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PhotoDiode Amplifier Design with Howland Current Inverter

A PhotoDiode Amplifier Circuit with Howland Current source and current inverter is analyzed in this circuit design video. This circuit has applications in optical receiver, optical signal amplification and optical current source design. This circuit is constructed with a pair of operational amplifiers, a Zener Diode, one photodiode and a potentiometer (variable resistor). The first op amp stage has two responsibilities. First, with properly biased Zener diode it provides a good reverse DC bias for the photodiode to operate and generate electric current proportional to the optical energy (signal) it receives. And two, using the adjustable or programmable resistor (possibly RDAC) the first stage provides a variable gain for its output voltage proportional to the current of photodiode (proportional to the received optical signal energy). The second stage is a modified Howland current source current inverter that that receives two input voltages V1 and V2 from the first stage and then produces an inverted-direction output current that is shown and computed to be proportional to the current of photodiode. With proper design, a practical approximation is simply the ratio of potentiometer resistor to the small internal resistor as computed in this analysis using combination of Kirchhoff's circuit voltage and current laws (KCL and KVL) and op amp virtual short. More video examples of sensor amplifiers and signal conditioning are posted in the Sensors & Instrumentation Amplifiers Playlist.