| 摘要 |
PURPOSE:To attain miniaturization and high efficiency mounting by using an FET circuit to realize a talking current loop. CONSTITUTION:When a relay contact is closed, a DC current from a station flows to a loop of (+) side of diode bridge DB-FETQS-resistor R3-(-) side of DB. The resistor R3 and a capacitor C1 constitute AC feedback to the FET circuit, and since the AC voltage at the point SR and the AC voltage of the output of the source S of the FETQ are in phase and nearly the same voltage, no AC current flows to the resistor R3 and then high impedance is attained apparently. Thus, the transmission loss is not almost increased through the insertion of the FET circuit. Further, since the DC loop circuit is realized by the FET circuit, no DC superimposition is applied to a talking signal coupling transformer T1. Thus, the diameter of windings of the transformer is made thin, the use of a permalloy core is made possible and the small-sized structure and small transmission loss are attained. |
