Linear CMOS transconductor with rail-to-rail compliance

摘要

An integrated transconductor circuit has an active load element and two parallel sets of transconductor stages. The active load element provides a high impedance without requiring a large area within the integrated transconductor circuit, and provides linear impedance when the active load devices are biased in the triode region of their respective characteristic curves. The multiple transconductor stages prevent saturation from reducing the dynamic range available at the output of the transconductor circuit. The parallel sets of transconductor stages are used to improve output linearity by reducing the differences between the threshold voltages of the serially-connected transconductor stages. In a particular example, the sizes of the active devices of the sets of transconductor circuit stages are chosen to provide a monotonic, substantially exponential transconductance to implement a control input to a current-controlled oscillator in a phase- locked loop that assures the frequency-independent stability of the phase-locked loop. The exponential function is selectively rolled off at the upper end to prevent excessively high-frequency operation at start- up from over- driving a divider logic and locking up the phase-locked loop. The transfer function curve of the transconductor circuit is robust to process variations over a wide range of operating conditions.