AN22022A-VT

General description

The amplifier is a simple, low cost add-on circuit to the SA253x single chip telephone family. It is supplied by the line current and amplifies the receive signal, taken from either RO1 or RO2 (Receive outputs), see Fig.1. Compared to more complex “real” handsfree circuits there is no channel switching or anti- larsen circuit implemented, therefore the receive and transmit gains must be lower to avoid howling.
However, at short distances between user and telephone (<2m, which is adequate for normal office work) there is even full duplex “handsfree” operation possible, when the handset is placed next to the telephone with the microphone facing to the user.
Due to the excellent sidetone cancellation of the SA253x circuits, howling between handset microphone and loudspeaker will only occur, when both are put very close to each other. In normal operation, no howling will occur, even with maximum loudspeaker volume.
Due to the unique structure of the SA253x family’s line adaption, adding the amplifier has no influence on the important PTT related parameters, like AC- impedance, DC-mask, etc. (see Pt. 5.1 below).

Key Features

• Low cost additional circuit enables use of loudhearing function
• Full duplex “handsfree” operation via handset microphone and loudspeaker (at distances <2m between telephone and user)
• Can be added to any SA253x- application
• Enables use of low cost 8Ω - Loudspeaker
• High output power with low distortion
• Entirely powered from the telephone line
• No influence of loudspeaker amplifier in AC-impedance, sidetone cancellation or DC-mask
• Loudspeaker volume adjustable by potentiometer
• Excellent sidetone cancellation of SAS253x circuits enables high loudspeaker volume without howling between handset microphone and loudspeaker