PURPOSE: To obtain a sufficiently large output amplitude even when power voltage is low by varying the current flowing to a load in response to the gain and decreasing a voltage drop caused in a load even when the gain is increased.

CONSTITUTION: Bases of transistors(TRs) Q10, Q11 being components of a differential circuit connect to bases of TRs Q3, Q4 to detect the ratio of collector currents IC3, IC4 of the TRs Q3, Q4. The collector current of the TR Q10 results in a current (i) flowing to the collector of the Tr Q4 through TRs Q8, Q9 forming a current mirror circuit to decrease the voltage drop of a resistor R3. When currents I1, I4 of constant current sources 2, 5 are set to proper values by taking a power voltage and output amplitude into account, the output amplitude is increased.