Integrated photodiode/transimpedance amplifier

Abstract

An integrated circuit photodetector includes a transimpedance amplifier including a differential amplifier stage with PNP emitter-coupled transistors and a PNP input transistor which are biased only by base currents of the emitter-coupled transistors, to achieve low input bias current. Low noise operation is achieved by bypass capacitors coupled between the bases and emitters of the input transistors, respectively. A constant current source supplies a current which develops a small pedestal voltage across a resistor to bias the non-inverting input of the transimpedance amplifier so as to avoid nonlinear amplification of low level light signals. A positively biased N-type guard tub surrounds the photodetector, which is formed in a junction-isolated N region on a P substrate, to collect electrons generated in the substrate by deep-penetrating IR light to prevent them from causing amplification errors. In one embodiment, a feedback network includes a resistor connected between the output of a buffer driven by the transimpedance amplifier and an inverting input thereof, and a capacitor connected between the output and inverting input of the transimpedance amplifier to provide low noise, fast settling operation.