Abstract: The invention relates to an apparatus for powering a loudspeaker amplifier (5) in a telephone set fed with a line current (ILO) from a telephone line (3). An incoming speech signal (S) is amplified (8) and sent to a loudspeaker (4) via the amplifier (5), (I6is powered via its feed connections (19, 20) from a reservoir capacitor (6). By the capacitor feed there are avoided variations in the line current (ILO) on amplification of the speech signal (S). The capacitor is recharged with the aid of the speech circuit (13), which has an amplifier means (16, 17) sensing a line voltage (UO) via a voltage sensing circuit (18). The amplifier means (16, 17) steers the line current (ILO) to a constant value responsive to the line voltage (UO) by sending a current ( to the line (3) via a resistor (R1), thus keeping constant a current I4 ) through the resistor (R1), this current being sensed by the amplifier means.

Abstract: An amplifier circuit with high input impedance and low output impedance intended for being supplied with current via the output signal line includes a pre-amplifier with a transistor of the field-effect type (Q12) and a bipolar transistor (Q13) directly connected thereto, where the base-emitter circuit of the bipolar transistor and the source-drain circuit of the field-effect transistor (Q12) are connected in series across the supply source. The gate of the field-effect transistor is connected to a biasing source (16), the temperature coefficient of which has substantially the same absolute value as the temperature coefficient of the base-emitter voltage in the bipolar transistor (Q13), but with reverse sign.

Abstract: A balanced amplifier output stage preferably intended to be used in a telephone instrument and which has its current supply from the telephone line includes two transistor amplifiers connected to parallel each having a top (T1, T2) and a bottom (T3, T4) transistor with series connected emitter-collector-circuits. Equally located transistors (e g T1, T2) in the series connections are driven in push-pull. To stabilize the current sum (I) passing through the two transistor amplifiers (T1, T3; T2, T4) connected in parallel there is a regulation circuit (T5) which compares the current sum (I) with a reference quantity (Vref) and feeds a correction signal in equal phase inputs of the amplifiers connected in parallel.

Abstract: A gain control circuit includes at least a first (T1, T2) and a second (T3, T4) transistor differential amplifier the input circuits of which are connected in parallel to a signal voltage source. The gain control is performed by distributing a constant current (Is) between these differential amplifiers as a function of a control quantity (Uc) for setting the transconductance of the stages in a complementary way. The two differential amplifiers are provided with negative feed back (F, R1, R2, R3) but with different feed back factors. As the transistors in the differential amplifiers carrying signals of the same phase are interconnected the output signal from the automatic gain control circuit will consist of a superposition of the output signals of the two differential amplifiers.