| 摘要 |
PROBLEM TO BE SOLVED: To provide a variable gain amplifier capable of suppressing variations in the output impedance when its gain is changed. SOLUTION: The amplifier is provided with source-grounded FETs 3<SB>1</SB>-3<SB>n</SB>whose gates are connected to an input terminal 1 in common and whose sources are grounded in common, a gate-grounded FET 5 whose gate to which a gate bias is supplied is grounded at high frequencies and whose source is connected in common to each drain of the source-grounded FETs 3<SB>1</SB>-3<SB>n</SB>in common, a load inductor 10 connected between the drain of the gate-grounded FET 5 and a drain bias terminal 8 to which a drain bias is supplied, an output terminal 12 connected between the drain of the gate-grounded FET 5 and the load inductor 10, and a gate bias control circuit 14 connected to each gate of the source-grounded FETs 3<SB>1</SB>-3<SB>n</SB>, to supply a gate bias selectively. Since the size of the gate-grounded FET 5 on the output terminal 12 side does not vary with a change in the gain, changes in the output impedance can be suppressed. COPYRIGHT: (C)2008,JPO&INPIT
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