An amplifier includes first and second transistors having emitters connected to a first current source, feedback from the second transistor collector to its base being supplied through a base-emitter path of a first Darlington pair in series with a first diode. A second current source is connected to the first Darlington pair emitter. Third and fourth transistors have emitters connected to a third current source with feedback supplied from the fourth transistor collector to its base through a second Darlington pair and a second diode. The second Darlington pair emitter is connected to a fourth current source and resistively coupled to the first Darlington pair emitter. Constant currents are supplied to the second and fourth transistor collectors and an input voltage across the first and third transistor bases controls allocation of current between the first and second Darlington pair emitter-collector paths to produce a differential output current proportional to the input voltage when both Darlington pairs are on. When the input voltage is highly negative, the first Darlington pair turns off and a biasing circuit reverse biases the first diode to prevent the second transistor from saturating and substantially discharging capacitance at the base of the first Darlington pair. Thus, the first Darlington pair can rapidly turn on when the input voltage is subsequently driven more positive. When the input voltage is highly positive, another biasing circuit reverse biases the second diode to prevent the fourth transistor from saturating.