App Note 45 part 2

Again, we have a collection of circuits here that captured Jim's imagination in some way or are improvements of circuits from previous app notes.

Figure 18 shows a quartz-stabilized oscillator, which is a different approach from the Hewlett-Packard-inspired oscillators in App Note 43. This circuit achieves 9 ppm distortion (Figure 48 in App Note 43 achieved 3 ppm distortion), but it requires a 4-kHz J-cut crystal.

Figure 19 is a single-cell-powered temperature-compensated crystal oscillator, similar to App Note 15, Figure 9. A boost converter is used to drive the varactor diode with bias voltages up to 4V.

Figure 21 appears to be an improved version of the "Zoo Circuit" V-to-F converter (App Note 23 Figure 16) with even lower power consumption (maximum 90 microamps). Figure 24 is another V-to-F converter, this one with a bipolar input (and a start-up circuit adapted from the Tek 547 trigger circuit).

Figure 27 is a 350-ps rise-time pulse generator. This circuit will be very useful in App Note 47 and other upcoming app notes (and it is much better than the 1-ns pulse generator in App Note 13 Figure D1). The pulse in Figure 28 is very clean, shown on his Tek 556 with the 1S1 sampling plugin. "I'm sorry, but 1GHz is the fastest scope in my house." (See Reference 7.)

Figure 30 is a low-dropout regulator using the LT1123 and the specially design (and now unavailable?) MJE1123 transistor. A germanium 2N4276 is explored as a replacement (but is no easier to obtain!).

Figure 36 is a power supply for a cold-cathode fluorescent lamp. Look at all the bottles! I count 48 of them. Yikes. Although Jim may not know it yet, this application is the beginning of a long-term obsession (or was it an assignment?). More praise for the Tektronix 556 and 547 on the bottom of page AN45-22.

Best quote (from Figure 36, a harbinger of future difficulties): "Do not substitute components."

More than half of the references on page AN45-23 (References 9 to 17) have to do with fetal heart monitoring (as shown in Figure 1). I feel a little sorry for Jim's wife and unborn son at this point. The App Note concludes with a great picture of Michael, perched atop a Tektronix 556.

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Related:

• App Note 45 part 1

App Note 29 part 3

This app note has several interesting appendices (of course, Jim's appendices are always interesting).

Appendix A discusses a common topology for a 5V-to-plus-and-minus-15V converter. The topology discussed is the Royer topology, and here, Jim thoroughly badmouths it. His main complaints are the noise (the switching residue) at the output and the current consumption (due to the magnetic saturation behavior). He says, "The converter's inherent operation ensures noisy outputs" and he suggests several schemes to minimize or avoid the noise. The "bracket pulse" and strobed-output operation are worthy of Rube Goldberg. He claims, "While useful, none of these arrangements offer the flexibility of the inherently noise free converters shown in the text." That sentence may be a bit of an overstatement. "Noise free" is an exaggeration.

The funny thing about this appendix is that he returns to the Royer topology for LCD backlight lamps, and expends quote a bit of ink extolling their virtues in that application. Of course, his complaints are still valid in that application, but I wonder if he ever wished that he had tempered his language here a little bit.

Appendices B and C we have seen before in previous app notes.

Appendix D discusses inductor selection. He says "Electromagnetic theory, although applicable to these issues, can be confusing, particularly to the non-specialist." He suggests the purchase of an inductor kit (such as shown in Figure D2) and a cut-and-try method for inductor selection. Jim sure loved his kits and decade boxes! (See previous app notes for praise for his General Radio 1432 resistor decade boxes.) This approach is a practical one, particularly if you can avoid the horrors shown in Figure D6. Yikes.

Appendix E covers converter efficiency and sources on inefficiency. There is good discussion and good advice all around here (with a brief mention of using germanium).

I already covered Appendix F last week.

