RECEIVED SPEECH AMPLIFYING AGC CIRCUIT

摘要

PURPOSE:To obtain a more natural speech receiving level, by providing a loop which applies AGC to a received speech amplifier and a quick discharging loop which is connected to a time constant circuit having larger time constant, with the outputs of two time constant circuits different in time constant to which the receiving speech signal is supplied. CONSTITUTION:The output of an amplifier AMP1 charges a capacitor C3 via a capacitor C<1>, a diode D3 and a resistance R7. This charging voltage controls a transistor TRQ3 and then an AGC. A discharging circuit of the discharging voltage is formed with resistances R8 and R9 and a TRQ4. The circuit noise given through a receiving speech input terminal T1 is amplified by the amplifier AMP1 to charge quickly a capacitor C2. The circuit noise is then discharged quickly by a resistance R6. Furthermore, a capacitor C3 is not substantially charged since the charging time constant is increased by a resistance R7 and the capacitor C3. Therefore no AGC is applied although the oposite subscriber having a low level receiving speech input is called. This eliminates an unhearable phenomenon.