In this second part of a two-part series covering the history of chips, you'll learn about the challenges of analog chips, Moore's Law, and more. Find out why we had a memory bottleneck on PCs, how Intel became so dominant, and what made it possible to put together a system on a chip. Tony Parker explains it from the perspective of his 30 years of experience as a professional chip designer.
A Few Words about Analog
Digital integrated circuits have usually been easier to build than analog integrated circuits. This is primarily because even with rather loose tolerances in the characteristics of the various circuit components (resistors, diodes, transistors, etc.), digital integrated circuits only needed the output voltage (or current depending on the technology) of the circuit to lie within the range that was either recognizably "1" or "0" based on the voltage (or current) received at each of the inputs to the circuit. With analog circuits came the need to control the circuit output characteristics to specified tolerances based on the whole range of possible input conditions. Nevertheless, in the late 1960s analog circuits began appearing on the market.
One of the first analog integrated circuits was the Fairchild mA709 operational amplifier introduced in 1965. I used it in one of my lab courses at MIT to build several circuits. It was slow and not very stable or easy to use. Soon a better operational amplifier, the 741, was introduced and pretty much set the standard for operational amplifiers in the years to come. The original version of the 741 was bipolar, but in the late 1970s an FET version was introduced, followed in the early 1980s by a MOSFET version.
These integrated circuit operational amplifiers were designed to mimic in a limited way the characteristics of a theoretical "ideal" operational amplifier with infinite open loop gain, infinite bandwidth, infinite input impedance (hence zero input current), zero output impedance, zero noise, and zero input offset voltage. The 741, like all actual implementations of a real operational amplifier, only approximated these "ideal" characteristics. Still, over its defined operating range, this "real" operational amplifier approximated the "ideal" sufficiently closely to be useful in a wide range of applications.
Another analog design destined to become ubiquitous was the 555 timer which was introduced by Signetics in 1971. The 555 has three operating modes: Monostable Mode, Astable Mode, and Bistable Mode, which correspond to "one-shot" timer, oscillator, and flip-flop respectively. Each of these modes can be used in a wide variety of practical applications. As a result over a billion of the 555 and its direct descendants were manufactured as recently as 2003.
Until recently, it has been common practice to design and build analog integrated circuits separately from digital integrated circuits for several reasons. Analog circuits are often sensitive to and difficult to isolate from the noise caused by the switching of digital signals on the same integrated circuit. Also, it is often advantageous to build specific analog circuits in a different semiconductor technology (Gallium Arsenide or Bipolar Silicon for instance) from the digital circuits associated with the design, which are usually more cost effectively implemented in a bulk CMOS process.
It is also more difficult to perform manufacturing tests of integrated circuits that contain both digital and analog circuitry. Today's SOCs (Systems on a Chip) need ever higher levels of integration, however. These needs, along with new technology developments like BiCMOS (Bipolar and CMOS Technology on the same integrated circuit), SiGe (Silicon Germanium Technology), and even better techniques for implementing analog circuitry on bulk CMOS technology have led to more "Mixed Signal" integrated circuits where analog functions and digital functions are mixed on the same integrated circuit.
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