Appendix G discusses magnetics and inductor selection (again!), and contains another great quote about his own inspiration (page AN29-44): "As a purveyor of switching power ICs we incur responsibility towards addressing the magnetics issue (our publically spirited attitude is, admittedly, capitalistically influenced)."

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Related:

• App Note 29 part 1
• App Note 29 part 2

App Note 15

"Circuitry for single cell operation." 8 pages.

This app note suffers from a curious self-contradiction, to which Jim almost admits in the last section: "No commercially available logic, processor or memory family will operate from 1.5V. Many of the circuits described [herein] normally work in logic-driven systems." In other words, there are some great circuit designs here that work within 1.5-volt power supply, but what circuits interface to the ADC, VFC, and S&H? The A-to-D converter and the V-to-F converter have "logic" level outputs, and the sample-and-holds have "logic" level inputs (where "logic" means high (1.5V) and low (0V) voltages), but there is no logic family that works at 1.5V (or at least there wasn't in 1985; ironically, these circuits are probably more useful today than they were then).

But, if you go ahead and use Figure 13's flyback regulator (based on a Bob Widlar design) to produce a five-volt supply to power your logic, why not run the whole system from five volts? In fact, App Note 11 has already covered clever tricks for five-volt circuits, and App Note 8 has already covered power-supply design techniques for battery-powered systems (especially App Note 8 Figure 18). So what are we doing here?

Why do people climb Mt. Everest? Because it's there. Why does Jim design circuits for 1.5V operation? Because he can. Getting active circuits to work at 1.5V is deep magic, and Jim has it. Part of the magic is the LT1017/1018 comparator, part of the magic is Widlar's LM10 op amp, and part of magic is clever circuit design. Figures 1 and 3 show how to use capacitor charging and the low-voltage comparators to implement a V-to-F converter and a single-slope A-to-D converter, respectively. Figure 6 is a ramp-and-hold circuit using the LM10 op amp.

The best circuit is Figure 7, a more traditional sample-and-hold with a JFET switch and a charge pump to get enough negative voltage to turn the switch off. (Is a charge pump cheating? Maybe.) This S&H requires careful hand selection of the JFETs to get a below-500-millivolt pinch-off transistor for Q1, and 500-microvolt matching for Q2 and Q3. Do you want low voltage operation? You pay for it!

Figure 9 is another temperature-compensated crystal oscillator (see App Note 12 Figure 6), using a voltage-boost stage that is shown in Figure 11 here (and also App Note 8 Figure 18 and similar to App Note 6 Figure 7; he must have had a box of Triad transformers).

The App Note concludes with a box section on "Components for 1.5V operation" which basically just says "LT1017, LT1018, and LM10; good luck, sparky!" (He also mentions the LT1004 and LT1034 voltage references.) He discusses the pros and cons of silicon and germanium diodes and transistors (if you have a stash of germanium diodes and transistors, that is). There is also a graph of battery voltage versus life for carbon-zinc and mercury, which is quite interesting.

Best quote (for the specifications, from page AN15-8): "The LM10 op amp-reference runs as low as 1.1V; the LT1017/LT1018 comparator goes down to 1.2V. The LM10 provides good DC input characteristics, although speed is limited to 0.1V/us. The LT1017/LT1018 comparator series features microsecond range response time, high gain and good DC characteristics." Amazing.

App Note 13 part 3

The appendices of App Note 13 include a wealth of practical information. Appendix A talks about bypass capacitors and includes five scope traces that warn of potential troubles. Figure A7 is particularly horrifying. (I wish he named some names here; I'd like to know what specific combination of capacitors caused that shameful ringing. I guess I'll have to experiment myself... Personally, I've been using a combination of tantalum and X7R for bypassing. I really should check it out, as Jim suggests.)

