LED Modulation Circuits

Posted on Feb 6, 2014

Common Light Emitting Diodes (LEDs) require a DC forward bias current of 10 to 20 mA for proper operation. Typically the maximum DC current is 30 to 50 mA. The color of light (or wavelength ») emitted from an LED depends on the semiconductor material used to fabricate the LED and this in turn determines the LED forward voltage at the operating cu

LED Modulation Circuits
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rrent. The forward voltage, which is roughly equal to the semiconductor bandgap potential of the active material in the LED is about 1 V for infrared LEDs with » ~ 880 - 920 nm such as those used in consumer remote-control devices, 1. 8 V for red LEDs at » ~ 660 nm, 2. 2 V for green LEDs at » ~ 560 nm and 3V for white LEDs. Infrared LEDs are available with analog modulation bandwidths of kHz to 20 MHz and higher. The circuits below demonstrate two basic approaches for analog current modulation of LEDs. The first circuit is useful for modulation up to about 5 MHz and uses a single transistor circuit biased as a firm current source with the LED in the collector circuit. With the circuit components shown, the high frequency cutoff is about 4 - 5 MHz, limited by the base-emitter forward biased capacitance. The quiescent DC current through the LED as determined approximately by (Vcc/2 - 0. 7)/R5 or about 24 mA. The low frequency cutoff frequency is about 15 Hz. The output current modulation amplitude in midband is approximately Vs/R4. A 100 mV modulation input provides a current modulation of 10 mA. The second circuit is useful up to about 50 MHz and uses a high-frequency analog op amp, a Linear Technology LT1363 with a GBW of 70 MHz, a slew-rate of 1000 V/us and an output current drive capability of at least 50 mA: The circuit is a transconductance or voltage-to-current converter configuration. The output is DC biased using a...

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