Optical transimpedance amplifier with high dynamic range

A preamplifier for use with currents developed by a photodetecting diode, said preamplifier comprising a first bipolar junction transistor having an emitter, base and collector electrodes, means for coupling the currents developed by said photodetecting diode to said base electrode, means for connecting said emitter electrode to a ground potential, collector load means for connecting a source of potential to said collector electrode, buffer amplifier means responsive to voltage signals at said collector electrode for providing an output of said preamplifier, for coupling a negative feedback signal to said base electrode, and for developing a potential whose value represents the average signal level of said voltage signals, the value of said feedback signal being dependent on said average signal level, characterized in that said collector load means includes a collector resistor and a first field effect transistor of one channel type having a gate, source and drain electrodes, said source and drain electrodes being connected to opposite ends of said collector resistor, and means responsive to said potential whose value represents the average signal level for coupling a potential to said gate electrode, said means for connecting said emitter electrode to said ground potential includes an emitter resistor and a second field effect transistor having a gate, source and drain electrodes, said source and drain electrodes of said second field effect transistor being connected to opposite ends of said emitter resistor, and said gate electrode of said second field effect transistor is directly connected to the gate electrode of said first field effect transistor, said second field effect transistor being of opposite channel type to said first field effect transistor, whereby said first field effect transistor decreases the effective collector load resistance for increased values of said average signal level, and said second field effect transistor increases the effective resistance between said emitter and ground potential for increased values of said average signal level.

A preamplifier for use with currents developed by a photodetecting diode, said preamplifier comprising a first bipolar junction NPN transistor having an emitter, base and collector electrodes, means for coupling said developed currents to the base electrode of said NPN transistor, a collector resistor connected between said collector and a source of positive potential, means for connecting said emitter electrode to a ground potential, buffer amplifier means responsive to voltage signals at said collector electrode for providing an output of said preamplifier, for coupling a negative feedback signal to said base electrode, and for developing a potential which represents the average signal level present at said collector electrode, the value of said feedback signal being dependent on said average signal level, characterized in that said preamplifier includes a p-channel metal oxide field effect transistor (MOSFET) having a gate, source and drain electrodes, said source and drain electrodes of said MOSFET being connected in parallel with said collector resistor with the drain connected to the collector of said NPN transistor, and means responsive to the potential which represents said average signal level for providing said gate electrode with a potential, said means for connecting said emitter electrode to said ground potential includes an emitter resistor and an n-channel MOSFET having a gate, source and drain electrodes, said source and drain electrodes of said n-channel MOSFET being connected to opposite ends of said emitter resistor with the source connected to ground potential, and said gate electrode of said n-channel MOSFET is directly connected to the gate electrode of said p-channel MOSFET, whereby said p-channel MOSFET effectively decreases the collector resistance of said NPN for increased values of said average signal level, and said n-channel MOSFET increases the effective resistance between said emitter and ground potential for increased values of said average signal level.