CMOS Current Amplifiers: Speed versus Nonlinearity

The tradition of implementing analogue circuits by means of voltage amplifiers is almost as old as the concept of electronic circuit design. The integrated electronic circuit, however, is a relatively new concept. Furthermore, integrated electronic circuits have significantly different limitations and strengths to the conventional discrete electronic circuits have. Since the active devices in integrated circuits amplify current rather than voltage, various current-mode circuit ideas have emerged after the introduction of the integrated circuit. This work deals with analogue integrated circuit design using various types of current-mode amplifiers. These circuits are analysed and realised using modern CMOS integration technologies. The dynamic nonlinearities of these circuits are discussed in detail as in the literature only linear nonidealities and static nonlinearities are conventionally considered. The most important open-loop current-mode amplifier is the second-generation current-conveyor (CCII). For this amplifier, a macromodel is derived that accurately describes all linear nonidealities. Unlike other reported macromodels, this model can accurately predict the common-mode behaviour of differential current-conveyor applications. The accuracy of the model is experimentally verified in the case of currentmode instrumentational amplifiers. This model is also used to describe the nonidealities of several other current-mode amplifiers because circuit structures similar to second-generation current-conveyors are common in such amplifiers. Push-pull class-AB realisations of the second-generation current-conveyor and the current-feedback operational amplifier perform efficiently when implemented in complementary bipolar integration technologies. However, in modern low-voltage CMOS integration technologies both amplifier types suffer from limited input and output voltage swing. Similarly, adequate distortion and input impedance levels are difficult to reach. Therefore, other current-mode amplifiers, such as the current-mode operational amplifier and the high-gain current-conveyor (CCII∞), are more suitable for modern CMOS-processes. Simple calculations show that, unlike with conventional voltagemode operational amplifiers, the large-signal settling behaviour of these two amplifier types does not degrade as CMOS integration technologies are scaled down.