| 摘要 |
<p>A circuit is for centrally powered subscriber stations in a telephone installation. There are receiver amplifiers, and the feed-back attenuation circuit takes the form of a resistance bridge with the microphone output in one diagonal and the receiver amplifier input in the other diagonal, and with the DC supply for the amplifier connected directly to the incoming line arranged as an arm of the bridge; and with a diode between base and emitter of the amplifier transistor arranged to ensure that the gain of the transistor remains independent of variations in the DC voltage. The voltage across the diode (D1) between the points (1) and (5) is also used as the operating voltage to control an additional unit (Z), which may be for example a charge indicator, is known fashion, by means of symbols transmitted along the line.</p> |
