| 摘要 |
PURPOSE:To obtain an accurate logarithmic amplifier with a small number of parts, by utilizing the fact that the resistance between the drain and the source of an FET and the voltage between the gate and the source are in logarithmic relations with each other. CONSTITUTION:The drain of an FET17 is connected to the inversion input terminal of a DC amplifier 16, and the source is connected to the earth. The gate of this FET17 is connected to the noninversion input terminal of the amplifier 16. The input signal is applied to this noninversion input terminal, and the output signal is taken out from an output terminal 2 of the amplifier 16. Since the resistance R1 between the drain and the source of this FET17 is changed logarithmically for the voltage between the gate and the source, the resistance R1 is expressed by h.expVGS (h is a constant) when the voltage between the gate and the source is denoted as VGS. If the resistance value of a feedback resistance 18 is selected properly, the input voltage and the output voltage of this circuit are in logarithmic relations with each other. |
