base electrode of the transistor 15 in turn being connected to ground by way of a resistor 20 the resistors 18 and 19 in this case are made equal to half the required output resistance while the resistors 3 and 20 have values respectively greater and smaller than the value of the resistors 18 and 19 by the factor g the emitter coupling of the transistors 1 and 15 to the current source 16 serves to provide differential drive to the output transistors 2 and 17 referring now to fig5 and 6 a differential input differential output embodiment comprises a symmetrical arrangement in which a pair of input transistors 1 and 21 have their respective collector electrodes directly connected to the base electrodes of output transistors 2 and 22 and their respective emitter electrodes connected to respective current sources 24 and 25 and interconnected by way of a resistor 26 the emitter electrodes of the transistors 2 and 22 are connected by way of resistors 3 and 23 respectively to the base electrodes of the transistors 1 and 21 respectively and by way of resistors 27 and 28 respectively to the emitter electrodes of the transistors 21 and 1 respectively the resistors 27 and 28 have values equal to half the required output resistance of the amplifier while resistors 3 and 23 have values greater than those of the resistors 27 and 28 by the factor g and the resistor 26 has a value smaller than the required output resistance by the factor g as shown in fig6 common mode feedback may be applied to the amplifier of fig5 by means of a sensing transistor 29 and a further transistor 30 which are arranged to.