During the transmission of a signal on a transmission line, signal energy is transferred most efficiently when no reflected wave is present.  Thus to obtain a transmission of energy with a maximum efficiency, it is necessary to provide means for matching any actual load impedance to the characteristic impedance of the transmission line.  However, only under exceptional cases will the load impedance be a resistance that is exactly equal to the characteristic impedance of the line.  In many instances a load can be matched to the characteristic impedance of a line by coupling to the load a network of reactances that tunes out the network and load reactance and simultaneously transforms to a value equal to the characteristic impedance of the line; in other instances where the actual load or number of loads on a line may be variable or not precisely known, active termination circuits may be used. 
U.S. Pat.  No.  4,748,426 issued May 31, 1988 in the name of Alexander Stewart discloses an active termination circuit having a resistor combination permanently connected to a bus that couples a plurality of peripheral devices to one another.  Other ends of the resistors are coupled to a positive voltage supply line and to a logic ground plane, respectively.  Stewart teaches active impedance matching by on/off control of switches which open or close paths to the resistors. 
Computer systems and telecommunication systems often have a plurality of source and destination nodes connected to and sharing a common data transmission facility such as a bus.  These nodes may simply be receivers and transceivers located on circuit packs electrically coupled to the bus.  Such systems are often designed to be flexible, permitting variable configurations; the capability is therefore provided for an operator to add or remove circuit packs according to the particular requirements.  In a system having loads which vary from time to time according to the configuration of circuit packs connected to the bus, it may be difficult to provide adequate terminations to eliminate unwanted signal reflections.  In addition, in systems where any particular node may transmit data to one of many possible receiver nodes and the distance between the respective nodes is variable, it is difficult to predict and provide the form of termination that will be required. 
In an attempt to overcome some of the aforementioned limitations it is an object of the invention to provide a circuit for attenuating unwanted signals. 
The invention provides an active self-switching clamp which is adapted to attenuate unwanted signals having a voltage level which varies from a logic zero level by a small increment.