Environmental Engineering Reference
In-Depth Information
Chapter
9
EXAMPLES OF FEEDBACK IN INTEGRATED
CIRCUITS
In this chapter we will describe some selected examples in which the
concepts developed previously are applied. These examples are select in the
sense that they are related to special aspects of feedback circuits which are
only marginally or never treated in standard textbooks. The aim is hence not
to repeat analyses that can be found elsewhere, but to stimulate the reader
with a number of case studies covering not only traditional IC analog circuits
(such as the differential amplifier and current mirrors) but also analog state-
of-the-art topologies (such as the current-feedback operational amplifier).
9.1
THE OUTPUT RESISTANCE OF A DIFFERENTIAL
AMPLIFIER WITH CURRENT-MIRROR LOAD
The
differential amplifier
is a fundamental building block in analog
integrated circuits. It is used in the implementation of the input stage of
operational amplifiers and its features are assumed here to be well known to
the reader. In the discussion to follow, we will consider a differential
amplifier with a current-mirror active load and, specifically, we evaluate its
output resistance.
The schematic diagram of the differential amplifier with current mirror
active load implemented with generic transistors is illustrated in Fig. 9.1. It
is made up of transistor couples T1-T2 and T3-T4, which compose the
differential pair and the active load, respectively, and the constant current
generator
Observe that the bulk terminal has been omitted to preserve
simplicity.
