The human eye can see any color as a mixture of blue, red, and green. The circuit in Figure 1 produces all three colors through an Broadcom ASMT-YTB0 tricolor LED. You can produce a wide range of colors by varying the current in the blue, red, and green LEDs.
|Figure 1.||Potentiometers P1 and P2 let you control the color of emitted light.|
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 0 V 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 0 V at both the stages, and collector currents IR, IG, and IB are almost equal.
The differential stages let you vary IR, IG, and IB over a range of 0 to IO, where
IR + IG + IB ≈ IO = 4.43 mA.
This value is approximate because IR + IG + IB 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, VA and VB, which you derive from resistive potentiometers P1 and P2, to the bases of Q2 and Q5 through voltage-follower-connected op amps IC3A and IC3B, 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.
The ADA4091-2 has a maximum input bias current of 65 nA, which causes a negligible voltage drop on resistors RBA and RBB. This voltage drop is less than 130 μV. You can achieve even more accuracy by inserting resistors of the same value as RBA 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 IO flows solely through Q2 and Q6. The red and green LEDs are therefore off. When P1’s wiper is at 0 V, 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 0 V, 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.
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