| 摘要 |
1,145,587. Substation circuits. INTERNATIONAL STANDARD ELECTRIC CORP. 21 July, 1967 [26 July, 1966], No. 33571/67. Headings H4K and H4R. In a substation which includes moving coil type transmit and receive transducers, outgoing speech is amplified by an amplifier which derives its D.C. power from the line and which consists of two stages of transistors that are D.C. coupled by a voltage dependent resistor which also serves for compensating for different lengths of lines, the emitters of the two transistors being coupled by a positive feedback A.G.C. circuit which includes a further voltage dependent resistor. Calling tone generator.-This comprises an audio transducer TG which forms part of the resonant circuit of an exchange battery powered transistor oscillator T1. The transducer produces a variable frequency tone continuously once it has responded to the initial negative pulse of ringing current. Transmit amplifier.-This consists of a dynamic microphone MA which is coupled via a low impedance network R19, C9 to a two-stage D.C. coupled amplifier T4, T3. (MA acts virtually as a constant current source and it is decoupled from the exchange line D.C. by capacitor C9). The amplifier includes a negative feedback path R13, a positive feedback path which provides A.G.C. (VR2) and is voltage gain stabilized because of the unbypassed resistors R12, R17. Operating current is derived from the line via a full wave rectifier D3-D6 and is supplied (at least to T4's collector and T3's base) via choke L1 and resistors R16, VR1. A measure of high frequency compensation is provided by C6. Anti-sidetone circuit consists of a transistor T2 which has a gain equal to that of T3 whereby a minimum of sidetone appears in the receiver RA whatever the length of line to which the set is connected. Receive amplifier.-This is only operative for long lines and comprises transistors T4, T2. A portion of the input signal is applied to T4 across R13, R17 and is amplified by T4 since T4's gain is increased considerably (via its positive feedback circuit) on long lines. The signal path to RA ordinarily consists in: A wire; junction D5, D6; RA; RB; R9 and R11, C5; C7; junction of D3, D4. Voltage dependent resistors VR3, VR4 act as click suppressors on incoming signals. Long line compensation is provided, by VR5 which stabilizes the collector current of T4, by VR1 which limits internal power dissipation on short lines and, by the resistance of L1, R16, R12 (and VR1) which govern the ratio of T4's collector current to T3's base current and hence the stability of the D.C. potential at the set's terminals. |
