Category Archives: App Notes

Jim Williams, AN13, & Op Amp Wizardry

Not too long ago I was reading through one of Jim Williams’ famous App Notes, AN13, High Speed Comparator Techniques. It’s an older App Note that was published back in 1985 and I didn’t really have a specific reason for reading it other than thinking it looked interesting and I wanted to learn more about comparators. For a comprehensive overview of AN13 I recommend reading Dr. Lundberg’s (aka Dr. Analog) three part summary over on his blog Reading Jim Williams.

The first section of AN13 is extremely informative and makes the app note well worth the read in my opinion. Entitled “The Rouges Gallery of High Speed Comparator Problems” this portion of the app note highlights common pitfalls of comparator circuits including bypassing, ground planes, probe compensation, and much more. As useful as The Rouges Gallery is what intrigues me the most in AN13 doesn’t actually have anything to do with comparators and is found in the first circuit of the Applications Section. The overall circuit is a Voltage to Frequency Converter shown in Figure 16 of AN13 and reproduced below with the part that fascinates me most boxed in red.

What Jim Williams has done is replace the input stage of A1, the LT318A, with a pair of 2N4393 JFETs. These JFETs drive the output stage of A1 via the two Comp pins of the amplifier. A1’s true inputs are shutdown by shorting them directly to the -15V supply leaving the rest of the amplifier free to serve Williams’ needs for this particular application. According to page 8 of the app note this trick was done “for low bias, high-speed operation.”  Now I don’t have a whole lot of experience using op amps with Comp pins to begin with let alone understand their internal architecture enough to hack them like Jim did. As far as I knew something like this wasn’t even possible and it definitely wasn’t brought up in any of the classes I took in school! Needless to say, upon seeing it done here in AN13 I was a little stunned.

After my initial shock I decided to look at the LT318A a little more closely to try to see how Jim Williams had pulled off this neat little trick. Linear Tech is usually pretty good about providing schematics  of their op amps with a description in their datasheets and I was hoping this was the case with the 318. As it turns out, the LT318A datasheet is a bit sparse compared to other datasheets from Linear but fortunately for me it does contain a schematic of the part. Unfortunately for me, it doesn’t appear to be a simplified version and there isn’t a functional description. Looking at the schematic in detail I could tell this wasn’t the basic op-amp architecture I was used to dealing with but I decided to dive in anyways. I’ve copied the LT318A schematic here and marked it up into functional blocks as best as I’m able to but if anyone out there has more info on this op amp or sees somewhere that I went wrong please let me know.

From studying the datasheet schematic of the LT318A I was able to understand more of how Jim’s op amp hack works. Connecting the amps inputs on pins 2 and 3 to the negative rail turns off the differential amplifier, the heart of which are the input transistors Q1-Q4 with Q13 and Q14 being the active loads. As a side note there may be some common-mode feedback on the diff pair but I’m not 100% sure. Thinking about what was said in the app note I would have to assume using two JFETs in place of the input circuit on the op amp would lower bias requirements and cutting out all those transistors speeds up operation of the overall application circuit.

The output from the input differential stage is then fed into what I assume to be a second gain stage  (not sure what Q21 does, this may be a weird folded cascode configuration too). From Figure 16 in AN13, you can see that Jim Williams has the JFET input transistors driving pins 1 and 5 on the LT318A. Sure enough these two pins correspond to the output of the diff pair on the amplifier and feed directly into the second gain stage. Following the gain stage comes what appears to be a Class AB output stage.

I enjoyed this little exercise of trying to understand some of Jim Williams’ techniques. While I may not have exactly figured out how the LT318A works I feel I did understand the high-level thought process behind this neat little trick. I also saw that my BJT design skills are a little rusty and that perhaps I should dust off Gray & Meyer or Sedra-Smith and brush up on the topic. Who knows, there could be a revisit to this post in the future…

Any neat op amp hacks of your own? See a mistake in my analysis of the LT318A? Let me know in the comments. Oh, and Happy New Year!

