Abstract
A model on alpha-power law MOSFETs based source-coupled differential pair (SCDP) is discussed and a simple design procedure for realizing a linear CMOS SCDP transconductance element is proposed. The proposed or modified SCDP circuit using this procedure is an alternative to that of conventional SCDP and the circuit discussed has superior linearity for a wide range ±(0-300mv) of input differential voltage at a supply voltage of 1.2v. The modified SCDP also includes the circuitry needed to suppress the variation in the quiescent current with respect to input common-mode voltage noise. The SPICE results are used to verify theoretical predictions. The results show close agreement between the predicted model behavior and the simulated performance. The simulated result on Total Harmonic Distortion (THD) shows that the modified SCDP circuit is better than the conventional SCDP by about four times at input differential voltage amplitude of ±100mv. An example circuit, a second order continuous time gm-C band-pass filter is constructed using the fully differential modified SCDP and the fully differential conventional SCDP circuit and the result shows that the modified transconductor circuit is better in linearity (THD) than the conventional SCDP by about two times at the input differential voltage amplitude of ±100mv. An automatic digital compensation scheme for temperature is also presented and the temperature coefficient of output current is reduced by about eight times to 250ppm/deg. C after compensation for the maximum change in temperature of 150deg.C and at the input differential voltage of 100mv.
Highlights
Linear transconductance elements [1]-[14] are useful in building blocks in analog signal-processing systems and the literature on this topic is rich
In [1] and [8], the linearity with input differential voltage is achieved by CMOS pairs and floating voltage sources
The objective of this paper is to present a model for the alpha-power law based CMOS source-coupled differential pair (SCDP) transconductors and a simple design procedure for the realization of linear CMOS modified SCDP transconductance block for both single-ended and fully differential outputs
Summary
Linear transconductance elements [1]-[14] are useful in building blocks in analog signal-processing systems and the literature on this topic is rich . A cross-coupled quad cell is proposed by [1]. In [9], a bias-offset cross-coupled transconductor is realized. In [1] and [8], the linearity with input differential voltage is achieved by CMOS pairs and floating voltage sources. In [6], the linearity is achieved with two additional PMOS SCDP pairs. A four MOS transistor cell to obtain a linear transconductor is realized in [12]. In [14], the linearity is obtained with a quadritail cell. In all of these transconductors discussed, only the square law devices are considered but in the present paper, a model for SCDP based on the alpha-power law devices is proposed
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