EXTENSION OF THE MODULATION BANDWIDTH OF LEDS BY POLE CANCELLATION

It is shown that the dominant pole due to the small-signal equivalent circuit of an LED can be nullified by a transmission zero provided by the source impedance so that the light intensity modulation may have a maximally flat frequency response and the 3 dB modulation bandwidth m ay be extended well beyond the limit set by the charge recombination t ime-constant of the device. Computer simulation has confirmed that the series parasitic inductance and resistance of the device play an impo rtant role in limiting the bandwidth. The pole cancellation method is equally applicable to circuits using an array of identical devices. In the case where the devices in an array have different poles, the comp ensated response is no longer maximally flat but is significantly broa der than the uncompensated one.