Enhancement of the low level detection limit of radiometric quality photovoltaic and photoconductive detectors

Abstract

When a current-to-voltage (I/V) converter is used with a photodetector, the ratio of the photodetector impedance (shunt impedance) to the input impedance of the detection circuit defines the ratio of the photogenerated current that will produce the output voltage. Ideally, the input impedance should be negligible in comparison with the shunt impedance as only in this case will the whole photogenerated current produce the output signal. With high quality, high band gap detectors in most of the cases this requirement is automatically met, but if lower band gap detectors are used this requirement may not be satisfied.The input impedance increases linearly with increasing feedback impedance of the I/V converter; i.e. at a sufficiently high feedback impedance value the output signal will deviate from the ideal case—it will be lower. Since the measurement of low intensity radiation requires a high feedback impedance, the lower the irradiance value on the detector the higher this deviation will be. If all the parameters of the detection system are kept constant during a measurement series, only calibration is required to achieve low uncertainty, but if any of the impedances or the gain of the operational amplifier changes during a measurement series this will result in increased measurement error. It should be noted that the values of these impedances can change rapidly with temperature. By applying a bootstrap circuit to the detector the impedance of the bootstrapped detector—depending on the frequency of the operation—will be 3–6 decades higher, compared with the detector alone; i.e. bootstrapping virtually increases the impedance of the detector. Consequently, this error will appear only at a much higher sensitivity range, greatly reducing this problem. Here we analyse the bootstrap solution.