Analog: Back to the future, part two

Analog Devices Inc. (ADI)

EDN directed some pertinent questions to Barry Gilbert, ADI Technology Fellow (Figure 5) and Bob Adams, ADI Fellow (Figure 7):

Figure 5: Barry Gilbert in his early days circa 1951

Barrie Gilbert

Background: Gilbert is one of the industry’s foremost experts in the development and application of analog circuitry. He now directs engineering at the Northwest Labs in Beaverton, Oregon, ADI’s first remote design center.

Gilbert’s 40-year affiliation with Analog Devices Inc. – dating back to 1972 – saw the company go from being principally a module maker to a producer of high-volume IC parts and digital signal processors.

One of the circuit cells that bears his name has for decades been used in all forms of communication systems, including ordinary radios, cell phones, microwave TV links, data modems, satellite communications and even radio telescopes.

The Gilbert cell—actually an entire class of versatile cell topologies used as basic analog function blocks—has served as the foundational design for products used everywhere in today’s electronic systems.  All invoke the now famous Translinear Principle. This fundamental theory in circuit design was discovered, formalized, refined and popularized by Gilbert. Translinear circuits perform pure-current-mode signal processing, a fundamental insight. Today, these ideas, whether in the original bipolar form or in CMOS embodiments, are found throughout analog design.

Figure 6: A schematic of the two-quadrant Gilbert Cell

Since its invention in 1967, what has become known as the Gilbert Mixer is now ubiquitous in radio transmitters and receivers. The compact nature and precise commutation properties of this mixer opened one of many important doors to the integration of radios in monolithic form, leading to the proliferation of modern indispensable communications devices. A closely related circuit, known as the Gilbert multiplier, overnight revolutionized the implementation of this important mathematical analog function. The 1968 Journal of Solid-State Circuits paper describing it became the first paper to be cited 100 times. Today, more than 40 years later, it remains one of the most-cited JSSC papers.

Gilbert believes that childhood hardships—including at age three losing his father in World War II, leaving his mother and three other children penniless—force one to be resourceful. Before and during his teenage years, he had access to a plethora of inexpensive military surplus gear which greatly helped to make him inventive. Gilbert laments that today's aspiring engineers are lacking the visceral experience of handling and hefting large coils and tuning capacitors, transformers and vacuum tubes, and such. Today’s surplus circuit boards are all but useless as a source of inspiration, or even “spare parts” to tinker with.

1. What initial analog developments from ADI’s past have helped shape ICs in the 21^st century industry?

In 1971-72, Analog Devices worked with and funded a start-up called Nova Devices to begin fabrication of linear integrated circuits. This collaboration carried through to become Analog Devices Semiconductor (ADS) division and started to deliver high-performance linear in 1972. Much of the early revenue came from op amps, including the AD741 – a near-copy of the historic precursor, but with stronger emphasis on precision and quality. And Analog Devices began making laser-trimmed FET op amps with much better performance than the industry standards of the day. This emphasis on providing high accuracy and advanced performance arose from the very earliest days. With the advent of nonlinear functions, such as the first high accuracy laser-trimmed analog multiplier, the AD534, the first monolithic RMS-DC converter, the AD536, and the first complete monolithic V/F converter, the AD537 – all using translinear techniques – another seminal emphasis came to the fore, namely the provision of precise calibration of nonlinear functions. Undoubtedly, it was the development of wafer-laser-trimming of ADI’s proprietary thin-film resistors that gave the company a considerable edge, in this regard.

Later, Analog Devices and the ADS division made further progress in the fabrication of linear ICs. Process 1 was optimized principally for op amp use; Process 2 was a little faster, and was used in I²L modes in the earliest ADCs. Further advances in speed came with Process 3. I felt the need for, and defined, a complementary bipolar process, which became “CB.” A radical later departure was the early adoption of silicon-on-insulator (SOI) processes, the “XF” series.

2. How did your innovation or an older architecture specifically set us up to achieve today’s 21^st century performance?

The growth to maturity of ADS, which rapidly became the largest revenue generator of ADI, and is no longer a separate division, came out of the seminal contributions of numerous talented people. It’s hard to identify crucial product developments that shaped the company at large. Each of these people would have a unique perspective on that issue.

But if I am to speak of from my own viewpoint, I believe it was my personal interest in “radio” – going back to childhood days – that drove me to push hard to provide the tools and ideas to develop chips for this sector of the business, at a time when the company was predominantly a provider of industrial and, to a lesser extent, military components. No one was using the word “gigahertz” at that time! One key development was of the first five-stage RF logarithmic amplifier, the AD640, sometime in the 1980s. Since that time, my team and I have development numerous multistage log amps for use in RF power measurement. We can boast that practically every cell phone and base station in the world uses these ADI products.

As for “older architecture,” products developed in the 1970s using translinear techniques – such as the AD534 multiplier, and other developments of that kind – remain in the catalog and continue to generate significant revenue.

Beyond that, I believe my insistence that we needed at least one scientific computer – and eventually a CAD team of our own – eventually yielded fruit. We initially purchased one VAX780 and time-shared it.

3.   What advice do you have for today’s 21^st century designer? What analog know-how does today’s designer need to create successful designs?

These are crucial questions, but they would need the wisdom of Solomon to provide adequate answers!

First, I would say this. Before any young person enters into a life of microelectronic design, he or she should be quite sure that this is going to be the beginning of a life full of joyous discovery and invention. There are many fields that can provide this sort of joy, so self-examination as to a career is essential at a very early age.

Second set out to be the best in your field. As an IC designer, you will need to wear many assorted hats. Yes, often you will be wearing your Circuit Designer’s Cap and Cape, but at other times, you will need to don your Pragmatist’s Hat, your Economist’s Hat, your Physicist’s Hat and many more. Deliberation over difficult trade-offs will frequently arise. In short, IC product design is not simply about transistors.

Third, beyond being “best in class” you must aspire to becoming Master of the Dance. By that, I mean that you will develop a deep sense of being in control of all that your mischievous little transistors do.

They will often want to sing, when you just need them to do a jig from left to right across your stage. When you choreograph your circuit on the screen, you must think like a transistor thinks. You must actually become a transistor!

Fourth, ask “What IF?” a thousand times a day. This question is the quintessential fountain of invention.