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!