What’s All This Neutralization Stuff, Anyhow?

Sept. 15, 2010
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Here’s a limerick for you:

There once was a man with a brain
Who decided to visit Fort Wayne.
He sold all the hoods
A complete bill of goods
And never went back there again.

Yeah, 40 years ago, I was called to go to Indiana to help solve a customer’s business problem. It turned out to be an easy problem, so we finished up, went to lunch early, and talked about analog stuff. However, the customer did remember that he had another little problem.

The customer had a variable-gain amplifier (Fig. 1) where the photoresistor changed to provide the right gain. But the inherent capacitance of the photoresistor was hurting the phase shift, especially when the resistance got high, up toward 2 MΩ. There was a built-in 5-pF capacitor that could not be defeated. Now what?

A Lunchtime Solution
I looked it over. Stray, fixed capacitance that is inaccessible can be tricky to cancel or counteract. H’mm. We had some new T82AH amplifiers from Philbrick, a variation on the Amelco 805BE. Inexpensive. And it could provide flat response out to over 2 MHz.

I proposed adding the circuit of Figure 1A. This is a classical form of neutralization. I forget if we built it and measured it, but after lunch, we convinced ourselves that it would work fine. And I still had more than an hour before I had to flee to the airport.

I sat there and admired this excellent circuit. But why couldn’t I get some ac positive feedback without adding a second amplifier? I thought about Figure 1B instead of 1A. I put in a little positive feedback at mid-frequencies to provide the desired phase-shift cancellation. Hey, it worked, too! And, this scheme didn’t even need a variable capacitor. It could be easily trimmed with the pot.

The engineer built it up, and it worked perfectly, and it went into production just fine. It just goes to show that there are many good ways to improve the performance of an analog circuit. Some are trickier than others. They aren’t all in books. Don’t be afraid to experiment.

“And I never went back there again.”

Recent Complaints About The “Smart Grid”
Recent gripes about “smart” power meters have centered on the observed fact that some customers’ bills have increased greatly. Yes, there have been a few new energy meters that read too high (such as 2×), as if the new meter has an error. However, if the new meter reads 2× too low (which can also happen), very few customers are complaining!

Finally, PG&E ran some tests using the old electromechanical meter, a standard smart meter, and a precision calibrated smart meter, all stacked up in series (see “Independent Report Clears Smart Meters, Faults Utility” at www.electronicdesign.com).

In most cases of error, the old meter was just reading much too low (due to friction?) with big scale errors (by 3×, 5×, or more). So, the customer was accustomed to paying low bills because of an old gain error and is now going to have to start paying more.

Some customers are still bleating that the new smart meters put too much electromagnetic “radiation” into the air, causing them much “danger.” They haven’t figured out that these new meters broadcast a few seconds per hour. If you sat on top of that meter, you wouldn’t get as much “radiation” as you get from your electronic wristwatch. But skeptics are hard to convince. Sigh.

Beast regrds! (sic) / RAP

P.S. More notes on limericks later.

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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