First npn amplifier on breadboard. Measured ≠ Expected. Also New oscilloscope Q. - Page 1
Forum section: electronics / beginners

author: renzoms
author_rank: Regular Contributor
timestamp: December 10, 2021, 02:05:53 am
content: The image shows the circuit I'm looking at.I expected Vce to be about 3.1V. But I'm getting like 6 something idk why.Also I just got a Siglent SDS1104X-E. Not sure how to measure current with it. Any resources? Also Idk how to make this display display numbers. I'm approximating values by looking at the y-axis lol. In other words Idk how to pick a point/display measured values on the scope.Thanks, hope this was acceptable quality post.Edit Troubleshooting myself: I'm gonna start by disconnecting the BJT and measuring 3V to see if it's there. I now realize I could work this out, independantly, but asking questions has always been an enjoyable experience for me, and sometimes I learn stuff from you all.

author: StillTrying
author_rank: Super Contributor
timestamp: December 10, 2021, 02:35:24 am
content: Ifyou're producing the 3V for the base from a 333k & 142k potential divider, that will add about another 100k to the R2 in series with the base, roughly having the base current ?"I'm gonna start by disconnecting the BJT and measuring 3V to see if it's there."Carefully adjust it to 3 V while the BJT is connected, what type of BJT is it, your expected current gain of around 100 seems low for a small low current transistor they're usually about twice that.

author: fourfathom
author_rank: Super Contributor
timestamp: December 10, 2021, 02:43:35 am
content: Quote from: StillTrying on December 10, 2021, 02:35:24 amNo, as I read it, that 333K / 142K divider *is* the 100K base resistor. Those numbers work out OK (3V, 100K). So what is the transistor? You are assuming a hFEof 100. Perhaps it's actually lower? That parameter isn't particularly well-controlled.

author: xavier60
author_rank: Super Contributor
timestamp: December 10, 2021, 02:54:49 am
content: A 6V reading is close enough to the expected 3V for that arrangement. For example, try heating the transistor.I hope that the next step is to add Emitter resistance or connect the upper Base bias resistor to the Collector to make the Collector voltage less affected by temperature and transistor parameter variations.

author: StillTrying
author_rank: Super Contributor
timestamp: December 10, 2021, 03:01:10 am
content: Quote from: fourfathom on December 10, 2021, 02:43:35 amGoing by the photo of the breadboard I think you're correct.The gain of 100 is the OP expected, if the gain really is only 58 I'm surprised, maybe emitter and collector are swapped.

author: renzoms
author_rank: Regular Contributor
timestamp: December 10, 2021, 03:14:28 am
content: Wow I was delighted by all your great answers

author: renzoms
author_rank: Regular Contributor
timestamp: December 10, 2021, 04:13:49 am
content: I checked for 3V, and found one of the resistors was off.Interestingly, it was pointed out that Beta may not be 100 like I assumed (I know Beta varies.)I plugged the BJT back in and still measure about 6.5 Vce (instead of 3.1 Vce)Thanks

author: fourfathom
author_rank: Super Contributor
timestamp: December 10, 2021, 06:27:39 am
content: Possibly:The equivalent (parallel) resistance of your two potentiometers isn't 100KYour collector potentiometer isn't 3KYour transistor has a much lower betaAs StillTrying suggested, the transistor is backwards (collector and emitter swapped)It *does* look like the transistor may plugged in backwards. Not all transistors are identical, but often when you look at the flat side of the body, left-to-right the wires are emitter, base, collector. Backwards, it's still an NPN transistor, just not a very good one.Do you have a DVM? If so, you can measure the resistance values and the current. You can't directly measure current with a 'scope. One of my many DVMs only cost me $29 (https://www.amazon.com/gp/product/B01NAVAT9S?th=1). It's not a great meter, but quite good enough for most of my needs.

author: xavier60
author_rank: Super Contributor
timestamp: December 10, 2021, 06:33:46 am
content: When the C and E are swapped, the 6V reading will now be due to zenering of the B-E junction rather than transistor action.

