Forgotten Circuits (that should be brought back)
Dennis Feucht - November 12, 2012
As electronics technology matures, it shows some signs of aging. As innovation wanes, breakthroughs grow farther apart in time and risk-taking decreases. In earlier decades, some IC companies were willing to put some unusual and conceptually novel parts on the market.
Signetics dared to put out a novel IC, Hans Camenzind’s 555 timer, and it has become a premier legacy IC. What about others that should also be? This article recounts a few of these parts and why it might be good to bring them back.AD639 Sine Converter
Long ago, Barrie Gilbert did some investigative work on the 2-BJT diff-amp circuit, which has a hyperbolic tangent (tanh) transfer function;
where I0 is the emitter source current and VT»26mV is the thermal voltage. To reduce nonlinearity, external emitter resistance, RE, is placed in series with the emitter in amplifiers. However, Gilbert applied the engineering adage that “if you can’t fix it, feature it” and put it to good use. Hyperbolic tangents are vaguely related to trigonometric functions, and Alan Grebene at Exar used a single diff-amp in the XR2206 function generator IC to convert a triangle-wave into a sine-wave.
The result was somewhat acceptable for a first-generation effort, though more refinement of the basic idea was carried out by Gilbert. He developed the multi-tanh concept of adding the outputs of diff-amps that had inputs offset from each other by a fixed voltage. This extended the function (and input range) and also led to other novelties, including the one that is used in the AD639 sine converter.
This IC is a trigonometric wonderland in 16 pins, and because its functional capability is so powerful, it was destined to become a legacy IC. Alas, ADI pulled the AD639 from the market without a replacement. Even Barrie did not know why. I do not know why. It seemed like a part destined to become a legend. It can synthesize all the basic trigonometric functions (sin, cos, tan sec, csc, cot) and their inverses.
The sine function is accurate to 0.02 %, better than most function generator sine outputs and better than the THD of many audio amplifiers. The IC has two of them, plus offset circuitry, and a multiplier and divider. It was given niche-market pricing and this IC did not find its way into FG instruments and others requiring accurate or low-THD sine-wave generation. It is specified to 1.5MHz.
Perhaps the only problem was that the AD639 was so appealing, ADI put a premium price tag on it which defeated its spread into the market as a commodity part. Perhaps Rochester Electronics (the leading supplier on the “trailing edge”) could revive it and reap the small fortune from it that it was destined to have. There is no reason to confine the mission of Rochester Electronics to that of a replacement parts supplier for obsolete equipment when they could also be considered the fulfiller of the destiny of great parts for new designs that did not catch on the first time around.
Next: CA3096 BJT Array