analog.freak

's profile
image

Biography has not been added


analog.freak

's contributions
  • 08.23.2017
  • In defense of the current-feedback amplifier
  • Koo Moo: In response to your rant : "The expression for the output impedance that you gave using two port analysis is incorrect. Go figure out why, since you think I am incompetent:" I never gave you any expression for the output impedance. But, thanks for stating that it is incorrect.
  • 08.23.2017
  • In defense of the current-feedback amplifier
  • Koo Moo: "you don't recognise" because you don't want to recognize, and you don't want to recognize because you just can't... The output buffer that you show as ideal in your Bazooka (short for "Bazoo Kiwanuka Amplifier") will no doubt exhibit a nonzero output resistance ro. You can't deny that. 2-port analysis approximates the node common to ro and RF as an "ac ground", so ro is kept out of 2-port calculations, indicating that it doesn't matter whether ro is or isn't there. This is one of the flaws of 2-port analysis, which you simply don't want to admit because it'd make your entire house of cards collapse. Return-ratio analysis, on the other hand, lumps ro with RF, and leads to exact calculations and/or measurements (note that lab measurements are impossible in 2-port analysis!). The presence of ro causes feedthrough around the gain node, which 2-port analysis fails to account for. Before you utter definite based-on-thin-air statements such as "they are plainly incorrect", you need to study the differences between the 2-P and R-R analyses in great detail! For this, you need not go too far: Like I did, you just study http://www.edn.com/electronics-blogs/analog-bytes/4430358/Two-port-vs--return-ratio-analysis- Do it, and when you're done, whether it takes you a week or a month, please do come back and let's move our conversation to a professional plane, ok?
  • 08.23.2017
  • In defense of the current-feedback amplifier
  • Koo Moo: what you claim as your latest revolutionary find has already been derived by Dr. Franco in the simple and direct way expressed by his Equations (8a) and (8b). He likes to express the closed-loop gain in the insightful and low-entropy form of his Equation (2), namely, A=(1/beta)/[1+1/(a*beta)]. All you've done was to multiply numerator and denominator by "a*beta" to obtain A=a/(1+a*beta), and this simple-minded manipulation has given you a vigorous impetus to restart your vain crusade against CFAs! As to your claim that the circuit is rubbish, rubbish are your utterances, not the circuit. Evidently you are not aware that the maximum gain achievable by a BJT is quite high, namely, gm*ro, where ro is the collector's ac resistance (the inverse of the slope of the iC vs vCE curve). Rewriting as gm*ro=(IC/VT)*(VA/IC)= VA/VT, where VA is the Early voltage and VT the thermal voltage, indicates that for a typical value of VA~100V, a BJT can provide a gain of almost 4000V/V! In the CFA case the role of ro is played by Req, which consists of the parallel combination of three terms, the output resistances of the npn and pnp mirrors (these resistances are on the order of beta*ro/2), and ther input resistance of the output buffer (which is also very high because of the Darlington configuration). So, your "rubbish" claims reveal an abysmal incompetence on your part - abysmal like the grave you're digging so sapiently for yourself. You think that using extreme adjectives and offending words will reinforce your claims: it only rinforces the pathetic state of your mind and your personality.
  • 08.23.2017
  • In defense of the current-feedback amplifier
  • Moo Koo: you are in the lab, and are given two ICs, along with a sheet showing that they have the same pin configuration. All we know is that one is a CFA, and the other a VFA. How do you go about finding out which is which? No tricks or tantrums, please. Just show us your experimental expertise.
  • 08.23.2017
  • In defense of the current-feedback amplifier