Useful App Notes: Part 2

AN104 – Load Transient Response Testing for Voltage Regulators, Linear Tech

An app note written by the late great Jim Williams back in 2006 discussing how to test linear regulators by applying a load transient to the output. These techniques discussed are also directly applicable to switching regulators as well making the app note doubly useful. My favorite parts of this app note are Figures 6 and 8 which show two interesting transient generator circuits, one BJT based and the other FET based. At the moment I’m trying to gather up the necessary parts to prototype one of the circuits and possibly throw it in a project box to use at work. A great resource for anyone doing work in power electronics.

ANA – The Monolithic Operational Amplifier: A Tutorial Study, National Semi

Want a fantastic reference about the intricacies of op amp design? Look no further than this app note written by Bob Widlar. Covering everything from input/output stages, to techniques for increasing slew rate and proper layout, any and all relevant topics related to op amp design are discussed.  There’s even a section on thermal feedback which is interesting as it’s not a topic typically covered all to often. While I don’t think I would recommend this app note as a stand alone reference for a beginner, it’s perfect for someone looking to expand their knowledge on op amp design. If you are a beginner I would suggest pairing ANA with either Johns & Martin or Gray & Meyer. Fun Fact: Gray and Meyer are cited several times in the bibliography.

The Data Conversion Handbook – Analog Devices

Not exactly an app note but still another good reference to keep at hand. If you’re looking for a general overview of ADCs and DACs  along with a bunch of other useful information I highly recommend this book. Chapter 1 is an interesting history of converter technologies throughout the ages. Chapter 2 discusses sampling theory and defines converter specs. Chapter 3 covers the various topologies of DACs and ADCs from a primarily applications standpoint rather than a design one but is still incredibly detailed.  Chapters 6 and 7 cover interfacing and all the various support circuitry that goes into properly using converters in your design. Chapter 9 has a some good information regarding board layout, passive components, prototyping, and more. I read through most of this handbook two summers ago during an internship while waiting for automated tests to run and I can say I learned quite a bit.

Useful App Notes: Part 1

Over the years I’ve accumulated a rather astounding number of app notes on my hard drive from various companies.  What few ones I’ve read so far are, in my opinion, very useful and should be shared.  The rest I’ve downloaded thinking “This looks like something I should keep around in case it ever comes in handy,” only to never open them and let them collect virtual dust.  Sharing the interesting ones on my blog seemed like a win-win-win situation, I learn things reading app notes, you learn things from reading app notes, and I de-clutter my hard drive/Dropbox account,everybody wins. So without further ado here’s a few of the most useful app notes I’ve come across.

AN1613: From SPICE Netlist to Allegro Design Sub-circuit, Intersil

If you’re like me whenever you come across a part that isn’t in any of the main component libraries when using P-Spice you think to yourself “Is this part really necessary? How well does my design work with this op amp instead of the one I want?”  I did this because up until I found this app note, even if I could find a SPICE model online for a part I wanted, I didn’t know what to do with it.  Enter AN1613.  It goes through an easy step by step process of how to take a SPICE model you’ve found online and actually make it work in a Cadence Allegro Design  simulator.   I’ve used this app note to help simulate parts from a few IC companies with great results.

Op Amps for Everyone, Texas Instruments

While not exactly a single app note, Op Amps for Everyone is still a great design guide to have handy when you need to quickly refresh yourself on a topic or get a basic overview of a concept before researching it in more detail.  It covers everything from basic circuit analysis and feedback theory to filter design, converter interface, and everything in between.  There’s plenty of examples to go along with the theory as well as a whole chapter on layout considerations.

AN95-1: S-Parameter Techniques for Faster, More Accurate Network Design, Hewlett Packard

Here’s a blast from the past, AN95-1 was first released in 1967 by HP. While slightly on the old side, I still found this app note pretty useful when trying to wrap my head around S-Parameters last summer.

Got any favorite app notes you can’t live without? Share them in the comments, I’m always looking to collect more of them.