| 摘要 |
PURPOSE:To obtain a wide band and to increase a variable gain range by connecting an element for gain variation in series to the input side of a transistor (TR) amplifying circuit, and then varying the impedance of said element for gain variation. CONSTITUTION:On the base side of a TR, a resistance R5 and a DC-stopping pass capacitor C3 are provided, and the value of the resistance R5 is made much less than the sum of the impedance of a pin diode D and the impedance R1 of an input signal source so that impedance when the input side of the TR is viewed from the base side never varies with the impedance of the pin diode D. Consequently, even when the pin diode D varies in impedance, the frequency characteristics of a TR amplifying circuit never vary. Therefore, a variable gain circuit D has a wide band and a >=15dB wide variable gain range in a 1.2GHz frequency band by utilizing impedance variation attained by varying a current flowing the pin diode D by the R5. |
