from the drain of transistor q 1 which is connected to an external control voltage vctrl through a user selectable external resistor rset amplifier a 1 therefore drives the voltage on the gate of transistor q 1 to a level such that the drain voltage of transistor q 1 equals the output of the reference voltage generator vref in the specific exemplary embodiment being described 1 25 volts thus the effect of the amplifier is to hold the voltage across transistor q 1 source to drain to a predetermined voltage 1 25 volts in the exemplary embodiment this is to be compared to simply using a current mirror to mirror a current through a resistor coupled to an external regulated voltage to the transistors coupled to the leds with such a current mirror alone the resistor current and thus the led current as well as the led drive transistor headroom would be highly dependent on the threshold of the transistors which has a substantial processing variation since amplifier a 1 has a very high input impedance the drain current of transistor q 1 will be equal to vctrl vref rset in a typical application the control voltage vctrl may use.