Automatic circuits for backing off the photodetector signal in kinetic spectrophotometry

Abstract

Three circuits are described which use a feedback loop to back off the photodetector signal in kinetic spectrophotometry. Their function is to feed back a signal equal and opposite to that produced by the photodetector anode current and in this way maintain the anode within a few millivolts of zero. Their settling time is a few tens of microseconds. During a sweep of the measuring oscilloscope, the feedback loop is held open and the fed back signal is kept at the value it had just before the start of the sweep so that transient signals can be observed. At or shortly after the end of the sweep, the loop is closed again. Provision is also made to allow the value of the anode current, existing just before the sweep, to be sampled, held and displayed on a meter. These circuits eliminate the need for careful manual balancing when the measuring system is d.c. coupled, and also greatly facilitate the experimental procedure when the light source is a pulsed lamp. The first circuit was designed for use with a 50 Omega measuring system with a rise time of a few nanoseconds. The second circuit operates at higher sensitivity and slower response time. The third circuit was designed for versatility and can be added to existing apparatus. Circuits of this kind are likely to be useful in other applications where small changes in an electrical signal are to be observed against an unpredictable background.