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 .