Common Emitter Bandwidth limitations (and general) - Page 1
Forum section: electronics / beginners

author: trevorhinkley
author_rank: Newbie
timestamp: August 22, 2025, 12:53:36 am
content: Hi all, this will be my first post on this forum, although I've been lurking a while. I've definitely learned an enormous amount from quietly reading the various posts here so thank you to everyone for contributing!First of all, here is the problem I'm trying to solve: I'm looking to construct a circuit that will output a square wave, ultimately sourced from a digitally controlled frequency generator (probably a 9833). However, there is one small subtlety: I would like to reduce the duty cycle by a fixed time interval (i.e. rather than have a fixed 45% duty cycle, I'd like to have each positive component of the wave be reduced by e.g. 100ns, so that the duty cycle at 1MHz would be 40% but at 100Hz it would be almost exactly 50%).The main method I've been trying is to feed an exactly-square wave (from my signal generator) into a basic common emitter, using an SS9018 and a 1k resistor, one at the collector and one at the base, plus a small capacitor on the output. This then feeds into a fast comparator. The thinking behind this is that the high-side resistor will form an RC with the capacitor and therefore slow down the rise but the impedance on the low side will be much lower and therefore the falling edge will be much tighter. This works wellin simulationand at lower frequencies. However, as mentioned, I would like to take this up to 1MHz, at least, and possibly up to 2MHz. As I approach these high frequencies though, the waveform goes to hell and pretty much disappears, which I assume is as a result of low-passing. I see this even without the capacitor. I'm measuring the output using a 50ohm terminator for my scope as I'm aware that the probe will have a high capacitance otherwise and I've also tried feeding the output into a fast op-amp, set up as a voltage buffer (AD8034 with GBW of 80MHz) before measuring the signal, to try to eliminate any impedance effects on the output. For openness, I'm building this on breadboard with through-hole components, although I haven't had any issues just throwing signals across at these frequencies so I'm assuming the capacitance of the board is probably not the issue here.Now, I'm aware of the Miller effect but my voltage gain here is effectively just -1 and, from the data-sheet, the SS9018 has a current gain bandwidth product around a GHz so I'm guessing that there's something I'm fundamentally misunderstanding about these components or this kind of circuit. It feels like I can get a decentwaveform up to around 100KHz but, in theory, I should be able to push this to 1 Mhz... So my question here is really "what am I doing wrong and what non-ideal behavior have I not understood in designing this circuit?"I should also note that I've tried sticking in a speed-up capacitor, and a baker clamp Schottky but that didn't really help.Thanks for taking the time to read!

author: Benta
author_rank: Super Contributor
timestamp: August 22, 2025, 01:08:01 am
content: You're mixing up GBW and switching time. Those are two completely different things.GBW applies to a transistor operating in linear mode.But you're apparently using it as a saturated switch, so completely different parameters apply.Post a schematic, a picture says more than a thousand words.Cheers. and welcome.

author: trevorhinkley
author_rank: Newbie
timestamp: August 22, 2025, 01:23:35 am
content: Thanks for responding; Generally I find that "switching time" is not really present on the data-sheets, at least for BJTs. I have tried an NMOS here as well but again, those seem to provide input capacitance but often not the switching time...The schematic is in the Falstad link I posted (which I'm repeating here). This is the bare minimum I've been able to push things down to (I've exaggerated the capacitor just to make the effect I'm going for more obvious).Is there a reasonable way of measuring the switching time? I've spent quite a while looking for resources to help characterize transistors but, beyond curve tracers (which don't seem like they'd capture these kinds of high-ish frequency effects), I haven't been able to dig too much up.I did come across mentions of saturation which is why I experimented with the baker clamp and speed-up but perhaps I'm not understanding those either? (please see here for what I mean)

author: David Hess
author_rank: Super Contributor
timestamp: August 22, 2025, 02:38:52 am
content: Transistors are only characterized for switching time if that is one of their intended applications.RF transistors do not make particularly good saturated switches. There is a special class of fast saturated switch which has higher Ft, but they switch much faster than RF transistors which have even higher Ft, even if Baker clamping is used.Fast rise and fall times also depend on low output capacitance, and they make special transistors for this also, although less so these days.If you want really fast switching, then use emitter coupled logic structures with differential pairs. With RF transistors, this will allow switching times below 1 nanosecond and inversely proportional to bandwidth. This was especially useful with RF PNP transistors, but these are no longer available.

