Digitally controlled precision current source with an open loop compensation circuit

摘要

A precision current source includes a current source transistor and a current steering circuit which selectively directs the output current from the current source transistor through either a first or a second current path in response to a digital input signal. A compensation circuit directs a highly accurate steady state current through first and second series coupled compensation transistors. The base of the second compensation transistor is coupled to the base of the current source transistor. The compensation circuit operates to control the voltage differential between the base of the current source transistor and a voltage source to maintain the output current intermittently steered through the second current path precisely constant regardless of changes in the beta of the current switching transistor or changes in the beta or base to emitter voltage of the current source transistor.