2 is connected in series with the diode 1 so that current flowing through the diode also flows through the resistor 2 and generates a potential across it a high input impedance amplifier 3 has an input 4 which is fed from a junction point 5 between the diode 1 and the resistor 2 with the potential induced across the resistor 2 by the infra red radiation received by the photodetector diode 1 as explained above since the amplifier 3 has a high input impedance it tends to integrate signals fed to it and therefore to compensate for this the amplifier 3 is connected to feed an equalizer 6 as explained above this front end has operational disadvantages and also is not easy to produce as an integrated circuit in fig2 there is shown an alternative known front end circuit in which current produced by the photodetector diode 1 on irradiation by infra red radiation is fed directly to a low input impedance transimpedance amplifier formed by an operational amplifier 7 provided with negative feedback in the form of a resistor 8 connected between its output 9 and its inverting input 10 a further amplifier 11 provides additional amplification since the input impedance of the transimpedance amplifier of fig2 is low the stray capacitance of the diode 1 causes no serious signal integration however the front end does not adequately reject low frequency noise due to background infra red radiation.