Abstract: A transmit amplifier which also stabilizes the line direct voltage of the audio transmission circuit of a telephone set with respect to a reference voltage, the amplifier gain being adjustable to a high value by means of a resistor which does not affect the further properties of the audio transmission circuit. Variation of the line direct voltage by the offset current of the microphone pre-amplifier preceding the transmit amplifier, and by temperature fluctuations of the reference voltage, is greatly reduced by increasing the reference voltage and adding a temperature-dependent current source to a voltage divider which is comprised in the amplifier.

Abstract: Transmission circuit for a telephone set for DTMF-tone selection. The required second-order tone filter is formed by arranging the speech filter (12) during the dialling procedure in series with a fixed first-order tone filter (10) by means of a switch (11). The resultant attenuation of the group II of high frequencies of the dialling tones is compensated for by increasing the amplitude of the dialling tones of the high-frequency group II and by an additional increase in the sub-group II(2) of the two highest frequencies of this group.