The following diagram is an example of a photovoltaic implementation. This op-amp circuit is called a transimpedance amplifier (TIA). It is designed specifically to convert a current signal into a voltage signal, with the current-to-voltage ratio determine. The following diagram is an example of a photovoltaic implementation. This op-amp circuit is called a transimpedance amplifier (TIA). It is designed specifically to convert a current signal into a voltage signal, with the current-to-voltage ratio determined by the value of the feedback resistor RF. The op-amp's non-inverting input terminal is groun. The basic output of a photodiode is current that flows through the device from cathode to anode and is approximately linearly proportional to illuminance. (Keep in mind, though, that the magnitude of the photocurrent is also influenced by the wavelength of the incident light—more on this in the next article.) Photocurrent is converted into a voltag. A major non-ideality that affects photodiode systems is called dark current, because it is current that flows through the photodiode even when no illumination is present. The total current flowing through the diode is a summation of dark current and photocurrent. Dark current will limit the system's ability to accurately measure low light intensiti. To switch the above detector circuit over to photoconductive mode, we connect the photodiode's anode to a negative voltage supply instead of ground. The cathode is still at 0 V, but the anode is at some voltage below 0 V; thus, the photodiode is reverse-biased.The performance of a photodiode-based detector system is influenced by the photodiode's biasing conditions. Photoconductive mode employs reverse biasing and provides higher sensitivity, wider bandwidth, and improved linearity. Photovoltaic mode employs zero bias and minimizes dark current. The next article in the Introduction to Photodiodes series.
