Low noise amplifier
An amplifier having multiple gain modes comprises a plurality of cascoded input transistors connected to an input and arranged in parallel, a degeneration stage connected to the input transistors and having a variable impedance, and switching means for switching between different modes of the amplifier by switching off one or more of the input transistors and varying the impedance of the degeneration stage.
1. An amplifier having multiple gain modes, comprising a plurality of cascoded input transistors connected to an input and arranged in parallel, a degeneration stage connected to the input transistors and having a variable impedance, and switching means for switching between different modes of the amplifier by switching off one or more of the input transistors and varying the impedance of the degeneration stage, and wherein the switching means is arranged so that the impedance of the degeneration stage is dependent on the number of input transistors that are switched off.
2. An amplifier according to claim 1 , wherein the degeneration stage comprises a plurality of degeneration elements and the switching means is operable to vary the impedance of the degeneration stage by bypassing one or more of the degeneration elements.
3. An amplifier according to claim 2 , wherein the plurality of degeneration elements are arranged in series.
4. An amplifier according to claim 1 , wherein the amplifier is arranged so that its input impedance is dependent upon the number of input transistors that are switched off and on the impedance of the degeneration stage, and wherein the switching means is arranged so that the impedance of the degeneration stage varies with the number of input transistors that are switched off such that the input impedance of the amplifier is substantially the same in each of the gain modes.
5. An amplifier circuit as claimed in claim 1 , wherein the switching means includes a transistor.
6. An amplifier circuit as claimed in claim 1 , wherein said degeneration elements are inductance elements.
7. An amplifier circuit as claimed in claim 6 , wherein said inductance elements include an inductor tapped along its length to provide two inductor portions.
8. An amplifier circuit as claimed in claim 1 , wherein said degeneration elements are resistance elements.
9. An amplifier circuit as claimed in claim 1 , wherein said input transistors are selected from the group consisting of MOSFETS and Bipolar transistors.