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Figure 1 was probably drawn by an EDN person. Bob's rough sketch is OK. I was taking the output from the wrong place.

You should also note, John, that there are errors in the EDN article.

The first picture shows the opamp in non-inverting configuration - this is, of course positive feedback, and sure enough it will latch to one of the rails.

Further down in the article, variations of the circuit are re-drawn in the inverting configuration - this is the correct version.

It is a quirk of AC analyses that sometimes an unstable configuration appears to be stable, when in fact it isn't. Always check in the time domain!

I presume Bob got it right. He would never have made that mistake. I'm surprised nobody else pointed that out.

--
Regards,
Tony

On 11/05/2025 16:20, John Woodgate via groups.io wrote:

I have uploaded the above as a .ZIP. According to Bob Pease, the gain at the peak should be close to 34 dB, independent of the tuned frequency. I don't get either of those results using version 24.1.8. However, the bandwidth is fairly constant with frequency, as predicted. I can't believe Bob Pease reported the performance in error.