The present invention uses two transistors instead of a sensing resistor
to provide a constant current source for a load such as an array of light
emitting diodes ("LEDs"). In the present invention, a bias current is
applied to a branch of the circuit. The drain-to-source voltages of two
transistors are matched. The voltage at the gate of both transistors is
controlled based on the bias current and the drain-to-source current of
the first of the two transistors. The second of the two transistors is
sized such that source current of the second transistor is a multiple of
the source current of the first transistor for a given gate voltage. By
the techniques of this invention, the load current in a circuit is
efficiently kept constant at a multiple of the input bias current.