Power-saving voltage-to-current conversion with the use of CMOS differential amplifier

Differential amplifiers are well known as input stage preamplifiers. This is because they exhibit the ability to reduce unwanted common-mode effects considerably. As a consequence, both noise and input signal of the amplifier can have low values. Proper operation of differential amplifiers is possible when implemented in chip form. For typical use of such CMOS amplifiers, input signals are delivered to differential-pair gate-terminals while tail terminal is used to ensure the required bias of the pair. The paper shows that the roles of gates and tail terminal can be changed. In other words, the tail current can be used as input signal while the gate ones as voltages controlling the amplifier gain. This enables to combine the achievable low noise with power efficient operation of the circuit. Necessary conditions for that are discussed in this paper. Suitability of atypically used differential amplifiers for voltage-to-current conversion is explained. Two examples of CMOS circuits implementing power economic conversion of this type are presented.