TABLE OF CONTENTS Current sources are basic electronic building blocks that are used extensively in the architectures of analog ICs, as well as in OEM circuit board designs. In both cases, current sources are created by combining diodes, resistors, and transistors (BJTs or FETs). They can also be created at the circuit board level by using discretes, matched pairs, transistor arrays, or by combining op amps with precision voltage references. The various techniques for doing so will be reviewed in Part 1 of this book. Although a few dedicated monolithic current source ICs are commercially available, it is often necessary to create one's own circuit to match the particular needs of the application.
Although most forms of today's instrumentation use either voltage or current references, the former are far more available. As a result, designers frequently use voltage references together with precision resistors, so that a stable reference voltage is converted into a precise current. Applications for current sources range from biasing and stabilization to reference and linearizing. For example, in the design of an op amp, the IC designer will use current sources to create active loads for the amplifier stages and to establish precise bias levels. By providing a constant current, this forces amplifier stages to stay at the Q-points within their active linear regions (see Figure 2.1). In a circuit board design, a current source may be used for linearly charging a capacitor with a constant current, as in a precision timing circuit or in a peak...
