| 摘要 |
PURPOSE:To improve S/N with simple circuit constitution by opening a switch circuit when an input signal is prescribed level or below, extracting the input signal from the 1st differential amplifier, closing the switch circuit when a prescribed level exceeds to extract a signal being a difference of the input and output signal of the 1st and 2nd differential amplifiers. CONSTITUTION:When the level of an input signal exceeds a control operating point L2, an output of a controller 27 shows a saturation characteristic because of distorted linearity with the input signal. Thus, a signal being a difference between a signal (a) from an input terminal 21 and a signal (b) of the controller 27, that is, a signal shown in characteristic (c) is obtained from an output side of the differential amplifier 23. On the other hand, both the input signal from the input terminal 21 and the output signal from the controller 27 are cancelled and the result is zero theoretically at a control operating point L2 or below, and an inverting input of the differential amplifier 22 is connected to common by a switch circuit 30, then the characteristic of the output signal obtained from the output terminal 31 is decided substantially by the input signal of the characteristic (a) from the input terminal 21, thus the S/N is improved.
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