TABLE OF CONTENTS Current sources and mirrors are among the most important building blocks of analog CMOS circuits and are being extensively used as DC current sources, DC current mirrors and AC current mirrors. For DC and AC current mirrors, the mirroring factor can be unity, or more than unity; in other words a current mirror can be considered and used as a current amplifier.
In this appendix, first the DC behavior of the DC current sources and current mirrors will be analyzed. Afterwards, the frequency characteristics of AC current mirrors, which have severe effects on the overall performance of many circuits, will be investigated in some detail.
The simplest MOS current source is a MOS transistor operating in the saturation region, i.e. V DS ? (V GS ? V T ), (Fig. D.1(a) ). Here load represents the circuit to be fed by this current source. The current is mainly determined by the gate source voltage V GS , but owing to the channel length modulation effect, also has a weak dependence on V DS . We know from (1.14b) that the load current can be expressed as
In this circuit, owing to the quadratic character of the load current control voltage relation, the sensitivity is high. Therefore, to obtain a certain desired load current it is necessary to adjust and maintain V GS
