Abstract: A telephone set having a voltage gain and control circuit element in series with the main current path of a line current control transistor, connected between first and second line end portions. The line current control transistor is connected between the first line end portion and the modulation terminal of the voltage gain and control circuit. A line current measuring resistor is connected between the reference terminal of the voltage gain and control circuit element, and the second line end portion. The rise of line current above a threshold is controlled by a second transistor connected between a threshold voltage source and the control electrode of the line current control transistor. The second transistor is also controlled by a compensating diode connected between the control electrode of the second transistor and the line current measuring resistor.

Abstract: A loudspeaking telephone set incorporating an anti-Larsen device which upon detecting instability controls the insertion of an attenuator in either or both of the listening and transmission channels. Such device incorporates a detection circuit (6) for detecting a start of instability, consisting of two analogue multiplier circuits (11), (15), two low-pass filters (18), (20), and two absolute-value convertors (19), (21), these elements being arranged in two parallel chains. One of the multiplier circuits (11) receives at its inputs the amplified signal from the receiver (4) and from the microphone (3). The other multiplier circuit (15) receives the same signals, except that one of them is phase-shifted approximately 90.degree. at the Larsen frequency, by a phase-shift circuit (16). At the output of the absolute-value convertors (19), (21), the signals are added together by an adder (23) which delivers at its output an attenuation control signal after passing through a threshold comparator (24).

Abstract: An interface is arranged between an integrated circuit which is provided for a 5 volt supply voltage and a variable-voltage telephone line. This circuit employs an MOS transistor whose current is controlled by a current regulating transistor and a current measuring resistor and includes a negative voltage feedback path for alternating current. A time constant is provided in order to disable the action of the current regulating transistor for transient or alternating currents.

Abstract: Telephone set with key for opening the loop for a predetermined time interval. A monostable triggering circuit includes an MOS transistor connected in series with the transmitter and dialing circuit and a bipolar transistor having a collector directly connected to the grid of the MOS transistor and a base connected to the drain of the MOS transistor through a capacitor and a diode. The capacitor together with a resistor connected in series therewith constitute a timing circuit. The voltage to which the capacitor is charged during the time that the circuit is open is limited by a Zener diode. Actuation of the key opens the MOS transistor thereby leaving the transmitting and dialing circuit disconnected. The transistor switches back to the conductive state when the timing capacitor has reached the voltage determined by the Zener diode. The MOS transistor is a low impedance in series with the transmitting and dialing circuit when conductive but presents a high impedance as its grid.