author: fourfathom
author_rank: Super Contributor
timestamp: December 10, 2021, 06:58:05 am
content: Quote from: xavier60 on December 10, 2021, 06:33:46 amGood point. It's still not a very good transistor when operating below the zener voltage. I do recall that the backwards saturation voltage (VECsat?) can be lower, but perhaps not always?

author: rstofer
author_rank: Super Contributor
timestamp: December 10, 2021, 06:24:22 pm
content: A scope is made to look at squiggly lines as a function of time. You are working with a DC circuit and a DMM (or 3) is a far better option. Yes, you can read voltages off the scope, just press the Measure button. See page 131 in the User Manualhttps://siglentna.com/USA_website_2014/Documents/UserManual/SDS1000X&Xplus_UserManual_UM0101X-E02A.pdfI think you can make 4 concurrent measurements (all ground referenced) by using all 4 channels. The manual only talks about 2 channels but it probably predates the 1104. Search around for a more current manual.

author: David Hess
author_rank: Super Contributor
timestamp: December 10, 2021, 06:41:13 pm
content: There is also a good chance of oscillation with that construction.

author: rstofer
author_rank: Super Contributor
timestamp: December 10, 2021, 06:45:11 pm
content: Quote from: David Hess on December 10, 2021, 06:41:13 pmAnd that should be visible on a scope.

author: David Hess
author_rank: Super Contributor
timestamp: December 10, 2021, 08:43:39 pm
content: Quote from: rstofer on December 10, 2021, 06:45:11 pmIs should be, but sometimes it will not because either the oscilloscope does not have sufficient bandwidth, or the probe loading is enough to stop the oscillation. This is one of the applications where high bandwidth and a low capacitance active probe have considerable utility.If oscillation is messing with the operating point, then capacitive coupling using a hand or finger while monitoring the output should reveal it.

author: fourfathom
author_rank: Super Contributor
timestamp: December 10, 2021, 08:59:08 pm
content: Quote from: David Hess on December 10, 2021, 06:41:13 pmI actually think there is a very poor chance of oscillation with that construction. I've breadboarded many simple transistor circuits that looked like that (messy) and very seldom have I seen RF oscillation. And I have GHz measurement capabilities.Has anyone else looked at the orientation of that transistor on the breadboard? EBC is the typical pinout (facing the flat side), at least for USA-sourced transistors. If so, it's plugged in backwards.So, renzoms, what is that transistor?

author: StillTrying
author_rank: Super Contributor
timestamp: December 10, 2021, 10:17:43 pm
content: Quote from: fourfathom on December 10, 2021, 08:59:08 pmI haven't because I don't think there's much point while we don't know all its numbers, I seem to remember finding out the hard way that for TO-92s you have to look at thecorrect manufacturer'sdata sheet.There's a mention here:"There is no standard pinout for the TO-92. The American BJTs use the E-B-C pinout while their Japanese counterparts use the E-C-B pinout and some RF devices use the B-E-C pinout."https://en.wikipedia.org/wiki/TO-92#Variants

author: magic
author_rank: Super Contributor
timestamp: December 10, 2021, 10:23:52 pm
content: Quote from: StillTrying on December 10, 2021, 10:17:43 pmAny example of that other than the infamous P2N2222?(Which is the opposite of every other xx2222 in TO-92.)I believe standardized part numbers like 2N3904/BC547/2SC1815 should be free of such problems

author: StillTrying
author_rank: Super Contributor
timestamp: December 10, 2021, 10:28:14 pm
content: "Any example of that other than the infamous P2N2222?"I don't know how many there are, but the very first one I thought of PNP BC213L has the base on the end.

