Q-meter by Luis Vleminq from EDN site - questions about transistors in CCS - Page 1
Forum section: electronics / beginners

author: piotr_salomon
author_rank: Newbie
timestamp: January 02, 2024, 12:33:31 pm
content: I'm preparing to build this Q-meter and I try to learn as I go:https://www.edn.com/novel-q-meter/This is the first HF circuit I ever attempted and I'm learning a lot about how HF differs from LF, board layout & such. That mainly means using HF components.To choose the replacement for old 2n5179 transistors author of this article suggested to look up their transition frequency, which should fall in the range 1GHz-1.5Ghz. I found a suitable transistor BFS17A. But then I looked at the circuit and started wondering does a current sink also require a HF transistor?Does that mean if I used a bc848 (Ft=100MHz) the CCS would stop sinking current at higher frequencies? I always thought Ft refers to switching frequency of the transistor & in CSS they act like resistors. But here I see the same HF transistor used in CCS.Can someone explain is it necessary or not? Does this work the same way as with LF diodes?Also in this venue - C8 holds the voltage on bases of CCS transistors - shouldn't it be bigger then 220p to increase stability of the CCS?Another thing I don't understand fully is the use of 3v6 zener diodes on the bases of Q1/Q2/Q3/Q4 - what is their purpose? Author describes this as giving transistors "space to breathe" - what does that mean? Is their zener voltage critical (say I use 3v3 or 3v9 diodes instead of 3v6)?

author: RoGeorge
author_rank: Super Contributor
timestamp: January 02, 2024, 02:50:03 pm
content: That's not exactly a Q-meter. I mean, it does not indicate Q.It indicates theseries resistance, and then you have to connect a frequency-meter, measure the frequency, measure the capacitor in the LC tank, then do the math with a spreadsheet or a pocket calculator.Then, the 2 measuring points must not have a DC path, so you can not measure Q for a coil alone, or Q for an LC parallel tank. It won't measure the Q of Quartz or resonators either, as suggested in the pics, only their ESR (Equivalent Series Resistance), and I know no direct formula between the ESR and the Q of a resonator. You will need a few other unknown parameters of the resonator, in order to deduce Q.Maybe I misunderstood the schematic, but that would be a terribly impractical Q-meter, though it doesn't display Q at all, so not sure why it is called a Q-meter.Interesting ESR-meter, though.I'm tempted to believe the author wanted a pretext to use the differential voltage-to-current circuit pointed in the bibliography (either in DOI: 10.1049/el:19730109 or at page 16 inhttps://cas.ee.ic.ac.uk/people/dario/files/E416/translinear-lores.pdf- Cross Quad)

author: piotr_salomon
author_rank: Newbie
timestamp: January 02, 2024, 05:03:14 pm
content: Thank you RoGeorge for your explanation.As I understand the usefulness of this circuit is comparing two coils of the same inductance with a fixed capacitor, a stable type, like a ceramic C0G type. The coil which will measure lower ESR has better Q.It is true you can not directly measure the value of Q, but it can aid me in designing a coil that will have better parameters & determine it's resonant frequency. Testing capacitors is also a good use for it.All that aside I still want to learn about HF circuits by building and testing it, because it has few components.I'm still wondering about the Ft of those bottom transistors. Does it influence their functionality as current sinks or not? Everything I read about Ft always gives example of signal being provided to the base of the transistor.

author: mawyatt
author_rank: Super Contributor
timestamp: January 02, 2024, 05:44:17 pm
content: This is indeed an interesting baseline use of the Caprio cell concept/technique, haven't found many folks that even knew about this cell, much less utilized such!!If you study the Caprio cell in detail, one finds that the input differential signal is "impressed" across equal but opposite side Vbe junctions and the subtle non-linear effects become more common mode rather than differential due to the self balanced of the signal path thru the various Vbe junctions. For this to work well, recall we resorted carefully differential balancing with Erdi Cross Coupled Common Centroid techniques across all the differential transistors.We employed this cell inside a Differential Cherry-Hooper Amplifier starting back in 2004 with IBM SiGe BiCMOS 8HP, then later 9HP, then much later in InP for even wider bandwidths and higher DR. The goal was very wide bandwidths from DC well into the MMW regions, and extremely high instantaneous Dynamic Range (EW use).Anyway, little known circuit with unique propertiesAdded:https://www.eevblog.com/forum/rf-microwave/the-nauta-circuit/Best,

author: mawyatt
author_rank: Super Contributor
timestamp: January 02, 2024, 06:08:16 pm
content: Quote from: piotr_salomon on January 02, 2024, 05:03:14 pmThose are current sinks and don't matter much except having a low Collector to Base, Emitter capacitances (and obvious matching). One could isolate the capacitance with larger R5 and R6, but would need to have a lower -12V to compensate for the IR drop.Interesting circuit, thanks for link.Best,

