A Very High Output Impedance Tail Current Source for Low Voltage Applications

Keitaro SEKINE

IEICE TRANSACTIONS on Fundamentals of Electronics, Communications and Computer Sciences   Vol.E83-A    No.2    pp.204-209
Publication Date: 2000/02/25
Online ISSN: 
Print ISSN: 0916-8508
Type of Manuscript: Special Section PAPER (Special Section on Analog Circuit Techniques and Related Topics)
analog signal processing,  MOS analog circuit,  tail current source,  high output impedance,  low voltage operation,  

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A tail current source is often employed for many analog building blocks. It can limit the increase of excess power. It 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 source in theory. Simulation results show that the output impedance of the proposed circuit becomes 1.28 GW at low frequencies. Applying the proposed circuit to a differential amplifier, the CMRR is enhanced by 66.7 dB, compared to the conventional differential amplifier. Moreover, the proposed circuit has the other excellent merit. The output stage of the proposed tail current source can operate at VDS(sat) and a quarter of VDS(sat) of the simple current source in theory and simulation, respectively. For example, in the simulation, when the reference current IREF is set to 100µA, the minimum voltage of the simple current source approximates 0.4 V, whereas that of the proposed current source approximates 0.1 V. Thus, the dynamic range 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.