| 摘要 |
A variable gain amplifier is provided with a first differential amplifier circuit and a second differential amplifier circuit each formed with transistors. A common signal is inputted to each of the differential amplifier circuits, and output signals of the differential amplifier circuits are added together for output via resistances. When a voltage of a variable voltage source is increased, a bias current of the first differential amplifier circuit having a high gain is increased, and therefore the gain of the first differential amplifier circuit is raised, resulting in an increase in overall gain. When the voltage of the variable voltage source is decreased, a bias current of the second differential amplifier circuit having a low gain is increased and therefore the gain of the second differential amplifier circuit is raised, so that the second differential amplifier circuit has greater effect and thus provides an amplifier circuit with a high saturation input level. The bias current of the second differential amplifier circuit is decreased when the gain of the variable gain amplifier is raised, and the bias current of the first differential amplifier circuit is decreased when the gain is lowered, whereby power consumption can be reduced.
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