LED Modulation Circuits

M. Gallant Oct 14, 2008

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 λ) 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 current. 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 λ ~ 880 - 920 nm such as those used in consumer remote-control devices, 1.8 V for red LEDs at λ ~ 660 nm, 2.2 V for green LEDs at λ ~ 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.

5 MHz Bandwidth Single Transistor Modulator

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.

50 MHz Bandwidth Op Amp Modulator

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 non-inverting input voltage divider network at a quiescent current of Vcc/2/Ri or about 15 mA using the component values shown with a supply voltage of +/- 10V. The modulation input is capacitatively coupled to the noninverting put via the RC lead network with a low-frequency modulation cutoff at about 3 Hz. Since dynamic resistance of the LED is low compared to Ri the feedback fraction is roughly unity, and therefore the expected modulation bandwidth will be on the order of 70 MHz. To achieve a high LED current modulation index in this configuration, a large output voltage swing across Ri is required and for operation at high-frequency, a very high slew-rate op amp is needed. For example, to achieve a peak modulation current of 10 mA (total is 25 mA) requires a modulation input of Vp ~ 3V. For modulation up to 50 MHz, the required slew-rate is 2*Pi*Vp*f or about 950 V/us which is just within the bandwidth and slew-rate specifications of the LT1363.

As with all high-speed op amp circuits, care is required in the layout and bypassing the power supply.