BJT transistor acting weirdly - Page 1
Forum section: electronics / beginners

author: bmxsesh
author_rank: Regular Contributor
timestamp: July 29, 2024, 09:22:08 am
content: I am trying to interface a Toshiba 2SC4250 npn bjt transistor. I am going to interface it with the CCD sensor and it acts as a current amplifier on the CCD output. In the datasheet the gain is specified for 10V but in this circuit it is powered at 15V so I am trying to get the gain value. It is a very small package that I was able to solder with some tiny wires to 2.54mm breakout board.Now I apply +15V from power supply to C terminal which is number 3, and power supply ground through the 4.7Kohm resistor to E terminal which is number 2 in the package.Base is left hanging, this is where I would like to apply a different voltage from the same power supply (it is a 2 channel 0-30V 0-5A supply). Now I measure voltage at the base with the scope and it says +15V as if it is shorted with C.Emitter voltage is also +15V as if it was shorted with C. B-E voltage is 0 and not 0.7V or anything like that.Did I fry the transistor by soldering or maybe some flux residue is conducting? Cannot get a grasp how are all three terminals shorted.Neither do I see that I exceeded any of the absolute maximum ratings, perhaps maybe except the maximum power dissipation in the collector which I've seen yesterday around 200-300mW. I do use the current limiting at 30-50mA for the +15V supply as per specs.

author: Benta
author_rank: Super Contributor
timestamp: July 29, 2024, 10:51:28 am
content: In your schematic, it looks like C and E are shorted?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: July 29, 2024, 11:36:48 am
content: It could be damaged from ESD, cross-wiring or slipped probes. But a device of this speed isn't exactly useful on a breakout board, even if undamaged and biased properly. An oscillator is far more likely.You can carve a microstrip amplifier circuit out of two-sided copper clad, using a utility blade to cut outlines of traces, or a rotary cutter tool.Tim

author: tggzzz
author_rank: Super Contributor
timestamp: July 29, 2024, 12:21:13 pm
content: Show a photo of your construction.What happens if the base is not left open, but connected to 0V?1.4GHz BJT, emitter follower, breakout board... They all make me think of this:https://entertaininghacks.wordpress.com/2024/03/16/practical-traps-with-a-one-transistor-audio-amplifier-solderless-breadboards-and-oscilloscopes/Oscillation plus inductance might lead to some of the transistor parameters being exceeded.

author: bmxsesh
author_rank: Regular Contributor
timestamp: July 29, 2024, 12:27:31 pm
content: Quote from: T3sl4co1l on July 29, 2024, 11:36:48 amYep, just went to check at my local store and two of four breakouts were not properly soldered and have short circuits between the terminals.Quote from: Benta on July 29, 2024, 10:51:28 amIt is from the Sony ICX415AQ CCD image sensor datasheet. That's a good point, I don't actually know what they mean by that. Here, too, MOSFET in this schematic which I guess is a simplified schematic of on-chip circuitry of the CCD, doesn't show whether it's a pMOS or nMOSFET.Quote from: tggzzz on July 29, 2024, 12:21:13 pmThese are two that the multimeter didn't find any short circuits in. All the voltages that are being applied are DC 0-15V so far. I think the other mistake I've made is that the '-' terminals of two channel power supply may be different grounds and if I am trying to have two power supplies in one circuit I have to make sure C supply and B supply have to have common ground and I'm not sure if they are electrically the same thing within the power supply.-Max

author: tggzzz
author_rank: Super Contributor
timestamp: July 29, 2024, 12:35:35 pm
content: You also need to show the inductors and capacitors that you will use to connect that transistor to the rest of your circuit. i.e. 1nH for every mm of wire length, plus a few pF depending on how long and close together the wires are. If you ignore that, you can see this kind of phenomenon:https://entertaininghacks.wordpress.com/2024/03/16/practical-traps-with-a-one-transistor-audio-amplifier-solderless-breadboards-and-oscilloscopes/

author: bmxsesh
author_rank: Regular Contributor
timestamp: July 29, 2024, 12:51:25 pm
content: Quote from: tggzzz on July 29, 2024, 12:35:35 pmThe rest of the circuit will be the one described in this article I am attaching in this post. This whole construction as far as I understand will be the emitter-follower of this bjt.Article:https://vintagetek.org/wp-content/uploads/2019/04/IntroToCCDs_Feb1987.pdf-Max

