Electronic Design

Bob's Mailbox

HI BOB,
I like your bridge amplifier (“What’s All This Bridge Amplifier Stuff, Anyhow?”). I can’t figure out what the VREF driven offset into the first stage does (It is just a tiny bias, so if the input goes ±12 µV, the circuit will keep on working. /rap) and I can’t figure out what the diode-configured transistor in the final stage does. I request your words of wisdom. (The output of a “rail-to-rail” amplifier can not swing all the way to ground. It might stop at +6 or 9 mV. The output of the circuit shown will go down to 2 or 3 µV. /rap)

As always, I am deeply interested in your trekking experiences. I just turned 60. I suspect you are a couple of years beyond that. (I am 68, but my trekking stories are still on my Web pages. /rap) Two years ago, my sons led me to the top of Half Dome, Yosemite. By the time I got to the top, I was dead, exhausted, finished. Trouble was, I still had to get back. Lucky for me it was all downhill, or they would have had to evacuate me. (I have never had any need or wish to go up Half Dome. /rap)

The last time I read about your hiking, you suffered a bit of frostbite. Are you okay now? (My toes are still uncomfortable, and so are the rest of my feet. My legs are in lousy, poor shape, but I am starting to get back. /rap) Still hiking? One of these days, I hope to cross paths with you in the Sierras. My sons want to do Mt. Whitney next. I think I am a bit long in the tooth for that. Have you been there?

BOB SIEFERT

HELLO, BOB,
I have hiked halfway up Whitney, but I like I said, I never got very interested in going to the top. I have hiked high enough in Nepal that at the saddle point of Thorong La (a col), I could spit down 3000 feet onto Whitney. And I’ve been 1000 feet higher than that at Kala Pattar and Everest Base Camp. And we had very good views. We ascended 1000 feet per day to get up there, with very good acclimatization. I think it is a brutal mistreatment of your body to rush up to 14k with no acclimatization. You are just asking for trouble, headaches, and altitude sickness when you go up fast as people do on those fourteeners.

RAP

HI BOB,
Great column (“What’s All This Bridge Amplifier Stuff, Anyhow?”). I measure my progress in analog skills by how far I can follow you and if I can spot the problems before you describe them. The feedback resistor size seemed low to me, more because I’ve been doing a fair amount of micropower stuff. Besides, we used 10k/20k resistors back with 741s.

(Well, I agree that 1k/2k was pretty low. But these days, many op amps can drive 1 mA without even 1 µV of input error, and older op amps couldn’t say that. Also, many modern op amps can go from 1k/2k to 100k/200k without causing even 10 mV of error due to IBIAS. /rap)

The rail-to-rail issue was learned painfully years ago—best to treat a rail-to-rail op amp like something halfway between an LM324 and a perfect op amp. (Yeah, and if that ain’t good enough, change the topology! /rap) If Figure 1 has the output of A1 and A2 each at a perfect 2.0000 V, what would be the output of A3, if we had a –1.0-V supply to run A3? Jim Stewart

HI, JIM,
Any one of the four resistors around A3 can cause a 13-mV error in VOUT if it is off by 1%. If all four R’s gang up and drift 1% in the wrong direction, VOUT could be as big as –52 mV or +52 mV—a truly sloppy performance. Even if the 1% resistors were improved to 0.1%, you would still have a ±5-mV error, max. No, we can’t tolerate that kind of error, even if the amplifier could swing to –5 mV, because the analog-to-digital converter can’t detect a –5-mV input! So, it just goes to show that putting some 1% or even 0.1% resistors in a circuit does not make a precision circuit!

RAP

HELLO BP,
I just read the bridge amplifier article. How about using 1% thick-film resistorpak with mostly matched and trimmed elements? I think that would help make a predictable performance product. (If you need pairs of matched R’s, same value, not so bad. But two R’s at 50 O and 1k will not have good tempco tracking or stability. It’s best to buy a couple of discrete R’s at low tempco. If you shopped for 50 O and 2k, you couldn’t find ’em! Even buying a thin-film network with those values is hard! /rap)

I recently read the reprint of “What’s All This Double-Clutching Stuff, Anyhow?”. I used to have a Datsun 2000 five-speed and could shift up and down as fast as you could throw the gear lever. I think those gears must have been turning together all the time. (Oh, the gears are in constant-mesh, but the synchros have to change. /rap) I only used the clutch to get it rolling or when I was behaving. And, I had hoped to catch one of your on-the-road shows. If you are traveling out this way, I would entertain!

GEOFFREY CAMPBELL

HELLO, GEOFFREY,
Thanks for the comments! RAP

Comments invited! [email protected] —or: r.a. Pease, 682 miramar avenue San francisco, Ca 94112-1232

BOB PEASE obtained a BSEE from MIT in 1961 and was a Staff Scientist at National Semiconductor Corp., Santa Clara, Calif.

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