author: magic
author_rank: Super Contributor
timestamp: December 11, 2021, 08:40:27 am
content: This appears to be another case when one seemingly innocent letter makes all the difference.CDIL even has both the BC212 and BC212L listed on theirwebsite.But then I found thishttps://html.alldatasheet.com/html-pdf/2882/MOTOROLA/BC213/256/1/BC213.html

author: tooki
author_rank: Super Contributor
timestamp: December 11, 2021, 01:34:27 pm
content: Quote from: renzoms on December 10, 2021, 02:05:53 amJust a little tip: right now, the way you hand-draw your resistor symbols is essentially identical to the inductor symbol. I’d get out of that habit ASAP before it becomes so ingrained you inadvertently do that on a test. (Since this looks like it’s a school problem.)

author: renzoms
author_rank: Regular Contributor
timestamp: December 11, 2021, 04:19:58 pm
content: The resistances are good*.The transistor is 2N2222 fromamazonShould be flat side E-B-C, So I had it backwardsAlso I draw my resistors curvy because I saw someone do it, and I ran with it.I see these as curved resistors. Lol.. It's been years, there's no going back. Compared to the years I drew resistors sharp, I enjoy curved resistor-drawing-process more. My inductors are clearly distinguished by loops. If we were coworkers, we would never have this problem [of possibly misinterpreting a component lol]. I seriously love drawing resistors curved.---So I expected VBE .7, VCE 3.1, Beta = 100.measured with the npn backwards) .7V, ~6.8Vmeasured with the npn right) .71V, 5.5VI analyzed. (Why 5.5V?). Probably Beta does not equal 100. I used ie = ib(Beta + 1) with measured #s to get Beta = 64.5I tested. Ya Replaced 3k trimpot with 4.6k. Expected VCE 3.1V - Measured = 3.5Vimage hereConclusions: 1. I am rounding up resistor values causing errors.2. I need more hands-on.This is fun.I'm learning a lot.3.Lab/oscilloscopeI analyzed (Why 6.8V?).VBE = reverse biased. VBC = forward biasedI would have said npn is cutoff. Clearly it's not.Given your comments , I'm curious about the VEBO/Breakdown Voltage / Zenering.Why 6.8? Is it 6.1 Zener VEB + .7 VBC? Edit: I guess one of the ways I could test this is change the R_E and see if I measure the same/similar voltage at a different current.image hereCurious about hFE (incremental Beta), now. Based on this data, Beta changes @ currenthttps://imgur.com/a/88W1g4bFinal comment thanks for the oscillo manual. Will be looking at videos/that. Edit: Actually I'm looking through the stickied oscillo thread. Probably the whole thing. Learn oscillo. (I am intentionally chatting in short phrases fyi lol)Thanks to you all!

author: rstofer
author_rank: Super Contributor
timestamp: December 11, 2021, 07:54:48 pm
content: Quote from: renzoms on December 11, 2021, 04:19:58 pmI haven't watched all of w2aew's scope video but what I saw seemed to revolve around analog scopes. DSOs are an entirely different thing. Sure, it's still V/div and t/div plus some triggering but DSOs have the really important single shot mode. I don't know of any particular video for but I get a large list when I google for 'learn electronics video siglent' including videos by Siglent.Maybe start with this one and see where you go:https://youtu.be/dy_i2qbgM3EOr this one by Siglent. I hope there is a series but I didn't check.https://youtu.be/L37KmoxqMt8

author: renzoms
author_rank: Regular Contributor
timestamp: December 11, 2021, 08:14:32 pm
content: alright cool.Thanks.Yeah. I noticed it was about analog scopes, too. So I just started watching it for other reasons like culture/exposure.Yup, I've got a bunch of videos open on my browser too. Thanks again

author: james_s
author_rank: Super Contributor
timestamp: December 11, 2021, 08:41:34 pm
content: Quote from: renzoms on December 11, 2021, 04:19:58 pmThis is not art, there is no room for creative license when drawing schematic symbols. These symbols are standardized for a reason, if you draw resistors that look like inductors you are going to confuse anyone you share these schematics with, I thought they were inductors too and wasn't sure where you were going with that circuit. It doesn't matter that your inductors look different, many of you schematics will have resistors with no inductors to compare them to. That said, in the real world schematics are almost never drawn by hand anymore anyway except for quick doodles. CAD software will have a library of standardized symbols.

