Multiplexed amplifiers commonly have several input stages that can be selected to be active one at a time. Such circuits behave as analog multiplexers with different signal sources connected to each input stage. One convenient configuration of an analog multiplexer is illustrated in FIG. 1 in which a common current source 9 is selectively connected to an activated input stage 11, 12, 13 of the main amplifier 15. The nonactivated input stages are capable of "floating" to some undetermined bias condition. One disadvantage of such circuit configuration is that the base-collector capacitance of the transistors in each input stage that is not active is capable of transferring high-frequency signal appearing on the input base to the main amplifier 15, thus resulting in poor signal isolation through the inactive input stages. Another disadvantage of such circuit configuration is that, when a previously inactive input stage is activated, its previous emitter voltage is unknown. Thus, undetermined time is required for the current source 9 to transfer charge and bias the emitters of the activated input stage to conduction.