Differential-pair-based current-mode analog signal processing

In CMOS electronics using differential pairs, input signals are usually supplied to gate terminals of MOS transistors creating the pair. Then, the tail terminal of the pair is used to ensure the required bias needed for proper operation of the transistors. In this paper, we show that roles of the gate and tail terminals can be changed. This implies, among others, that the tail can be used as a terminal delivering the input signal. In such a case, new possibilities of decreasing the circuit power consumption occur, especially when making use of current mode techniques. This is because the power consumption can drop to zero if the input signal disappears. Combining current mode technique with the differential one enables to achieve important circuit advantages like the required small headroom and simplicity of realizing signal summation with the ability of a considerable reduction of common-mode effects, characteristic of differential methods. The last feature allows us to process even very weak signals, like it is the case at input stages of the popular differential amplifiers. Necessary conditions for successful implementation of the proposed idea have been explained. Example of a simple CMOS circuit operating according to theoretical predictions made is also presented.