| 摘要 |
<p>A telephone set (1) is fed with line current (IL) via a telephone line (2) from final amplifiers (17, 18). In order to transmit an undistorted speech signal at restricted battery voltage (UB) when the line (2) is long, the final amplifiers (17, 18) are controlled in the following manner. A voltage sensing circuit (11) senses a line voltage (UL) and controls first and second current-generating circuits (12 and 13) which respectively send a first (I11) and a second (I12) current. A first difference-forming circuit (14) generates a first difference current (I2) between a first reference current (Ir1) and the first current (I11). A second difference-forming circuit (15) generates a second difference current (I4) between the second current (I12) and a second reference current (Ir2). When the second current (I12) is less than the second reference current (Ir2) the second difference current (I4) is diverted and assumes a zero value. A third difference current (I6) between the first and second difference currents (I2 and I4) is amplified in an amplifier means (16) and controls the final amplifiers (17, 18). When the line (2) is long, and the line voltage (UL) exceeds a desired, fixed value, the second difference current exceeds its zero value and restricts the third difference current (I6). The line current (IL) is thus restricted and voltage space for the speech signal is maintained.</p> |
