| 摘要 |
PURPOSE:To attain a linear attenuation characteristic with less temperature change by providing an operational amplifier to the feedback section of which a resistor and a diode having the same characteristic as that of a diode of a pi type diode attenuator are provided and to the input of which a control current is impressed and applying its output voltage to a control terminal via an RF choke circuit. CONSTITUTION:The feedback section of the operational amplifier 6 consists of the resistor R11 and the diode D11, the controlling current Ii is impressed to an antiphase input, and its output voltage is impressed respectively to control terminals 4, 5 via the RF choke circuit (acting like a voltage follower function) comprising an operational amplifier 7, capacitors C21, C22 and resistors R21-R23. Through the constitution above, the attenuation AT to the RF signal is expressed as ATT=20log10[(2R0/R11)xe<x>+1], where x=(q/KT)R11Ii and R0 is the characteristic impedance of the RF circuit. When the (x) is sufficiently large in the equation above, the ATT is expressed as ATT 20log10(2R0/ R11)x+alphax, (where alpha is a constant), and even when the control Ii is changed, the change in the 1st term is less in the equation above and the ATT is changed nearly in proportion to the current Ii. Further, the effect of the temperature change is similar to the impressed current change in proportion to the absolute temperature. |
