| 摘要 |
PROBLEM TO BE SOLVED: To provide an amplifier circuit, in which the changes in the output impedance in both high-gain mode and low-gain mode and gain adjustment are conducted exactly. SOLUTION: The high-frequency signal of an input terminal 1 is inputted into the base of a transistor 102 and is outputted to the collector thereof. In high-frequency mode, a switch 110 is turned off to make a transistor 104 into an off operation. All the high-frequency signals are inputted into the emitter of a transistor 103, outputted to the collector, and then are outputted to an output terminal 2. In the low gain mode, the switch 110 is turned on to turn the transistor 104 into an on operation, so that the base voltage which is identical to that of the transistor 103 is applied. Since the current ratio of the transistor 103 to the base grounding transistor 104 is determined by the size ratio of the transistor 103 to the transistor 104, the current can be adjusted exactly, by adjusting the size current of the transistors 103 and 104, and the adjusted current is outputted to the terminal 2. At this time, since load conditions are the same in both the high- and low-gain modes, changes in the output impedance are small.
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