AC AMPLIFIER CIRCUIT

PURPOSE:To attain amplification without being affected by a DC output level of a pre-stage circuit by adopting the configuration such that a DC input level to a differential amplifier circuit depends automatically on a level where an input current of the differential amplifier circuit and an output current of a bias circuit are equal to each other. CONSTITUTION:The differential amplifier circuit 1 consists of NPN bipolar transistors(TRs) Q1, Q2 whose emitters are connected in common, whose bases are used for 1st and 2nd input sections and whose emitters are connected to a constant current source 2 and the emitters allow a predetermined bias current I1 to flow. A bias circuit 3 is made up of PNP bipolar TRs Q3, Q4 whose emitters are connected in common and bases are used for outputs and connect to input terminals 4, 5. A current I2 is supplied from a constant current circuit 6 to the bias circuit 3. According to the circuits above, the DC input level of the differential amplifier circuit 1 depends automatically on a level where a DC input current to the differential amplifier circuit 1 is equal to an output current of the bias circuit.