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Differential Amplifier Characterization Using Mixed-Mode Scattering Parameters Obtained From True and Virtual Differential Measurements

This paper examines the differences in large signal mixed-mode scattering parameter characterization of differential amplifiers arising from virtual and true differential probing. The analysis is carried out by means of differential gain compression curves obtained from exemplary amplifier test assemblies with variable common mode rejection ratio. Based on analytical derivations involving basic nonlinear circuit theory, mathematical closed form transfer functions of these amplifiers are presented that enable an a-priori estimation of the occuring measurement error. Numerical simulations complete the theoretical investigations by giving additional physical insights onto the individual amplifier nodal voltages effecting the compression. Experimental results obtained from true and virtual differential compression curve measurements of the considered amplifier topologies are finally compared to the results gained from the theoretical considerations. Based on these results, the reason for the deviation between virtual and true differential measurements is addressed and upper and lower bounds for these deviations are given that are in accordance with the results reported in literature.