A Linearization Technique for Variable Transconductors Using Emitter Coupled Pairs and Its Application to a 1-Volt Active Filter

A family of linearized transconductors consisting of N bipolar transistor differential pairs are introduced. The basic idea of the linearization technique is to synthesize a single linearized dc transfer characteristic from the nonlinear transfer characteristics of individual differential pairs operating in parallel. Maximally flat transfer characteristics can be obtained by giving each pair an appropriate input offset voltage and by weighting their tail currents properly. A high transconductance to operating current ratio can be obtained by using this technique. It is particularly useful for low-voltage and/or low current designs, because it is not based on a negative feedback technique or a trans linear technique. For example, when N=4 is used, the linear input range is expanded to about 100 mVp-p at 1% error. The conditions for obtaining a maximally flat transconductance are presented. A 1-volt 5th order active lowpass filter IC using this technique was designed and fabricated. The measured results are also presented.