We have analyzed the characteristics of flux modulated voltages of a dc SQUID for both positive and negative bias currents. It is shown that the change in the modulated voltage caused by fluctuations of the junction critical currents is eliminated after subtraction between the modulated voltages for the two biased currents. In the flux feedback mode using an alternating bias current, a large reduction of 1/fnoise is observed for dc SQUIDs made of NbN junctions with amorphous Si tunnel barriers.