  • Koo Moo No, you haven't proved anything new. Dr. Franco himself showed that in the particular circuit under consideration, the loop gain obtained via 2-port analysis (an approximate form of analysis) just happens to coincide with that found via return-ratio analysis (an exact form of analysis). He even said that there is no point repeating the closed-loop plots, as the results would be the same. Please read! Yet, you decided to proceed with your simulations anyways, in the desperate hope of disproving the results of an exact analysis using the results of an approximate analysis (which in this particular case just happens to be exact). No way that you can make this kind of dagerous generalizations, and I think Dr. Franco himself states this explicitly at some point (plese read his blog and let me know where...) Wanna see an example in which your 2-port anaysis fails in a more evident way? Consider the case in which we wish to account also for the output impedance of the output buffer (call it ro). Return-ratio analysis would give a loop gain of Trr = z(jf)/{RF+ro+rn[1+(RF+ro)/RG]} whereas two-port analysis would continue to give Ttp = z(jf)/{RF+rn[1+RF/RG]} Your closed-loop gain would then be Vo/Vi = (1+RF/RG)/(1+1/Ttp), whereas the exact closed-loop gain would be Vo/Vi = (1+RF/RG)/(1+1/Trr)+aft/(1+Trr) where aft is the feedthrough transmission gain around the amplifier, that is, the gain with z(jf) tending to 0. For our circuit, aft~ro/(RF+ro). At high frequencies both loop gains tend to 0 because z(jf) does so. So, your Vo/Vi would tend to zero, whereas the (exact) Vo/Vi tends to aft, which is different from zero!!! No way that you can fit a CFA into a VFA form. Please read and study the blog (as well as its references) with great attention before you keep on making false accusations. You have splattered the entire comment area gibberish posts. You seem to be digging your grave deeper and deeper. Good luck to you.
  • 08.23.2017
  • In defense of the current-feedback amplifier
  • "Current feedback" means that what comes back to be summed at the input is a current, regardless of whether it is caused by a voltage-sample or a current-sample action at the output. Look at the last two pictures of the article. Look at the actual physics, not at the unfortunate terminology propagated by two-port analysis. Bode's treatment of feedback predates two-port analysis. If I were to get a feedback of zucchini instead of a current, I'd call it zucchini-feedback amplifier (ZFA).
  • 03.11.2015
  • Quest for the Ideal Transistor?
  • Oh, here we go again. Alyward and Koo Moo, you are just obsessed with the CFA, and are all over the internet trying to discredit this wonderful device. The only plausible reason is that you both are seeking notoriety... If there is any misconception in here, that's the one YOU are trying to spread, Alyward! What comes back to a low-impedance summing node is mostly current, period (contrary to what two-port analysis purports it to be). If instead of current you got back zucchini, then you'd call it "zucchini-feedback amplifier (ZFA). All datasheets list CFA'a as high-speed amplifiers. Whether it is because of their "so called class AB" input stage, or because it rains a lot in Seattle, CFAs are inherently fast devices, and enjoy a speed advantage that normal VFAs don't, namely, that of providing a virtually noise-gain-independent bandwidth. If you two are really convinced that the CFA is so wrong, you should contact the thousands of engineers who have applied them successfully, and tell them that no, their applications don't work.
  • 08.23.2017
  • In defense of the current-feedback amplifier
  • The input circuit, like the output circuit, IS a Darlington-type push-pull voltage BUFFER. It has a voltage gain of slightly less than unity, a fairly high output impedance, and a fairly low output impedance (tens of Ohms to just a few Ohms). I can use it to drive a load, or I can use an active load to source to or sink from it current. I just found out that I indeed CAN MODEL it as a voltage buffer in my computer simulation, and have even tried it out in my lab, without getting any smoke (or any genie telling me that I am wrong). Koo Moo, where did you get your "must never" ideas?
  • 08.23.2017
  • In defense of the current-feedback amplifier
  • If there is any misconception in here, that's the one YOU are trying to spread. What comes back to a low-impedance summing node is mostly current, period (contrary to what two-port analysis purports it to be). All datasheets list CFA'a as high-speed amplifiers. Whether it is because of their "so called class AB" input stage, or because it rains a lot in Seattle, CFAs are inherently fast devices, and enjoy a speed advantage that normal VFAs don't, namely, that of providing a virtually noise-gain-independent bandwidth. Please read this blog attentively, and if you are still mad a CFAs, ask one of the designers who also posted here to provide you a list of his 100-million CFA users, and contact them individually to tell them that no, their applications don't work. Unless you are using your comments to promote your own website, in which case, well....