Circuit with reference source to control the small signal transconductance of an amplifier transistor

摘要

A circuit has a reference source (12) for supplying a bias signal to set a small signal transconductance of an amplifier transistor in an amplifier (10) to a predetermined value. The reference source has at least one reference transistor (120a-b, 30). A feedback circuit (128, 129, 38) has an input coupled to the main current channel of the reference transistor or reference transistors (120a-b, 30) and an output coupled to the control electrode of the reference transistor or reference transistors (120a-b, 30). The feedback circuit controls a control voltage at the control electrode, so as to equalize an offset current and a difference between main currents flowing through the current channel of the reference transistor or reference transistors (120a-b, 30), obtained with and without a small voltage offset added to the control voltage. The main currents flowing with and without a small voltage offset may be obtained by using a first and second reference transistor (122a,b), matching each other and an offset voltage source (126) coupled between the control electrodes of the first and second reference transistor (122a,b), to apply the small voltage offset between their control electrodes.