The PBL 3853 is a biopolar integrated speech circuit with specific parameters making it highly suitable to be used as a line interface and speech circuit in a telephone line powered electronic payphone. Emphasis has been put on low current consumption in the IC thus facilitating that a greater part of the available line current can be used to power other electrical functions in the design. From a minimum line current of 16 mA, 12 mA at 4.5 V can be used to power auxiliary functions. The circuit can optionally create an active impedance towards the telephone line, set by external passive components, to reduce the current consumption for the transmitted signal. The circuit is designed to be used with a low impedance dynamic microphone but can be used with an electret type as well. The receiver drives a low ohm dynamic transducer directly. External clamping diodes are required on the receiver output. Both transmitter and receiver can be muted separately (receiver cut off). Line length regulation of the gain is possible in both receiver and transmitter. It is also possible to use the circuit without regulation. Payphone tones and confidence tone in the receiver at DTMF dialling can be injected directly into the receiver amplifier where their levels are not affected by line length regulation and without the signal going out on the line. All pin numbers refer to DIP package unless otherwise noted.