Logging Photodiode JFET Amplifier with 1pA Input Bias Current

The 1pA input bias current contributes only 0.4fA/rtHz of current noise or 0.4nV/rtHz voltage noise with a 1Mohm source impedance. Photodiode current is converted to a voltage by the first op amp and D1 and amplified by the first, second and third logarithmic compression amplifiers.