Sergio Franco

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Professor

I am an author and (now emeritus) university professor, who was drawn to analog electronics by unusual circumstances. After graduating in physics in Italy, I secured a Fulbright Scholarship to do graduate work as a member of the venerable ILLIAC III Computer Project at the University of Illinois... only to find that all the graduate research positions (digital) had already been taken, except for one (analog) that nobody wanted. So, I had to sit down in the lab and learn the ropes on my own (transistors, op amps, data converters, log amplifiers, analog multipliers), and am glad that my physics background helped me look at circuits using physical insight, with math being only a tool for a more rigorous corroboration (when necessary). I applied the analog expertise thus acquired to the design of an electronic system for the composition of music in real time (SalMar Contruction). After my doctorate, I left academia to work in industry. Then, in 1980, I returned to academia (summers off!), where I contributed to the formation of many hundreds of analog engineers, most of them now happily and gainfully employed in Silicon Valley. Along with book writing, teaching has been my most satisfying career highlight. I enjoy writing and explaining things, especially when it comes to using physical intuition to demystify unduly contrived concepts. More about me at http://online.sfsu.edu/sfranco.


Sergio Franco

's contributions
  • 03.11.2015
  • 9 Comment(s)
Quest for the Ideal Transistor?
  • 02.11.2015
  • 1 Comment(s)
Feedback with Bidirectional Blocks
  • 12.03.2014
  • 6 Comment(s)
Circuit paradoxes – Or are they?
  • 10.28.2014
  • 6 Comment(s)
Miller Compensation and the RHPZ
  • 09.13.2014
  • 3 Comment(s)
Loop gain measurements
  • 06.03.2014
  • 1 Comment(s)
Feedback and Impedances
  • 05.21.2014
  • 4 Comment(s)
Two-port vs. return-ratio analysis
  • 03.11.2015
  • Quest for the Ideal Transistor?
  • D Feucht: Your statement 'Whether feedback is called "voltage" or "current" is misleading' does not hold in the case of Fig. 6b. If you ground the input (Vi = 0), break the loop at the CFA's output, and inject a test signal into the feedback network to see what comes back to the CFA's inverting input, you see just a current - no voltage is fed back, as the input buffer keeps Vn fixed at 0 V. So, you can only refer to this state of affairs as "current feedback". You then easily see that the forward gain is zc, the feeback factor is 1/R2, and the loop gain is T = zc/R2. In practice, the input buffer is not ideal as in Fig. 6, but exhibits some small non-zero output impedance zn. You can in principle re-analyze the circuit as a 'voltage amplifier' with respect to the input difference Vi-Vn = zn*In, but you get far more useful insight (and far more quickly so) if you continue to view it as a current-input amplifier and focus instead on how zn causes it to 'deviate' from the idealized case of zn = 0 (the effect of zn is to reduce the feedback factor somewhat, and, hence, the loop gain - see Ref [3]). As far as the dynamics go, you can view the entire CFA as a mere R-C network (R = R2, C = Cc), where the resistor current (vI-vO)/R2 is conveyed to Cc not 'directly' via R2 itself, but 'indirectly' via the current mirrors. It is precisely because of the current mirrors that a CFA is 'inherently fast'. Except for the C node, each node in the circuit exhibits a low resistance (~1/gm), so the pole that each of these nodes forms with its own stray capacitance occurs at a much higher frequency than the pole associated with the C node; this, so long as R2 is much greater than 1/gm. (This situation is similar to the CMOS op amp of the folded-cascode type, whose open loop response is dominated by the pole of just one node.) There is no slew-rate limiting in an R-C network; likewise, thanks to its unique internal architecture, the CFA is virtually slew-rate free.
  • 12.30.2014
  • Book Review: Analog Circuit Design: Discrete & Integrated
  • The LSC Analog Circuit Design was a spiral-bound preproduction version designed for class usage while waiting for the production of the book reviewed above to be completed. The preproduction version contains errata that have been eliminated from the final version. Please see http://online.sfsu.edu/sfranco/BookAnalog/AnalogErrata.pdf
