| 摘要 |
A first stage amplifier circuit includes an amplifier, a variable feedback resistance, a comparator and a shift register. The amplifier amplifies an input signal supplied thereto. The variable feedback resistance includes two or more feedback resistors and one or more switches for changing the connection of the feedback resistors and thereby changing the composite resistance of the feedback resistors. The variable feedback resistance is connected between the output terminal and the input terminal of the amplifier. The comparator judges whether or not the output signal of the amplifier exceeds a predetermined threshold value. The shift register has one or more stages corresponding to the switches of the variable feedback resistance. The shift register operates the switches of the variable feedback resistance one by one and successively reduces the composite resistance of the variable feedback resistance as long as the output signal of the amplifier is judged by the comparator to exceed the threshold value. Therefore, the gain of the amplifier is controlled adequately by the digital feedback resistance control circuit, thereby both a wide dynamic range and high speed response of the first stage amplifier circuit are realized, and thereby saturation of the amplifier is prevented and thus duty distortion of high speed signals in the amplification is eliminated.
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