Having lately been involved in the archiving of old articles and various correspondence, some frustration has been noted. My contact info is not up-to-date! But this is understandable, as the associated time periods predate email and today’s Internet.
So, I’d love to hear from anyone with whom I shared letters way back when, especially from the 1980s and later. Please pass on this note if you know anyone in this category (email contact here).
Recently, AudioXpress published an interesting article on the use of feedback amplifiers. The article is Michael Kiwanuka’s “Current Feedback and Voltage Feedback Fallacies” which appeared in June 2017, p32-37. The article purports to clarify certain ‘fallacies’ regarding four basic feedback configurations, using op amps.
The conclusions reached by the article really are most curious. Quoting directly from the final summation: ‘The terms “current feedback amplifier” and “voltage feedback amplifier” as presently used are wholly unfounded. ‘
This is a very odd conclusion, as it flies directly in the face of 3-4 decades of current feedback amplifier (CFA) history! In the attachment cited below, the bibliographic references document this point quite well, and will serve as Useful Background Reading. After careful study, readers should then be able to draw their own conclusions as to whether the feedback system used in conjunction with a given amplifier architecture employs current or voltage. A couple of CFA data sheets will help immeasurably, of course. In fact, this exercise should show just how CFAs operate.
Author MK also is critical of certain book authors on these subjects, namely Professor Sergio Franco and Walt Jung. Accordingly, we have authored a technical rebuttal document, cited below. This document is supported by other signatories, as noted within. This rebuttal has been forwarded to AudioXpress, along with a request that it be made available to their readership. We are hopeful that in time they will honor this request.
This is an update to this posting, as of 7/12/2017, including a newly revised technical rebuttal document. This revision includes links to the latest errata information from AudioXpress.
This notice is to inform the users of this website about current and or pending changes to Walt’s Blog. It is back!
The blog site ceased operation over the past months, with most if not all PDF content moved into an archival format. This content will continue to be available, via the Site Pages link at www.waltjung.org.
We are happy to note some recent Walt Jung articles, just below. Click the highlighted link to download a copy. Comments are welcome.
Most Recent WaltJung.org Articles
2017: ‘A Sources 101 Update’ Is a new and updated article related to the original ‘Sources 101’ as referenced below. It appeared in AudioXpress in June 2017, and includes detailed analysis of the test methodology. It also includes both SPICE results and new high performance circuits with their associated lab results. Note that the Sources 101 Kit (below) includes all relevant materials, including LTSpice test files.
2016: ‘Sources 101 Kit’Is a ZIPfile summary of ‘Sources 101’ and the related follow-up letters from AudioXpress; April 2007, May 2007, September 2007, and April 2009. Download this for a complete summary.
Watch these blog pages for additional content of importance.
I recently received a very interesting email from Walt Kester, my friend and former co-worker at Analog Devices Inc. (ADI). In it he was announcing the online availability of about 30 ADI books. I am paraphrasing his note as follows:
This link will take you to a PDF file that allows you to download material out of any or all these books:
The PDF file has active links to the table of contents and the actual files for all the books published over the 50 year history of ADI, including a collection of op amp articles by Ray Stata from the 1960s.
Thanks for assembling such a powerful collection, Walt! I’m sure many readers will find this most useful.
Looking for a low noise reference circuit for an audio regulator, at 2.5V? But you’ve found bandgap circuits too noisy? Read on!
Consider the simple circuit to the right, which I call the GLED431. Just 3 low cost parts, all easy to get. It acts like an extremely low noise 2.5V zener. On my setup, noise measures around 2nV/√Hz, so if you take out the measuring system noise, the actual noise is likely below 1nV/√Hz. Really quite good. We’ll have more on this later on, in 2016.
While the GLED431 performance is very high for noise, you will need to apply about 5mA (or more) to make it work. Yes indeed, this current threshold is much higher than that of the TL431. But, it also has around 1/100 the noise! Caveat(1): The voltage won’t be as tight as typical bandgap ICs, nor as low for temperature drift. Those are conscious tradeoffs.
Here are some Vout measurements on a sample set of 5 LTL-4231Ns, in the lab prototype shown, after 1 minute warmup:
Not too shabby! In the schematic, the leftmost R values are just as shown from lab tests, as trimmed for the 2.500V target Vout. Obviously, just use a single 150Ω RN60D unit for this R. Note that the forward voltage of the LTL-4231N green LED (LiteOn) and the Vbe of the ZTX951 (Diodes Inc.) conveniently add, producing the desired Vout of 2.5V. Caveat(2): These two parts should not be changed if you expect to get close to 2.500V!
In use, if you are building say, a 5V regulator, select a series resistor so that 5mA is supplied to the GLED431 cell (499Ω). With this, also be sure to select a very low noise op amp, and reduce all the surrounding resistances, so as to minimize their noise contributions. Finally, be careful to minimize capacitive loading.
I am now releasing this simple version, as a Christmas present to the readers. Stay tuned for more, have fun with the GLED431, and have a great holiday!
We are pleased to note that Gary Galo has recently made a very large Guest Contribution to this website. This is in the form of a series of his notable preamp modification articles, that appeared in AudioXpress. The articles are listed below in the sequence they appeared. Click the individual link for a given article. A ZIPfile with all the articles is available as well, at the end.
The four (+) part series on modifications to the Adcom GFP565 preamp:
After fighting various forms of SPAM comments for way too long, we’ve decided to try a new system for comments from users. It is effective now, and will enable a comment sent via email, on any aspect of this website.
Simply use MakeWebComment at WaltJung.org as your target email, after changing the ” at ” appropriately (no spaces, subbing an uppercase 2). That’s it. Write on!