thank you!eg DTA143XUA (PNP) or DTC113ZUA (NPN)ROHM do an comprehensive list of options for input and pullup/down.

author: Zero999
author_rank: Super Contributor
timestamp: May 26, 2021, 01:13:21 pm
content: Quote from: exe on May 26, 2021, 12:38:19 pmIt means the leakage between the collector to base, would be multiplied by the hFEand turn it on slightly.I don't know what you mean by themionic emission, which sounds like it's something applicable to vacuum tubes, rather than solid state devices.

author: 741
author_rank: Frequent Contributor
timestamp: May 26, 2021, 04:20:31 pm
content: The D.S. (e.g for DTC113ZUA (NPN)) says this can be used as an inverter "without connecting exernal resistors".I don't see it: It's open-collector, surely I'd have to add a pull-up?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: May 26, 2021, 07:53:04 pm
content: There are open-collector inverters. Where your output gets the pull-up from, is up to you...Tim