A very high output impedance tail current source for low voltage applications

A tail current sourer is often employed for many analog building blocks. It can limit the increase uf excess power. Il can also improve CMRR and PSRR. In this paper, we propose a very high output impedance tail current source for low voltage applications. The proposed tail current source has almost the same output impedance as the conventional cascode type tail current sou rce in theory. Simulation results show that the output impedance of the pro posed circuit becomes 1.28 G Omega at low frequencies. Applying the propose d circuit to a differential amplifier. the CMRR is enhanced by 66.7 dB, com pared to the conventional differential amplifier. Moreover, the proposed ci rcuit has the other excellent merit. The output stage of the proposed tail current source can operate at V-DS(sat) and a quarter of V-DS(sat) of the s imple current source in theory and simulation. resprctively. For example. i n the simulation, when the reference current I-REF is set to 100 mu A. the minimum voltage of the simple current source approximates 0.4 V. whereas th at of the proposed current source approximates 0.1 V. Thus, the dynamics ra nge can be enlarged by 0.3 V in this case. The value is still enough large value for low voltage applications. Hence, the proposed tail current source is suitable for low voltage applications.