Tricolor LED emits light of any color or hue

Vary the current in three LEDs to produce any color.

Marián Štofka, Slovak University of Technology, Bratislava, Slovakia; Edited by Martin Rowe and Fran Granville -- EDN, September 9, 2010

The collector outputs of bipolar differential stages form the current sources. A classic symmetrical differential stage with two equal bipolar transistors is a backbone of almost all bipolar analog ICs. In this case, however, the differential stage is asymmetrical, with a 2-to-1 collector-current distribution instead of the common 1-to-1 ratio at 0V base-voltage difference. The circuit produces the 2-to-1 current ratio by paralleling a third equal transistor, Q₃, to Q₁. The common collector of the paralleled transistor pair connects to the common emitter of the Q₄/Q₅ differential stage. Thus, the base differential voltages equal 0V at both the stages, and collector currents I_R, I_G, and I_B are almost equal.

The differential stages let you vary I_R, I_G, and I_B over a range of 0 to I_O, where I_R+I_G+I_B≈I_O=4.43 mA. This value is approximate because I_R+I_G+I_B is lower by a relative value of 3/β, where β is a current gain of the bipolar transistors. The relative error is less than 1%. Transistor Q₆ equalizes Q₂’s collector voltage with those of the Q₁ and Q₃ collectors. This approach preserves the matching of the base-emitter voltages of Q₁, Q₂, and Q₃. The base currents of bipolar transistors in this case can reach to as much as 100 μA. For this reason, you route the color and hue control voltages, V_A and V_B, which you derive from resistive potentiometers P₁ and P₂, to the bases of Q₂ and Q₅ through voltage-follower-connected op amps IC_3A and IC_3B, two halves of an Analog Devices’ ADA4091-2. The ADA4091-2 has low power consumption and input offset voltage of less than 500 μV with a typical value of 80 μV.

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The ADA4091-2 has a maximum input bias current of 65 nA, which causes a negligible voltage drop on resistors R_BA and R_BB. This voltage drop is less than 130 μV. You can achieve even more accuracy by inserting resistors of the same value as R_BA between the respective inverting inputs and outputs of both the A and the B followers. This step brings reduction of input-bias-current-caused errors to one-sixth worst case—down to 1/600.

Potentiometer P₁ controls the blue LED’s intensity. At the upper-end position, when the LED is 100% blue, transistors Q₂ and Q₃ are off, which turns off Q₄ and Q₅. Thus I_O flows solely through Q₂ and Q₆. The red and green LEDs are therefore off. When P₁’s wiper is at 0V, output current flows exclusively through paralleled Q₁ and Q₃ and distributes itself to Q₄ and Q₅, depending on the position of the wiper of potentiometer P₂. With P₂’s wiper at its upper end, the circuit emits 100% green light. At 0V, the emitted light is fully red. An intermediate position of the wiper yields a mixture of red and green. By moving P₁’s wiper from the ground position, the circuit produces a mixture of red, green, and blue.

Transistors Q₁, Q₂, and Q₃ should tightly match. You need a difference in base-emitter voltages of less than 1.5 mV. The same requirement holds true for the Q₄/Q₅ pair. Matching requirements are less stringent for Q6. You should use a bipolar NPN matched-transistor pair for Q₁ through Q₆, or at least Q₁ through Q₅, whereas Q₆ is a single transistor. Eventually, you can use three matched-transistor pairs.