Harmonic distortion induced by the λ-effect in MOSFET current mirrors

This paper discusses the large-signal harmonic distortion (HD) caused by channel-modulation (λ-effect) on four elementary MOS current mirror (CM) configurations. The third-order equation that relates the incremental gate-source voltage to signal current in CM input transistor is conveniently simplified into a simpler quadratic dependence, so that a tractable expression for the output current is obtained, and therefore, closed-form formulae for HD₂ and HD₃. The study is supported in good conformity by a set of comparisons between data of HD₂ and HD₃ from a numerical FFT of the cubic equation and those from simplified expressions. HD is calculated in function of circuit and transistor parameters, providing a deep insight for the choice of CM that best trades off layout area and power consumption to meet a distortion level. A comparative HD analysis from distinct current mirrors is also provided. Among the configurations investigated, the high-swing CM presents better linearity, with HD₂ below 0.05% across the range of analysis.