Electronic Design

Bob's Mailbox

Dear Bob:
In your recent "Mailbox" (electronic design, April 15, p. 69) you wrote, "On a good day, I teach by asking questions." This Socratic Method stuff has been around for a while now. It works well. But somehow it hasn't caught on much outside of the people who learned it first hand. Maybe a new tech name will revive it. How about "Socraticological Methodology?"
Paul Butler
via e-mail

I dunno, Paul. Maybe I'm just syllable-logically challenged.—RAP

Hi Bob:
I was just reading your "What's All This Multiplication Stuff, Anyhow? (Part IV)" column (electronic design, July 8, p. 72). As a fellow analogophile, may I remind you that the good-old, industry-standard 1496 multiplier (modulator/demodulator) chip has been brought back into production?

As you may recall, it's a basic eight- transistor differential multiplier cell in a 14-pin package. Both ON Semiconductor (n'ee Motorola) and NJR are making them (MC1496 and NJM1496, respectively), which retail for under 50 cents. (I think I got my last ones for 35 cents each.) If I recall, National also used to make these as LM1496s. They're a lot cheaper than the AD534, and multisource too. (Yeah, but they lose linearity REALLY FAST. So if you get good s/n ratio, it's at the cost of poor dynamic range. The LM13700 is better when you consider what system specs you need to meet. /rap)

For basic multiplier functions, not much is cheaper and simpler to use. The input biasing needs a little care, and it works well into the many MHz. (True! /rap) I've used this part for assorted signal-detector circuits with great success, along with a few additional transistors or op amps. It's great for synchronous detector circuits for instrumentation applications where you want to detect a tiny modulated signal in a noisy environment. (I'm toying with a modulated laser/light-beam guidance system for a work colleague's "Battlebot" using such circuitry. No more fiddly joysticks to operate.)

The 1496 is only reasonably linear for small signal levels. (Its siblings, the 1494 and 1495, provided many volts of range.) But these shortcomings are usually possible to design with. (The LM13700 still has advantages, I think. /rap) You've caused me to put my thinking cap on for an elegant, cheap solution to the voltage-controller pot using one of these and a few discrete transistors.
John Lord
via e-mail

But it will be hard to hold good linearity! Best wishes!—RAP

Dear Mr. Pease:
I take back every uncomplimentary word I've ever spoken or written to/about you. (I don't let that bother me. So long as fewer people are cussing at me than at Bill Gates, I don't feel bad. /rap) I'm a '71 transistor, IC, µP, ASIC, FPGA circuits kind of guy who has lately gravitated to electron/photon interactions. They used to think the photons were digital, eh, but there are enough subdivisions to make any analog guy happy. (If you're interested in photons, have you looked into Philip Hobbs' book on electro-optical stuff? It's $125 and worth it. /rap)

Lately, I've been having fun building every one- and two-transistor circuit that I can find. Colpitts, Hartley, blocking, Theremin mixers, starved-current pre-amps, linear LED talkers, etc. It's so cool to "hear" thunderstorms 60 miles away with a 5087-88 pair amping an FET electret mike. My wife and I once hid under the bed because we forgot the mike was "on" and feeding our stereo. (My wife and I hide under the bed for a different reason—to get away from the kids. /rap) The tornado was 35 miles east wrecking Brett Favre's place in Hattiesburg, but my mixer/t-storm listener made it sound like it was right on us.

Your "Wattmeter" (electronic design, May 13, p. 86) is one of the most fun, single-page, two-transistor with tempcomp multiplier, circuit-cum-explanation events that I can remember. (Thank you! It IS a fun circuit. I'm glad you like it. /rap)

With all due respect to Mr. Widlar (may he rest in cerveza-soaked partying peace), and you, I would change my first name to "Bob" if I didn't already have a great cuz by that name. You are "the man" as the (probably obsolete) kids would say, and I thank you.
Dan Garcia
via e-mail

Well, I try to be a good teacher about analog STUFF. Thank you for the comments.—RAP

Dear Bob:
I need an article on "What's All This Bandwidth Stuff, Anyhow?" There are a bunch of TV commercials about increasing your bandwidth.
Leroy Lakey
via e-mail

Leroy, I heard that Global Crossing and Enron had some things in common—besides just being a couple of HUGE bankruptcies surrounded by fraud, etc. These guys were doing trades with each other—trades of bandwidth. They traded the (potential) bandwidth back and forth, back and forth, raising the prices and the volume. Nobody ever USED the bandwidth: They just traded it. Just like the sardines—the "buying-and-selling sardines" that were only suitable for trading and buying and selling, but not for eating.—RAP

All for now. / Comments invited!
RAP / Robert A. Pease / Engineer
[email protected]—or:

Mail Stop D2597A
National Semiconductor
P.O. Box 58090
Santa Clara, CA 95052-8090

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