Appendix B further discusses probes and oscilloscopes. Again, he doesn't name any specific makes and models of oscilloscopes, but we can guess what he's using (a Tek 547 and a Tek 556). It's funny how he suggests that the oscilloscope should have 150 MHz of bandwidth, after admitting that "90% of the development work was done with a 50MHz oscilloscope." More space is devoted to discussing probes, FET probes, current probes, and (of course) grounding. I think that I will steal the test circuit in Figure B1 to use at the basis of a lecture demo and/or lab project. It is simple, yet instructive. The picture in Figure B5 shows a wide variety of probe types ("Note the ground strap on the third finger."). 

Appendix C discusses some suggestions for ground planes. In short, use them and love them. 

All three of the above appendices will appear again (in one form or another) in App Note 47.

Appendix D shows an interesting and strange circuit for producing very fast pulses. First comment: the LM301A is only specified for a maximum voltage of 36V. The military-grade version, the LM101A, is specified to 44V. I wonder why he didn't suggest the LM101A? Second comment: the circuit uses a TD-263B tunnel diode! That's cool (it's the right tool for the job), but I don't think that Germanium Power Devices even makes tunnel diodes any more. Does anyone? In Figure D2 and the accompanying caption, we learn that the heretical HP scope that we occasionally see is a 275-MHz unit. (I don't know my HP scopes very well. Can anyone identify this model? Is it an HP 1725A?)

Appendix E discusses high-speed level shifters. Figure E2 shows a TTL-inspired level shift with a 15-volt output. I like figure E3 with the speed-up capacitor and the Baker clamp. I really do have a soft spot in my heart for old logic-circuit topologies. I'm curious about what application requires that power FET switching one amp(!) in 9 nanoseconds in Figure E4.

Best quote (page AN13-27): "Probes are the most overlooked cause of oscilloscope mismeasurement." Yep.

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Related:

• App Note 13 part 1
• App Note 13 part 2

App Note 8

"Power conditioning techniques for batteries." 10 pages.

This app note includes useful circuits for battery-powered systems, including voltage doublers, voltage inverters, and rail splitters. The LTC1044 is used in several circuits, as is Bob Widlar's low-voltage op amp, the LM10 (Figures 3, 16, and 18).

Figure 6 is an interesting low-dropout regulator, using a voltage doubler to supply the op amp to drive the base on the NPN pass transistor, achieving a dropout voltage of Vce,sat. Figures 9 and 11 show low-power switching regulators (Figure 11 includes a linear-regulator stage with a JFET pass transistor). Figure 12 shows an "inductorless" switched-capacitor regulator (which is exactly the same as Figure 23 in App Note 3).

Germanium! Excellent! Figure 18 uses germanium diodes and transistors. How often do you get to use 2N1194 transistors and 1N100A diodes? Can you even buy 2N1194 transistors anymore? I bet Jim was hoarding them, waiting for an opportunity to use them. (At first glance, I thought that Figure 18 was awfully strange, given that it doesn't use any Linear Technology parts. After a little more digging, I found out that Linear did second-source the National LM10 for a while (along with some other parts). Interesting. Given the contentious (and litigious) relationship between Linear and National in the early days, I'm surprised there was that much technology transfer.)

(With my over-active imagination, I can imagine how this situation arose: Linear Tech asked Bob Widlar to design a low-voltage op amp for them, and Widlar replied, "Been there, done that, it's called the LM10." When Linear said, "No, that's a National part; we want one of our own to make and sell," Bob gruffly replied, "Then get it from National." Disclaimer: I totally made this scene up.)

The best circuit is the single-inductor, dual voltage flyback regulator in Figure 22. I just think it's a neat topology to get a bipolar 15-volt power supply from a 6-volt battery using a single inductor, although I'm surprised that the clock input on the 74C74 is just labeled "30kHz input". I'm shocked that he didn't design a Wien-bridge oscillator to provide the 30-kHz input. Shocked.

Best quote (page AN8-8): "This circuit [Figure 18] will supply a 5V, 150uA load (about 25 CMOS SSI ICs) for 3000 hours from a single 1.5V D battery." Impressive.