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9VASMT-YTB0OPTICAL OUTPUT(R,G,B MIX)5.6kREDGREENI RI GIC 1BLUEI B82 nF2.5V+ ADR1581VOLTAGEREFERENCE−V REF1.25VIC 2Q 4≈I O –I BQ 3Q 5Q 6Q 1 Q 22.2kR BB2.2kV +IC 3B+V B100 nF15k0% REDP 215k0% GREEN82 nF+ADR1581−I O120 R BA2.2k2.2kIC 3A+V ADA4091-2100% BLUEV A15kP 115k0% BLUEFigure 1 Potentiometers P 1and P 2let you control the color of emitted light.The differential stages let you vary I R,I G, and I Bover a range of 0 to I O, whereI R+I G+I B≈I O=4.43 mA. This value is approximatebecause I R+I G+I Bis lower bya relative value of 3/β, where β is a currentgain of the bipolar transistors. Therelative error is less than 1%. TransistorQ 6equalizes Q 2’s collector voltage withthose of the Q 1and Q 3collectors. Thisapproach preserves the matching of thebase-emitter voltages of Q 1, Q 2, and Q 3.The base currents of bipolar transistorsin this case can reach to as much as 100μA. For this reason, you route the colorand hue control voltages, V Aand V B,which you derive from resistive potentiometersP 1and P 2, to the bases of Q 2andQ 5through voltage-follower-connectedop amps IC 3Aand IC 3B, two halves ofan Analog Devices’ (www.analog.com)ADA4091-2. The ADA4091-2 has lowpower consumption and input offsetvoltage of less than 500 μV with a typicalvalue of 80 μV.The ADA4091-2 has a maximuminput bias current of 65 nA, whichcauses a negligible voltage drop on resistorsR BAand R BB. This voltage dropis less than 130 μV. You can achieveeven more accuracy by inserting resistorsof the same value as R BAbetweenthe respective inverting inputs and outputsof both the A and the B followers.This step brings reduction of input-biascurrent-causederrors to one-sixth worstcase—down to 1/600.Potentiometer P 1controls the blueLED’s intensity. At the upper-end position,when the LED is 100% blue, transistorsQ 2and Q 3are off, which turns offQ 4and Q 5. Thus I Oflows solely throughQ 2and Q 6. The red and green LEDs aretherefore off. When P 1’s wiper is at 0V,output current flows exclusively throughparalleled Q 1and Q 3and distributes itselfto Q 4and Q 5, depending on the positionof the wiper of potentiometer P 2.With P 2’s wiper at its upper end, the circuitemits 100% green light. At 0V, theemitted light is fully red. An intermediateposition of the wiper yields a mixtureof red and green. By moving P 1’s wiperfrom the ground position, the circuitproduces a mixture of red, green, andblue.Transistors Q 1, Q 2, and Q 3shouldtightly match. You need a difference inbase-emitter voltages of less than 1.5 mV.The same requirement holds true for theQ 4/Q 5pair. Matching requirements areless stringent for Q 6. You should use abipolar NPN matched-transistor pair forQ 1through Q 6, or at least Q 1through Q 5,whereas Q 6is a single transistor. Eventually,you can use three matched-transistorpairs.EDNSEPTEMBER 9, 2010 | EDN 53