An amplifier module for radio frequency signal circuits includes an electrically conductive housing. Two coax connectors are electrically linked to an amplifier circuit contained within the module. A separate transformer is external to the module. Both the module and the external transformer are mountable to a chassis frame. Other modules are usable in the system in addition to the amplifier module. The transformer can be wall mounted to the chassis frame, or the transformer can be internal to a module which also mounts to the chassis frame.