DIFFERENTIAL AMPLIFYING CIRCUIT

PURPOSE:To control simply the amplification factor of a differential amplifying circuit with no trimming of a resistance which requires a complicated and exclusive device, by attaining the variable setting of constant current value for a constant current source which limits a working current flowing in common to a pair of active elements. CONSTITUTION:A differential amplifying circuit consists of a pair of transistors TRQ1 and Q2 connected with a load resistance RC respectively and a constant current source 10 which controls a working current flowing in common to the TRQ1 and Q2. Plural constant current circuits consisting of TRQ3, Q4... and resistances R3, R4... connected in series to the emitters of said transistors are connected in series to each other to constitute the source 10. Then an optional constant current circuit selected by operation signals e1, e2... sent from outside and supplied to the bases of the TRQ3, Q4... is actuated. Thus the constant current value is set variably. Thus the amplification factor is controlled variably for the differential amplifying circuit.