Large signal performance of class-AB transconductance stage

This paper presents a simple mathematical model for the differential output-current/ differential input-voltage characteristic of the class-AB CMOS transconductor. The model, basically a Fourier series, yields closed-form expressions for the amplitudes of the harmonic and intermodulation components of the differential output-current resulting from a multisinusoidal differential input-voltage. The special case of a two-tone equal-amplitude differential input-voltage is considered in detail. The results show that the harmonic and intermodulation performance of the class-AB CMOS transconductor is strongly dependent on the values of the bias current of the transconductor and the amplitudes of the input tones with the third-order intermodulation component dominating over a wide range of the input voltage amplitudes and bias currents. The results also show that while the third-order harmonic and intermodulation products are monotonically increasing with the increase in the differential input-voltage amplitude, the second-order components are exhibiting maxima.