author: tooki
author_rank: Super Contributor
timestamp: December 11, 2021, 09:06:08 pm
content: Quote from: renzoms on December 11, 2021, 04:19:58 pmHonestly, that’s just… a really awful practice. It’s not about your “enjoyment”, it’s about clear intent.It’s one thing to be stylized but unambiguous, but in this case, you’re directly and unambiguously using a symbol with another established meaning. I don’t care what bad habits of yours that your coworkers have become accustomed to. A schematic should be clear to anyone who reads it, not just those who’ve learned how to interpret your personal style.Schematic symbols, these days, come in two styles, one where the resistor is a zigzag centered on the trace, the other where it’s an empty rectangle centered on the trace. Those are your choices, no more, no less.The resistors in that video are very sloppy, but still angled, they’re not the fully-formed curves of the inductor symbol like you’re doing.

author: fourfathom
author_rank: Super Contributor
timestamp: December 11, 2021, 10:13:48 pm
content: Sorry to pile on, but yes, those resistor symbols are completely unacceptable. Do yourself and everyone else a favor and draw them so they *don't* look like inductors. It's not that difficult, and the purpose of a schematic is to communicate.

author: renzoms
author_rank: Regular Contributor
timestamp: December 12, 2021, 01:40:09 am
content: I can change the way I draw these symbols for you guys. That's part of the whole fun of being part of a community, innit

author: vk6zgo
author_rank: Super Contributor
timestamp: December 12, 2021, 02:10:50 am
content: The voltage divider isn't really doing anything in this application, as you can't lift the base to emitter voltagevbeabove 0.7v.With 0.7v across R2, the current through that resistor will be so small that its contribution to the total voltage drop across R1 can be ignored, so you could delete R2.if you do that, with R1 at its present value,ibwon't change much & should behigherthan the original designed value for the 3v supply, as will Collector currentIc, so it still seems that there is something wrong, either with the transistor, resistor values, or connections.One suggestion:- Series up a couple of AA alkalines as a source of 3v, (with the expected values ofib, it should last for ages), & revert to the original design.If you still get "funny readings", the problem isn't to do with your version.

author: xavier60
author_rank: Super Contributor
timestamp: December 12, 2021, 02:23:37 am
content: Also, the Base bias could be temporarily removed to confirm that the Collector rises to the full supply voltage.

author: fourfathom
author_rank: Super Contributor
timestamp: December 12, 2021, 02:36:40 am
content: Quote from: renzoms on December 11, 2021, 04:19:58 pmTransistors from Amazon may be excellent. Or they may be floor-sweepings from some fab in China. I wouldn't be surprised if they didn't meet published gain specs. Even floor-sweepings can be useful, just be careful how you use them.Building test circuits to measure transistor parameters is excellent education, but one thing you should learn from it is to not design circuits that rely on transistor parameters. Sure, you care about the parameters and will design for ballpark numbers, but in most cases you will want various forms of negative feedback to establish the operating point. Look at emitter degeneration resistors, and connecting your base-bias resistor to the collector rather than to Vcc. There are plenty of other ways to design stable and predictable circuits, but these two techniques are a good place to start.

author: vk6zgo
author_rank: Super Contributor
timestamp: December 12, 2021, 04:42:04 am
content: Quote from: fourfathom on December 12, 2021, 02:36:40 amYes, even back in the tube days when unused examples of such devices from different production runs were much closer in parameters than is common with BJTs, good engineering practice was to design circuitry, the performance of which was as much as possible, independent of device parameters.It was the same solution, but to a different problem, as tubes were "thermionic devices".One aspect of this was that when cathode emission decreased, over time, their characteristics changed, first imperceptibly, but ultimately, radically.