author: trevorhinkley
author_rank: Newbie
timestamp: August 22, 2025, 02:53:59 am
content: Quote from: edavid on August 22, 2025, 02:18:39 amIt's going to be used to drive a sample-and-hold. I'd like a small, fixed delay after the clock pulse so that I'm sampling the signal after all the clock nastiness. Since I'm going to be digitally sweeping the clock itself, an analog method seemed like the easiest way here. To be completely honest, now I've gone down this path, it seems like I've run into a whole mass of stuff that I don't fully understand so there's also an aspect of wanting to solve this particular problem in order to gain a fuller understanding of transistors in general.Quote from: David Hess on August 22, 2025, 02:38:52 amI was wondering if you had any examples of these kinds of transistors?Quote from: David Hess on August 22, 2025, 02:38:52 amI'm not sure that this approach would allow for the impedance difference between the high and low side. The original motivation for using the common emitter was specifically so that I could have a slower rise time than fall time. I will look into the theory though as it seems like it might be a useful technique somewhere else. Thank you!

author: trevorhinkley
author_rank: Newbie
timestamp: August 22, 2025, 05:46:58 am
content: Quote from: edavid on August 22, 2025, 05:06:42 amDelaying both edges would mean that the sampling period would have a clock edge inside it. That clock is also driving a push-pull driver IC so there's significant amounts of noise associated with the time just after both rising and falling edges. The idea of using the S&H is to dodge that noise spike. By introducing a small delay to the onset of the sampling, it will skip the noise on that transition and, since the noise occurs just after the transitions, it should dodge the next one as well, whilst retaining the maximum length of sampling time regardless of clock frequency.

author: PGPG
author_rank: Frequent Contributor
timestamp: August 22, 2025, 08:04:53 am
content: If you want to play with ns take a look at LVC serie digital circuits like:https://www.mouser.com/ProductDetail/Nexperia/74LVC1G14GW-Q1001?qs=sGAEpiMZZMuyBeSSR239Iaz5ElWEWkZtN27B%252BxMsLi4%3DIf Mouser has 300 000 pcs of some part it is popular one.I am using LVC1G serie ICs to detect pulse length changed by 2ns.You can charge capacitor with resistor and discharge it much faster with diode.

author: MariuszD
author_rank: Regular Contributor
timestamp: August 22, 2025, 09:37:05 am
content: In an RC circuit with a diode, you'll get asymmetrical slopes.A transistor operating in saturation is unnecessarily complicating things. I used a 2N2222 transistor to delay one edge (without a capacitor), but it was a 1us delay.

author: MrAl
author_rank: Super Contributor
timestamp: August 22, 2025, 11:15:42 am
content: Quote from: trevorhinkley on August 22, 2025, 12:53:36 amHello there,I am not sure if you are aware of this, but a perfectly clean square wave wavefront has frequency components that go to infinity. In terms of bandwidth, that would mean that to get a super clean rise and fall time you'd have to have very high bandwidth.That said, you should really be checking out the rise and fall times of the transistor. If it does not have it on the data sheet, then you'd have to get a different transistor most likely. I believe you understand what storage time is (Baker clamp) but if not look into that also.Also what helps is a snap off circuit.Another thing to think about is impedance. Higher frequencies work better at low impedances. That means that a 100k collector resistor is probably not a good idea. 1k maybe, 100 Ohms better.What else you can do is look for some designs around the web that uses transistors for switching fast signals and see what transistors they are using, how they are biasing them, and how they are driving them, and what the output impedance is.

author: David Hess
author_rank: Super Contributor
timestamp: August 22, 2025, 11:21:07 am
content: Quote from: trevorhinkley on August 22, 2025, 02:53:59 amI see these from Mouser, but no PNP ones anymore.PN2369A TO-92 NPN2N4264 TO-92 NPNPMBT2369 SOT-23 NPNBSV52 SOT-23 NPNMMBT2369A SOT-23 NPNSMMBT2369A SOT-23 NPNI do not see these as a solution for anything you are trying to do because any common transistor should be more than fast enough.

author: trevorhinkley
author_rank: Newbie
timestamp: August 22, 2025, 03:00:06 pm
content: Quote from: MariuszD on August 22, 2025, 09:37:05 amI did actually try this circuit as well, and will probably end up going with that one. As you say, it is a lot simpler (although I suspect it might be better to go with a schottky; please correct me if I'm wrong about that). At this point though I kinda want to get the transistor arrangement working because I just want to understand what it is I'm misunderstanding about the operation of those components.Quote from: MrAl on August 22, 2025, 11:15:42 amI'm aware that square waves have frequency components that go to infinity and am not trying to get a perfectly clean transition (especially given that I'm inentionally trying to low-pass one of the edges). For my purposes, a 50ns transition on the "clean" edge would most likely be fine, which I should be in the ballpark of 20MHz. I thought the components I was selecting would be able to handle that and it seems like decent logic ICs go down to the single digit ns transitions so this doesn't feel impossible...I'm using 1k resistors and am a bit wary of going lower than that due to excessive current draw. I will give 100 a go just for the purposes of testing though.I've spent a lot of time looking for information on running the common emitter configuration at high frequencies but the preponderance of AI-generated junk has made finding specific information much more difficult, which is why I've been leaning more and more into the posts on this forum!Quote from: David Hess on August 22, 2025, 11:21:07 amThank you. I've picked up some PN2369A transistors (although I found them in TO-18 packaging on ebay...) as that does look like what I'm after. Will report back when I've received them and had a chance to try them out. I wasn't aware that this was a specific class of transistor, thank you for putting that list together. Much appreciated. As you say, though, it does feel like a common transistor should be enough here. Generally I've stopped even really considering discrete PNP circuits in general as it seems like they just aren't really made any more. Likewise for PMOS...

