A high isolation low noise amplifier with near unit gain up to 100 MHz

The authors report results obtained with a circuit in which three cascaded common base BJT (bipolar junction transistor) stages have short base leads directly connected to ground. Alternating NPN and PNP transistors biased with a bipolar supply were necessary for this purpose. The bases act as shields within the device, preventing any direct capacitive collector-to-emitter coupling; the only reverse path for the signal is represented by the small internal base resistances. The results obtained with a small number of prototypes are reported. Highlights of these are unity gain with a 3 dB bandwidth of 350 MHz, isolation in excess of 140 dB up to 100 MHz, phase noise floor at -168 dBc/Hz with corner frequency of about 300 Hz (at 13 dBm drive level), delay stability in excess of a few ps/°C at 100 MHz, and very good input and output matching to 50 Ω