This chapter begins by explaining briefly why there is still a need for analog design and introduces its main tradeoffs. The need for accurate component modeling follows. Then, the essentials of p–n junctions and bipolar and field-effect transistors (FETs) for circuit design are recalled. The main differences between bipolar-transistor and FET operations are emphasized and drawbacks of some popular FET models for circuit design are commented on. Finally, two single-stage amplifiers, one in bipolar and another in MOS technology, are designed in order to make the differences between these technologies clear.

Analog design

The need for analog design

Analog circuits such as audio and radio amplifiers have been in use since the early days of electronics. Analog systems carry the signals in the form of physical variables such as voltages, currents, or charges, which are continuous functions of time. The manipulation of these variables must often be carried out with high accuracy.

On the other hand, in digital systems the link of the variables with the physical world is indirect, since each signal is represented by a sequence of numbers. Clearly, the types of electrical performance that must be achieved by analog and digital electronic circuits are quite different, and for this reason they are generally studied in separate university courses.

Nowadays, analog circuits continue to be used for direct signal processing in some very-high-frequency or specialized applications, but their main use is in interfacing computers to the analog world.