| 摘要 |
1,237,089. Community aerial systems; transistor amplifying circuits; transistor bi-stable circuits. PHILIPS ELECTRONIC & ASSOCIATED INDUSTRIES Ltd. 25 Nov., 1968 [28 Nov., 1967], No. 55814/68. Headings H3T and H4R. In a television community aerial system a repeater amplifier is provided which comprises a normal amplifier (1, Fig. 1, not shown), and a standby amplifier (4, Fig. 1) the output of the normal amplifier being monitored and compared with its input and the standby amplifier switched into circuit if the normal amplifier fails. An oscillator (Fig. 4, not shown) is also connected to the circuit and energized with the standby amplifier to provide an indication of the normal amplifiers failure. A pilot signal may be transmitted from the central station of the system to provide a signal for the amplifier failure detector, the pilot signal being derived from a generator (Fig. 5, not shown) including a normal oscillator and a standby oscillator brought into operation if the normal oscillator fails. Both the amplifier and oscillator changeover are effected by bi-stable circuits which when triggered to energize the standby circuits remain in that condition until the power supply is interrupted for a short time, when they switch to the normal operation state. The monitoring of the input and output is effected by comparison of D.C. voltages derived by rectifying the respective signals, the comparison result operating the bi-stable to effect switching of the power supply to the amplifiers. |