  • 09.13.2014
  • Loop gain measurements
  • Jverbrug: Here's my next blog: http://www.edn.com/electronics-blogs/analog-bytes/4438647/Feedback-with-Bidirectional-Blocks
  • 10.28.2014
  • Miller Compensation and the RHPZ
  • I think that if you open the document in its pdf format, all figures show alright. Sorry for the inconvenience. There are also a few typos that we'll try to fix soon.
  • 09.13.2014
  • Loop gain measurements
  • Thanks for pointing out. When I started this series of blogs on negative feedback, I indicated explicitly my intention to "progress through levels of increasing complexity" [A], or a bottom-up approach. So, in [A] I began with what is usually referred to as "conventional feedback", consisting of two unidirectional blocks. In [B] I expanded the scope by including forward transmission through the feedback network. In [C] I used what we might call "The 1975 Middlebrook Method" to investigate circuit stability. "[This] method usually provides pretty accurate results, [even though it does] not take into account backward transmission through the loop. [Moreover] it will always tell you correctly whether a circuit is stable for small perturbations or not" [D]. The next logical step of my negative-feedback series will be the GFT, or what we might call "The 2006 Middlebrook Method". But, one thing at a time... [A] http://www.edn.com/electronics-blogs/analog-bytes/4424393/The-magic-of-negative-feedback [B] http://www.edn.com/electronics-blogs/analog-bytes/4427143/Feedthrough-in-negative-feedback-circuits- [C] http://www.edn.com/electronics-blogs/analog-bytes/4434609/Loop-gain-measurements- [D] https://sites.google.com/site/frankwiedmann/loopgain
  • 05.21.2014
  • Two-port vs. return-ratio analysis
  • Yes, of course there are plenty of other sources with more detailed and rigorous information on the subject. However, 10+ lectures of 50 minutes each, is a lot of time for the stated scope of my tutorials on negative-feedback review, which I try to present in an informal, intuitive, and faster fashion: http://www.edn.com/electronics-blogs/analog-bytes/4424393/The-magic-of-negative-feedback "Negative feedback is full of fascinating nuances that some students, rushing through homework and tests, don’t get the chance to absorb in full. Many will master them on the job after graduation, but others may not get the opportunity to go deeper. To honor the genius of Harold Black, I intend to post a series of tutorial blogs specifically for these engineers. My “analog bytes” will progress through levels of increasing complexity, from the very basics of the present byte all the way to a byte on the oft perceived as intimidating topic of frequency compensation in the presence of a right-half-plane zero."
  • 02.02.2012
  • Implement an audio-frequency tilt-equalizer filter
  • Interesting circuit - never saw it before. If the pot wiper capacitance is an issue, you can try alleviating it by inserting a dummy resistance R in series between the wiper and the inverting input - just keep R low enough so as to limit its noise contribution (thermal noise of R plus op amp's noise current flowing through R itself).
  • 06.18.2014
  • Can you find the missing 1?
  • Yes, indeed, yours is yet another useful way to look at it, and thus help dispel possible confusion by the beginner. The return ratio of the circuit is independent of whether you use it as an inverting or is a non-inverting amplifier (assuming the resistors remain the same). The only (usually minor) difference is in the feed-through gain of the two configurations (typically more severe in the inverting case), also an issue covered in one of my blogs at: http://edn.com/electronics-blogs/analog-bytes/4427143/Feedthrough-in-negative-feedback-circuits-
  • 06.18.2014
  • Can you find the missing 1?
  • An insightful alternative for emphasizing the differences between the inverting and the non-inverting configurations is to focus on their respective feedback topologies: series-shunt for the noninverting configuration, and shunt shunt for the inverting one: http://edn.com/electronics-blogs/analog-bytes/4426049/Inverting-vs--Noninverting-