Linear current modulation with feedback on Light Emitting Diodes in Optical Wireless Communication

For LED-based transmitters used in optical wireless transmissions, the relationship between the input voltage and the output light intensity of an LED is nonlinear. Hence, voltage modulation cannot be used directly on an LED for continuous modulation such as OFDM without signal distortion. Compared to voltage modulation with a predistortion circuit, current modulation is simpler to apply since the output light intensity of an LED is linearly proportional to the input current over a wide range. This paper considers BJT-based transconductance circuits used for current modulation in LED-based transmitters, and proposes the use of a feedback circuit topology to overcome the problem of high variation in the forward current gain parameter of BJTs. In particular, the current-series feedback topology is analyzed, yielding a circuit design that can be used for current modulation and is insensitive to the current gain variation.