author: tggzzz
author_rank: Super Contributor
timestamp: August 22, 2025, 04:15:39 pm
content: Quote from: trevorhinkley on August 22, 2025, 02:53:59 amHow much jitter can you tolerate? In some applications that is the limiting factor, and can limit the ENOB.Have you considered programmable delay lines? This one, picked atrandomfirst search result has a delay which can vary from 2.2ns to 12.5ns in 10ps steps.https://www.renesas.com/en/products/8s89296

author: trevorhinkley
author_rank: Newbie
timestamp: August 22, 2025, 05:00:27 pm
content: Quote from: tggzzz on August 22, 2025, 04:15:39 pmIt depends in which direction. If I'm introducing a 100ns to onset of sampling then I can tolerate an additional +200ns extra delay but probably only -20ns early triggering at the very maximum (and yes I could probably just push that 100ns back a bit to 150ns to accomodate symmetrical jitter).I did consider programmable delay lines but:1. It wouldn't solve the problem at hand, directly, since I'm only looking to delay one of the edges but not the other.2. It seems like massive overkill for a situation that can probably be solved with a diode/transistor, resistor and cap + Schmitt trigger / comparator. I really don't need 10ps of accuracy at all.3. They all seem crushingly expensive. That part, for example, is 18 bucks. Compared to 1 buck for a dual comparator (and pennies for the semi + passives).This is one of those things where I thought that the fundamental idea would be very simple to implement (asymmetric impedance + capacitor = asymmetric low-pass) but turned out to reveal a lot of gaps in my knowledge. This is actually the first time i've pushed into MHz territory so I'm generally encountering a lot of push-back from material reality... Which is great, actually, because I'm learning an absolute ton here

author: PGPG
author_rank: Frequent Contributor
timestamp: August 22, 2025, 11:39:16 pm
content: Quote from: trevorhinkley on August 22, 2025, 05:00:27 pmUsing symmetric delay you can then use AND, OR, XOR gates to get from both signals the one you need.

author: MrAl
author_rank: Super Contributor
timestamp: August 23, 2025, 06:13:11 am
content: Quote from: trevorhinkley on August 22, 2025, 03:00:06 pmHello again,Yes, when I said infinite frequency that just shows that the faster response you need the higher the highest frequency is that you need to get that faster response. When we talk about infinities we are usually talking about a limit in theory or just an indication of what is happening. If you really needed an infinite frequency, you'd be in serious troubleIn some cases we need a second stage also. This allows a little more wiggle room in each transistor stage. An NPN+PNP stage can work really well with the right biasing and feedback setup.When experimenting with resistor values such as with the collector resistor, it works pretty well to change them in a logarithmic pattern. If you start with 1000 Ohms, then the next would be 500 Ohms, then 250 Ohms or 200 Ohms, then 100 Ohms, etc. This helps with the balance between getting the right current level and not having to draw excessive current. Of course the input drive would have to change also as you go lower, and the Baker clamp would become more important, and there are different Baker clamps if one does not seem to work very well.

author: trevorhinkley
author_rank: Newbie
timestamp: August 24, 2025, 11:15:03 pm
content: Quote from: PGPG on August 22, 2025, 11:39:16 pmThat actually sounds like it might be a better solution than the RC + diode option (which seems to have quite a few issues related to the reverse recovery time of the diode). I'm guessing if I went down this route then I'd want to use logic gates with Schmitt inputs to avoid intermediate input voltage related instability issues?Quote from: MrAl on August 23, 2025, 06:13:11 amNot sure I understand this bit. Are you saying that we can get a higher bandwidth with a Sziklai pair?Quote from: MrAl on August 23, 2025, 06:13:11 amI'm only familiar with the two-diode baker clamp I posted earlier and the Schottky transistor configuration. Are there other alternatives?

author: MrAl
author_rank: Super Contributor
timestamp: August 25, 2025, 12:54:18 am
content: Quote from: trevorhinkley on August 24, 2025, 11:15:03 pmThe two common Baker clamps are the single diode and the two diode types. There's also a cutoff clamp that prevents the input from going too high. High speed diodes or Schottky diodes.The Sziklai pair can be used up to 100's of Gigahertz, but that's probably with MOSFETs. MOSFETs will be faster than many bipolars.The one that should work pretty well for you is the modified bipolar version which includes resistors to set the gain. It should be very fast because it's all transistor action with no op amps. Also you get low output impedance.