author: bmxsesh
author_rank: Regular Contributor
timestamp: July 29, 2024, 01:13:24 pm
content: Is this ok to tie '-' terminals of two power supply channels with that little grey wire, or I rather not do that? I want to have power supply at C and power supply at B and capture the VTC with the oscilloscope. GND and '-' have no potential difference and so does '+' and GND have no potential difference.-Max

author: tggzzz
author_rank: Super Contributor
timestamp: July 29, 2024, 01:23:49 pm
content: Quote from: bmxsesh on July 29, 2024, 12:51:25 pmNo, you don't understand.If you have 1mm long wires then you alsounavoidablyhave 1nH inductance. Read and understand the construction of a "gimmick capacitor", and ensure you don't accidentally create one. Calculate the resonant frequency of 10mm of wires (i.e. 10nH) and 5pF of capacitance, and compare that with the transistor fTto see whether oscillation might occur.Read the link I have given you twice.

author: bmxsesh
author_rank: Regular Contributor
timestamp: July 29, 2024, 01:28:20 pm
content: I do understand that wire length for different type of signals can affect performance and should be viewed as transmission lines. In short, instead of ideal wire, how should I view the wire than? Some R-L-C network?

author: bmxsesh
author_rank: Regular Contributor
timestamp: July 29, 2024, 01:46:51 pm
content: Plus, I am not even applying any AC bias so far. I will worry about frequencies later. Only want to see if the large-signal model works and plot the gain for +15V supply. Something like this from the datasheet but for +15V supply.

author: tggzzz
author_rank: Super Contributor
timestamp: July 29, 2024, 01:57:10 pm
content: Quote from: bmxsesh on July 29, 2024, 01:28:20 pmWhether transmission line models or RLC models are more relevant depends on the length and frequency and construction details.Without seeing a photo of your construction (all of it!) we cannot know.

author: tggzzz
author_rank: Super Contributor
timestamp: July 29, 2024, 02:05:58 pm
content: Quote from: bmxsesh on July 29, 2024, 01:46:51 pm"AC bias" is a contradiction in terms.Whether or notyouare "worrying about frequencies" is completely irrelevant.It is clear that you haven't bothered to read the link given.Read it.Be aware that the phenomenon illustrated in that article (and the Horowitz and Hill inspiration) occurs with only bench power supplies; no external oscillator is necessary.

author: bmxsesh
author_rank: Regular Contributor
timestamp: July 29, 2024, 02:07:38 pm
content: Quote from: tggzzz on July 29, 2024, 01:57:10 pmRed wire to power supply channel I (+15V) purple wire to power supply channel II (I will tune it by hand from 0 to 15V), 4.7kOhm resistor from emitter to ground. Black wires all ground. For some reason these bjts are always shorted, base and collector. Whether I can't solder it properly by hand or something else. I don't see it as a frequency problem at the moment

author: ledtester
author_rank: Super Contributor
timestamp: July 29, 2024, 02:11:36 pm
content: Quote from: bmxsesh on July 29, 2024, 01:13:24 pmI suppose you want to drive the base with one power supply and supply the collector with the other. This is fine... grounds of both power supplies will be the same and you'll attach the GND clip of your oscilloscope probe to this common ground.Add a base resistor to limit the current through the base.QuoteI don't understand this comment.

author: tggzzz
author_rank: Super Contributor
timestamp: July 29, 2024, 02:16:37 pm
content: Quote from: bmxsesh on July 29, 2024, 02:07:38 pmSo, same construction as in the link I provided; no surprise there.Oddly enough, your circuit will behave similarly. But you won't realise that unless you bother to read the link!

