An amplifier system having a first pair of transistors connected between a pair of differential input terminals and a pair of differential output terminals, and a second pair of transistors having emitters connected to the respective input terminals and having collectors connected to a bias source. For a linear change in current gain a collector current sensing circuit is provided which puts out a voltage representative of the sum of the collector currents of either the first or second pair of transistors. This voltage is delivered to an input of a differential feedback amplifier, to another input of which is applied a variable gain control voltage. The differential amplifier delivers its output voltage, proportional to the difference between the two input voltages, to the bases of either the first or second pair of transistors. With the two input voltages of the differential amplifier thus made equal to each other, the current gain of the amplifier can be changed linearly with the gain control voltage.