author: piotr_salomon
author_rank: Newbie
timestamp: January 02, 2024, 10:39:22 pm
content: QuoteThank you mawyatt.A quick look through the datasheets I have:HF transistors I considered:2N5179 - Cob=1pF max (Ft=900Mhz-2GHz ) (the original design)BFS17A - Cob=0.6pF typ. (Ft=1.5Ghz typ.) (my replacement)SS9018 - Cob=1.3pF typ. (Ft=1.1GHz typ.) (maybe)MPSH10- Cob=0.7pF max (Ft-650MHz min, so prob. 1GHz typ.) (maybe)LF transistors for Q5/Q6 CCS:BF494/BF495 - Cob=1pF max (Ft=120Mhz min) (hfe=67-220) 30VBF422 - Cob=1.6pF max (Ft=60Mhz min) (hfe=50 min) 250VBF720- Cob=0.8pf typ (Ft=100MHz typ ) (hfe=50 min) 300VBC848 - Cob=3pF max (Ft=100Mhz min) (hfe=150 typ) 30VAs I understand in the CCsink, low hfe and low Ft will contribute to the current stability by rejecting HF noise. From what I gathered the BF495 BF422 BF720 seem like a good current source candidates for this circuit.But I will probably test BC848 as well, to see how much difference a 2pF can make.Can I increase C8 value, or does its capacitance somehow transfer from base to collector so that's why it's rather low?All this consideration of capacitance is interesting new thing for me.QuoteThis name alone is so cool I'm going to attempt to implement it. Can you recommend a good source to learn about this technique? I hope 100 transistors will suffice to find 4 that match.

author: RoGeorge
author_rank: Super Contributor
timestamp: January 02, 2024, 11:20:08 pm
content: Quote from: piotr_salomon on January 02, 2024, 12:33:31 pmFor stepping into RF and measuring coils, you may want to build an RLC bridge, or a grid-dip-meter, much simpler and way more useful.The trans(conductance)-linear(ity) circuits work well when transistors are nearly identical, which makes them ideal structures for integrated circuits, where transistors can be well matched. With discrete transistors they don't work that well.That circuit is not for beginners. Not a good learning exercise. In fact, it looks overly complicated and with no real benefits. There are a few comments on the EDN saying the circuit didn't work for them, while no comments are saying they did the circuit and are happy with it.If you still want to try that schematic, then the constant current sources have to be able to deliver constant current at all times, including at an eventual fast variation in the load. If the load is fast, then the constant current transistor has to be able to respond fast, to compensate for the fast changes in the load, so they have to be RF transistors, too.C8/220pF is not for holding the voltage constant in the base. The voltage is kept constant not at the base, but at the emitter of Q6, and not by C8, but by the U1a opamp and its feedback loop. C8 is to make sure the feedback loop won't oscillate. A C8 too big might make the opamp to self-oscillate.Transistors do not "breath", so leave the diodes as they are, 3.6V, or else in case the circuit doesn't work, you'll never know if it doesn't work because you changed the diodes, or because of something else.In general, it is a very bad idea to use uncertain language in engineering. Metaphors are for steering emotions. Technical writings must stay precise and articulated. In science and engineering, in general, figurative speaking style can only bring confusion. It usually means one doesn't have the vocabulary. Also, self praising own work is a big warning sign, I'm talking about another EDN article where the same author was claiming best frequency range ever in VCO, or something like that.I don't like the writing style and the stretched claims in the EDN, sorry for the criticism.The Caprio cell is new to me, seems very interesting, thanks for bringing the subject. Will try to understand the bibliography in the next days, so for now I can't speculate about the value of 3.6V Zener, or about how much variation in the constant current can be tolerated before compromising the measurement, but a BC848 shows a significant current swing at 100MHz vs at 100kHz, see the attached LTspice simulation.

author: piotr_salomon
author_rank: Newbie
timestamp: January 03, 2024, 02:29:17 am
content: RoGeorge,Thank you so much for taking time responding to my post. The video about Bram Nauta is very interesting. I read his article and I will need few weeks to absorb it. It's so dense. I need to sit with with those equations a while.I take your advice very seriously and am thankful for it. Still I think one attempt at building the circuit won't hurt, even if I fail. It will go into the box and maybe I will come back to it later to fix it when I learn more. At least I can practice the board design.QuoteAs I suspected there was something to it. It must be related to the properties of the pn junction not just capacitance. Thanks to You I learned the Ft of transistor applies to all 3 terminals.As you said the circuit is very interesting in itself, and worth studying. What better way then to build it? I think I can manage matching 3 pairs of transistors to some decent degree to at least give it a go. I have only cheap 20MHz DSO, so I won''t be able to truly see anything past few MHz, but still it might show some interesting stuff.I'm looking at RLC bridges and grid dip oscillator. The latter especially seems like a thing I certainly can make as well. I even played with the circuit with 2 cross coupled BJT's that runs on single 1.5V cell.I remember it gives a nice stable sinusoid with small 10uH inductors. But I never been able to run it past 2Mhz or so. I guess because I used bc848's. I need to switch to HF transistors now.Since I don't have an analog uAmmeter I maybe use LM3914 LED bar with this design:I might try it this week since I have couple BFR93A 6GHz transistors that I never used before. I wonder if that's overkill.As for the original article - different people have different styles. It is indeed a rushed and scarce presentation, no images of the actual circuit board, etc. But hey - it's out there for free. Who am I to judge. Over the years I learned to be cautious with circuits on the internet, that's why I'm looking for opinions from people like You.But there are times you come across something intriguing like this, which has some familiar yet perplexing topology & you can't help but to try it for yourself.

