Page 52 - Analog Circuit Design Art, Science, and Personalities

Samuel Wilensky

Figure 6-2.
creative. but do it by building on the present base of knowledge. A quote from Sir Transistor-
Isaac Newton beautifully describes how this design process works: transistor
switched binary
If 1 have seen farther than others weighted 12-bit
it is by standing on the shoulders of giants. DAC.
-.--Sir isaac Ncwion to Roheri Iiooke. February 5. 1675

A less tasteful, but some would say more honest, illustration of how electronic
circuits are designed is contained in a humorous 1950s song by Tom Lehrer:

PI agi ark, PI ag iarize ,
Let no one else's work evade your eyes.
Remember why the good Lord made your cyes.
So don't shade your eyes but.
Plagiarize, Plagiarize.
Only be sure always to call it please.
Research.
---Song by Torn Letirer

1 quoted the lyrics of the Lehrer sorig tongue-in-cheek, but circuit design is an
evolutionary process where one must draw on past developments. The digital-to-
ariaiog converter (DAC) of the early 1960s is a classic example of how a circuit de-
velops, changes, and improves as it moves through the hands of different designers.
For those of you not familiar with DACs a quick explanation is in order. The
DAC is a device whose input is a digital number, usually in a binary format, and
whose output is an analog signal. The analog output is usually a voltage or a current
whose value is a function of the digital input and a reference voltage (see Figure 6-1 j.
The DAC was one of the first circuits developed for linking the analog and digital
domains. and even today the DAC plays a large role in computer graphic terminals,
music synthesizers, and the many other applications in which a digital processor
must communicate with thc analog world.
During the early 1960s transistors were replacing vacuum tubes, and digital inte-
grated circuits were just becoming available. Analog integrated circuits were not
widely available, and those that were available were expensive. Almost all analog
circuit design was carried out with discrete components and an occasional integrated
amplifier. The transistor was becoming available, and since it closely approximates
an ideal cui+ent source, it was an excellent candidate for the switch in a current out-
put DAC. The first DACs built with transistors used emitter coupled current

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