PURPOSE: To obtain a wide operating frequency range by setting an input and a complementary input bias level to a high and a low level respectively in response to an input frequency and varying the state in the inside of an IC automatically.

CONSTITUTION: The gain frequency characteristic of an amplifier circuit 3 is set so that the gain is large at a low frequency input and the gain is small at a high frequency input and when a gate input level of a FET 8 is low, the gate bias is set by resistors 5, 6 so as to cut off the FET 8. Thus, as the input frequency gets lower, the FET input level is increased, the DC level of the drain terminal is lowered and the input and the complementary input bias levels get lower, then the operation at a low frequency region is attained.