it down when the common voltage of the input differential voltage pair is higher than a reference voltage the output circuit receives the input differential voltage pair outputted from the input circuit to pull high or low an output voltage accordingly application_number US 7148508 A description fig2 shows a circuit diagram of the preamplifier 200 according to the first embodiment of the invention the preamplifier 200 is for preamplifying an input differential voltage pair vin and vip to pull an output voltage vo 1 for a receiver high or low preamplifier 200 includes an input circuit 210 and an output circuit 220 the input circuit 210 receives the input differential voltage pair vin and vip in the first embodiment when the common voltage vcom of the input differential voltage pair vin and vip is higher than a reference voltage vr the input circuit 210 pulls the input differential voltage pair vin and vip down to produce a differential voltage pair vin u2032 and vip u2032 and then transfers the differential voltage pair vin u2032 and vip u2032 to the output circuit 220 when the common voltage vcom of the input differential voltage pair vin and vip is not higher than the reference voltage vr the input circuit 210 directly transfers the input differential voltage pair vin and vip as the input differential voltage pair vin u2032 and vip u2032 to the output circuit 220 without pulling the input differential voltage pair vin and vip down the output circuit 220 then.