series as for the circuits shown in fig2 and 3 and as shown in fig1 a desired gain characteristic can be obtained as shown in fig4 fig1 shows a circuit diagram of another embodiment of the agc amplifier of fig6 and corresponds to fig9 in fig1 the differential circuit 40 is formed by a well known constant resistance circuit the constant resistance circuit comprises a capacitor c1 having a capacitance c1 resistors r1 to r3 each having a resistance r and an inductor l1 having an inductance l1 in fig1 l 1 c 1 r 2 and a total impedance z r fig1 is a circuit diagram of a specific agc amplifier of the present invention in fig1 the amplifiers 10 and 20 shown in fig9 or 11 are formed by differential operation type amplifiers and the adder 30 shown in fig9 or 11 has current adding points n1 and n2 for example the amplifier 10 includes a pair of transistors q11 and q12 a constant current source cs10 and load resistors r11 and r12 connected to a power supply v cc the agc amplifier shown in fig1 does not have a gain changing circuit for changing the gain a 0 and or a 1 but the change of the gain for example a 1 can be easily realized by adding a gain changing circuit 21 consisting of control transistors and supplied with a gain control signal gc as shown by dotted lines in fig1 the differential coefficient u03c4 1 of the differential circuit 40 is determined by the frequency f and expressed by the following relationship when the frequency f is 32 mhz or mb s the time constant u03c4 1 0 05 u03bcs to obtain the total gain characteristics as shown in fig4 a predetermined number of the.