Differential amplifiers

DETAILED DESCRIPTION It is an object of the present invention to provide a differential amplifier in which this suppression is improved.
According to the invention, a differential amplifier comprising two transistors having their collectors connected through respective loads to one supply line, their emitters connected together and to a controllable constant current source which supplies a bias current and which is connected between the common connection of the emitters and a return supply line, and their bases connected to receive a control voltage, is characterized in that the common connection of the emitters is also connected through a by-pass resistance to a bias voltage source, the circuit parameters being such that flow of bias current will result in a small current being diverted through the by-pass resistance instead of flowing through said transistors, this small current approximating in value to the value of any residual bias current when the current source is switched off, and having a value which is negligible relative to the value of the bias current when the current source is switched on.
The operation of a differential amplifier according to the invention is not significantly affected when the bias current is large.
However, for small values of bias current, when it is intended that the bias current should be zero, the bias current is mostly diverted into the by-pass resistance so that the current into the transistors is approximately zero, and the output current is therefore also approximately zero.
Thus, when the differential amplifier is intended to be `OFF`, suppression of the control of the output current by the control voltage is improved.