author: bmxsesh
author_rank: Regular Contributor
timestamp: July 29, 2024, 02:18:32 pm
content: Now that I look at the datasheet I see that DC bias at base I will only be able to vary from 0 to 3 V. Then the manufacturer assumed that the output of the on-chip op-amp of the CCD does not exceed 3 V. I see that with +15V I am not exceeding the C-E voltage. When I power it up base is floating unconnected initially and maybe that was the problem, that I should have at least grounded it? I did limit the currents as per specs in the power supply every time.Quote from: ledtester on July 29, 2024, 02:11:36 pmThe power supply I got you can see each channel has three terminals '+' '-' and 'GND' and I am typically using '-' as my circuit ground thinking that 'GND' is the ground of the appliance itself in the power outlet it is using. But I hooked up a scope between GND and '-' and it showed 0V as in no potential difference between appliance ground and channel ground. It's how I understand this, but I used to work with power supplies that didn't have the third 'GND' terminal.

author: bmxsesh
author_rank: Regular Contributor
timestamp: July 29, 2024, 02:53:25 pm
content: My assumption right now is that they burn due to latch up when power is applied. They mention it here but do not mention how to avoid thishttps://toshiba-semicon-storage.com/content/dam/toshiba-ss-v3/master/en/semiconductor/knowledge/reliability/handling-precautions-tdsc-en.pdfThe transistor I soldered and checked at the store with multimeter did not have any shorts. Now after I applied power every terminal is shorted.

author: bmxsesh
author_rank: Regular Contributor
timestamp: July 29, 2024, 03:08:18 pm
content: Quote from: tggzzz on July 29, 2024, 02:16:37 pmMaybe it is the wire stuff that's causing a latch up and crushes the tiny transistor. But then again, capacitors and inductors that I'm gonna use to alleviate it also are through hole and have long and thick legs.I read the article and I see screenshots and jokes but no practical solution like a formula or a single circuit diagram. Sounds like an author is some kind of a anti-breadboard shill. PCBs are good especially if you own one of those fabs in your backyardwe ain't that big at the current moment

author: tggzzz
author_rank: Super Contributor
timestamp: July 29, 2024, 05:30:53 pm
content: Quote from: bmxsesh on July 29, 2024, 03:08:18 pmRead more carefully: there aretwoschematics. Plus a reference to a full explanation. That should be sufficient to start the process of understanding.The first schematic of an emitter follower, like yours, is not a good representation of reality: simulations don't show what happens in real circuits. The second, more complete, schematic with ""parasitic" components has simulations that follow the behaviour seen in a real emitter follower circuit like yours.The relevant general formula (of an LC resonant circuit) is taught in school, or during a first year university course. An electronic engineer should know them.You would benefit from understanding why in some cases 0603 size capacitors are insufficient, and manufacturers make (otherwise identical) 0306 capacitors, e.g.https://www.digikey.co.uk/en/products/detail/kyocera-avx/06033C103K4T2A/1599359https://www.digikey.co.uk/en/products/detail/kyocera-avx/03063C103KAT2A/776648You may not like the physics, but the emitter follower circuit (and power plane decoupling) doesn't care about that.

author: bmxsesh
author_rank: Regular Contributor
timestamp: July 29, 2024, 11:19:56 pm
content: Did the diode test on the fried ones with a dollar store multimeter. B-E and B-C diodes are reading 2 mV, but on the fresh one it reads proper 800-823 mV on both B-E and B-C.I was doing few things wrong, now the collector sustains +15Vdc.My mistakes were:1) Base was floating initially2) No Rb3) In my tests was applying +15V rapidly instead of rising it gradually4) Bad mechanical and electrical connection on BJT pinsNow everything works fine and I even got a better construction and like I said it had nothing to do with frequencies as everything is DC at this pointThe scope capture is me slowly rising Vc to 15V and as you can see the base stays unaffected so the transistor hasn't burned and can continue operating

author: tggzzz
author_rank: Super Contributor
timestamp: July 29, 2024, 11:47:36 pm
content: Quote from: bmxsesh on July 29, 2024, 11:19:56 pm3 is almost certainly irrelevant.1,2,4 are relevant.Your "everything is DC" point is incorrect. H&H used a power supply as the base bias voltage - and thatcausedthe VHF oscillation by allowing the transistor+parasitics to operate in common base mode. Adding Rb or a ferrite suppresses the common base mode.

author: Benta
author_rank: Super Contributor
timestamp: July 30, 2024, 12:03:38 am
content: Instead of those "solderless breadboards", just stick you components in a slice of toast.Result will be the same.