transistors could be used besides fets including bjts the positive terminal 402 receives a reference voltage vref 1 404 which is determined based on the desired current that is to flow through the serial led string 310 the negative terminal 404 is connected to the source of the fet 408 at node 410 which is connected to one terminal of a resistor 412 the second terminal of the resistor 412 is connected to a ground 414 resistor 412 is preferably a highly accurate stable resistor so that a voltage measurement at node 410 will be used to accurately determine the current through the led string 310 the drain of the fet 408 is connected to the cathode of the last led 309 of the corresponding serial led string 310 at a node 316 as shown still referring to fig4 during operation opamp 416 detects the voltage drop across resistor 412 by measuring the voltage at node 410 and comparing it to vref 1 404 the opamp 416 generates an output voltage 418 that controls the gate voltage of the fet 408 and therefore the conductivity of fet 408 based on the difference between the voltage at node 410 and the reference voltage 404 more specifically the opamp 416 measures the voltage across the resistor 412 and drives the fet 408 so that the voltage across the resistor 412 substantially matches the reference voltage 404 as such the conductivity of fet 416 and therefore the current flow through the corresponding led string 310 can be adjusted higher or lower i e regulated by adjusting the reference voltage 404 the.