way power divider 52 which splits the input signal into two channels 54 and 56 one channel output is connected to a voltage controlled attenuator 60 the gaas devices 62 64 and 66 comprising the attenuator 60 function as voltage variable resistor elements and are configured in a t attenuator configuration by appropriate selection of the series 62 64 and shunt 66 elements the attenuator 60 can provide variable attenuation while maintaining a constant input and output impedance power dividers suitable for use as divider 52 are commercially available such as the model ds 331 available from adams russell 80 cambridge street burlington mass 01083 this power divider splits the input power equally between the two channels 54 and 56 other power split ratios could be used provided the gains in the feed forward loop are appropriately adjusted attenuators suitable for the purpose of device 60 are commercially available such as the model ma4gm301 gaas attenuator device available from macom 100 chelmsford street lowell mass 01851 this device comprises the three gaas field effect transistors fets 62 64 and 66 which are depletion mode devices and are used as voltage variable resistors the gaas fet is a three terminal device with the gate being the control element the gate is inherently isolated from the conducting channel while charge present at the gate controls the resistance of the channel as the gate bias of the fet is made more negative than the source the fet channel resistance increases the resistance of the fet devices can be changed in less than a nanosecond these devices also provide very low capacitance from source to drain and therefore approach the performance of ideal resistors other types of attenuator device could be used in a similar attenuator configuration for example pin diodes.