| 摘要 |
PROBLEM TO BE SOLVED: To provide a variable gain amplifier with no change in the impedance in gain switching. SOLUTION: In a transistor 2 and transistors 5 and 6, the transistor 2 is turned on during high gain, and the transistors 5 and 6 are turned on during low gain, so that the areas of transistors 2 and the transistor 5 match with each other. Then, the output load conditions become the same for both high and low gains, and the output impedance will not vary; and as for the input impedance, the high-gain pass transistor 2 is turned off in a low-gain mode, and a current in the transistor 6 made to flow though the input impedance of a transistor 22 becomes high, and the input impedance will not vary, because the collector of a transistor 1 becomes low in impedance.
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