TABLE OF CONTENTS Eric A.Vittoz and George Wegmann
The current mirror is a ubiquitous building block in analog integrated circuits. First applied in bipolar technology (see Chapter 6), it is now extensively used in CMOS to duplicate, multiply or divide bias currents or signal currents. Unfortunately, the precision of an MOS mirror is degraded by the large values of threshold offset and 1/f noise of MOS transistors. The classical approach to deal with these limitations will be reviewed in Section 7.3. Section 7.4 will introduce the principle of dynamic current mirrors (or current copiers), which eliminates completely the previous limitations and moves the achievable precision to new limits. Sections 7.5 to 7.9 will discuss these limits, that are essentially due to charge injection by switches, drain conductance and drain-to-gate capacitance, leakage current and sampled noise. Sections 7.10 and 7.11 will show how multiple copies and multiplying or dividing mirrors can be created. Applications are discussed at length in Chapter 14 but some of them will be mentioned in Section 7.12, which also suggests some possible extensions of the dynamic scheme to other functional blocks.
The symbols and definitions that will be used for n- and p-channel transistors are shown in Figure 7.1. Since the device is symmetrical, source, drain and gate voltages V S, V D and V G are referred to the local substrate which is either the general substrate of the circuit, or a separate well. The...