author: RoGeorge
author_rank: Super Contributor
timestamp: January 03, 2024, 10:51:54 am
content: I might have been too verbose and too aggressive yesterday, and might have left the impression that I know how that schematic is supposed to be working. I don't.In the linked topic about the Nauta Circuit, mawyatt was just starting to tell about the Caprio Cell, and how it works, just that I didn't register that. I've put the Caprio Cell on my wanna read least, and eventually forgot about it. My point is, pay attention to mawyatt's comments, because if something he wrote seems puzzling, this means you'll learn something new from him. I barely know what I'm talking about in general, let alone about that quite unusual schematic from EDN.For example, right now I have about the same question as you regarding the constant current sources.My reasoning yesterday was that, in order to have a constant current transistor, one will need a transistor. When the frequency is too high, the transistor effect is overwhelmed by the other effects, for example by parasitic capacitance of the junctions, so the device is no longer a transistor. No transistor, no CCS.But now, how about the opamp? LM358 is a low frequency opamp, for less than 1MHz. Certainly won't work at 100MHz, and the BE of the transistor is in the feedback loop of the LM358. I'm tempted to say Q6 won't act as a CCS even if it were to be a high frequency transistor, because that opamp can't work at 100MHz.I'll search for spice models of the LM358 and 2N5179, and simulate that later.If your main goal is learning/experimenting rather than measuring Q, then go for it, the design idea from EDN seems intriguing, and even if it doesn't work, the Caprio Cell is a very interesting circuit to play with anyway.Quote from: piotr_salomon on January 02, 2024, 12:33:31 pmAbout the PCB, in the EDN comments the author said the build was on a double layer copper clad, with one ground plane and isolated islands carved on the other side, I guess something like this:w2aew - #123: Build a crystal oscillator from schematic thru prototype construction and testing - DIYhttps://youtu.be/blalAktxFoIother prototyping methods shown by w2aew:#122: Electronic Circuit Construction Techniques: review of some prototype circuit building methodshttps://youtu.be/kH110yjYZ2g

author: RoGeorge
author_rank: Super Contributor
timestamp: January 03, 2024, 10:54:30 pm
content: Found online a spice model for 2N5179.The opamp speed is not critical because the Vbe is almost constant anyway. The most of Vtest 100MHz perturbation enters in the opamp's feedback loop through the parasitic CB capacitance of the transistor. RF transistors have lower Ccb, so the 2N5179 (Ccb=1pF) would make a better CCS than BC847 (Ccb=3.5...6pF).C8 drives most of the perturbations from base to ground. A bigger C8 would help, though a too big C8 will make the loop less stable, as seen in the magenta trace (C8=2.2nF), and even worst in the gray trace (C8=22nF).Lower plot panel is a detail of the last 50ns, red box from the upper plot panel zoomed in, so to show the Ipp swing through R5, when the C8 value is stepped as: 0.22pF, 2.2pF, 22pF, 220pF, 2.2nF, 22nF.Middle plot panel is the measured peak-to-peak variation of the CCS current through R5, vs the value of C8. For C8 bigger than approx. 100pF, there is no visible reduction of the CCS swing. 220pF seems to work best.

author: RoGeorge
author_rank: Super Contributor
timestamp: January 11, 2024, 10:48:14 pm
content: Quote from: piotr_salomon on January 03, 2024, 02:29:17 amThe key reading to me was "Current-mode Circuits From a Translinear Viewpoint: A Tutorial" by Gilbert Barrie himself.(was also published in a book, as chapter 2 in "Analogue IC Design the Current Mode Approach" - editor Toumazou-Lidges)I did look at first at the bibliography from the EDN article, then went to read the Caprio cell from the indicated DOI (available on sci-hub), then the indicated pdf class notes, and it was not very clear. My luck was that the class notes were based on the Gilbert's tutorial (and the tutorial's title was mentioned there), so I could look it up. Once I've read the Gilbert's view on translinear principle with BJTs, it all became clear, and not only about the Caprio Cell, but about many other building blocks in linear ICs, too.At this point I'm even tempted to try the EDN schematic, so to fix the learning with some practice.Thanks for bringing the topic.P.S.In the Caprio cell paper from that DOI, the AN-489 from Motorola (1969) is in the bibliography. Took a while to find that AN from 1969, though AN-489 (Analysis and basic operation of the MC1595 multiplier) was not an essential reading. To spare the search, it's at page 176 of 228 in this collection of Motorola ANs:https://dn790006.ca.archive.org/0/items/bitsavers_motoroladactronics32APPNOTES_13467413/32_APPNOTES_text.pdf