Mendelsohn & Assoc.
Multi-gain amplifier with input impedance control
In one embodiment, an amplifier circuit has at least one branch and current-source circuitry providing a tail current to the branch, which has at least one load tank, at least one input transistor coupled to the load tank, and variable-impedance circuitry coupled between an input node of the amplifier circuit and the gate of the input transistor. The transconductance of the input transistor can be altered to achieve two or more different gain settings for the amplifier circuit. The variable-impedance circuitry can be controlled to contribute any one of at least two different levels of impedance to the overall input impedance of the amplifier circuit. If the transconductance of the input transistor is reduced, then the variable-impedance circuitry can increase the level of impedance contributed to the overall input impedance of the amplifier circuit such that the overall input impedance of the amplifier circuit remains substantially unchanged.
Agere Systems Inc.
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Jinghong Shaorui Lawrence A.
Chen Li Rigge