The Optically-Coupled Current-Mirror: a Novel Approach for Linear Transmission of Fast Analogue Signals via Fiber Optics

We report recent improvements in the performance of a novel circuit topology, the optically-coupled current-mirror (OCCM), that includes optical links in its feedback and forward paths. In this way the input-stage remains Galvanically isolated from the rest of the circuit. The input stage consists on a passive dipole composed only by two components: a LED and a photodiode connected back-to-back. When it is cooled down, eventually close to 77 K, the optical power versus LED-current characteristics assumes a perfectly linear law and the open-loop gain increases by a factor at least x 2, due to the increase of the LED responsivity. The input stage can be connected in series with the anode of different types of ionizing detectors. Originally conceived and operated with success for the readout of the DC component of PMTs biased with cathode grounded, the OCCM has now been optimized for the transmission of fast analogue signals. At present the signal bandwidth approaches 32 MHz. The dynamic range can reach large values by connecting two or more input stages in series, each one associated to a single channel with appropriate gain. To be noted that the only semiconductor devices eventually subject to radiation damage are the LED and the photodiode of the input stage. In this paper we describe recent achievements of the OCCM, and discuss the latest results obtained.