In this chapter, fundamental building blocks are described. These blocks include a variety of current mirrors, single-stage amplifiers with active loads, and differential pairs. A good knowledge of these building blocks is critical to understanding many subjects in the rest of this book and for analog IC design in general. CMOS mirrors and gain stages are emphasized because they are prevalent in modern designs. Fortunately, most of the small-signal analyses presented can be applied to bipolar circuits with little change. In addition, rather than using resistive loads and ac coupling, the gain stages covered are shown with current-mirror active loads since such loads are almost always used in integrated circuits.
When analyzing electronic circuits containing transistors to determine their small-signal behavior, it is implicitly assumed that signals are small enough that linear approximations about an operating point accurately reflect how the circuit operates. These linear approximations may be represented schematically by replacing transistors with their small-signal equivalents, whose parameters (
ds, etc.) are related to the device’s operating point currents and voltages and are summarized in Section 1.3. The general procedure for small-signal analysis is therefore: