| 摘要 |
PURPOSE:To set the titled resistor easily to an objective resistance value with external control by utilizing two field effect transistors (FETs). CONSTITUTION:When a control signl Vc is applied between a control signal input terminal 3 and the 2nd power terminal 2, an output of a differential amplifier 7 applies a voltage where a drain voltage of an FET6 is erual to the control signal Vc to a gate of the FET6. In adopting the FET6 having a peoper current gain beta in this case, it is possible to locate the operating point of the FET6 to a region II' in the figure and the equivalent resistance is regarded as a resistance expressed in an equation I . Since a drain-source voltage of the FET6 is the Vc and a current flowing to the FET6 is a current IB decided by a constant current source 5, an R6 is expressed in an equation II. Similarly an FET8 is equivalent to a resistance expressed in an equation III. When the matching between a threshold voltage VT6 of the FET6 and a threshold voltage VT8 of the FET8 is excellent and the FETs having nearly equal value are selected, the equation III is expressed by utilizing the equations II and I . Thus, a control signal input terminal 4 and the 2nd power terminal 2 act as a variable resistor having a value proportional to the control signal Vc expressed in an equation IV. |
