| 摘要 |
A variable FET attenuator uses feedback in a control loop to correct the attenuator automatically for input/output return loss as the attenuator is varied with a control signal input. Both the series and shunt variable FETs are set to the proper conductance to maintain a proper input match with a desired attenuation value. A second, reference attenuator circuit provides feedback control of the primary attenuator circuit through an operational amplifier. The reference circuit is designed to be electrically equivalent to the attenuator circuit in terms of characteristic impedance. The control nodes, and attenuation control signal input and an output from the operational amplifier, are paralleled so that both the reference and primary attenuator circuits behave identically, without having to RF decouple the feedback loop from the attenuator circuit. Preferably, identical bridged-T variable attenuators are used for both the attenuator circuit and the reference circuit. Precise matching is obtained by incorporating both circuits in the same monolithic integrated circuit substrate, together with the operational amplifier.
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