| 摘要 |
PURPOSE:To obtain a constant impedance characteristic stable in terms of temperature by using a constant voltage value referencing a voltage drop of a diode having the same characteristic as that of a diode being a component of an attenuator as a bias voltage fed to a DC bias impressing terminal. CONSTITUTION:A bias voltage generating circuit to a DC bias impressing point 3 consists of diodes D5, D6, an operational amplifier 4 and an RF choke circuit 5. A DC control voltage -V1 is impressed to a non-inverting input of the operational amplifier 4 via a resistor R3. The diodes D5, D6 have the same characteristic as that of diodes D1-D4 constituting the attenuator. In the constitution above, the relation of V0=2VDS exists. On the other hand, the relation between the rectifying characteristic ID of the diodes and an RF (high frequency) resistor R is expressed as R=(KT/q)(1/ID). In a pi type diode variable attenuator, the constant impedance characteristic is kept when the product of the resistance values of the diodes D1, D2 (D3, D4) is nearly equal to the square of the RF characteristic impedance. The impedance product RD1.RD2 of the diodes D1, D2 is expressed as (KT/q)<2>(1/IDS<2>) from the equation above and then nearly constant. Thus, the characteristic is improved considerably more than a conventional attenuator. |
