Self-Biased Logarithmic Photoreceptor With Low Settling Time

A self-biased logarithmic photoreceptor configuration is presented. The photoreceptor employs positive feedback for self-biasing to allow for operation under a large range of inputs, but does not require large compensation capacitors to ensure stability, thus achieving a reduced settling time in comparison to previous implementations. Various trade-offs involved in the photoreceptor operation are analyzed and compared to earlier similar designs. Detailed measured results from a photoreceptor implementation in the AMS 0.35 μm technology are reported. The proposed photoreceptor has a saturation-free output for an input range larger than 100 dB, a current consumption only twice the mean input current level and a small-signal settling time more than 10 times smaller than that of the corresponding open-loop system. The presented implementation occupies a relatively large area and is thus suitable for single-pixel systems, rather than pixel arrays. Alternative implementations with reduced size are also proposed.