Abstract: A method and apparatus for controlling current flow in current sources includes a current source FET (18), a control FET (20), and a driver circuit which includes complementary logic (310). The use of complementary logic for control advantageausly allows commonly available logic functions to control the current flow in individual current sources while maintaining a substantially constant bias voltage on the gate of the current source FETs. A chip-wide bias generator can be maintained substantially constant while controlling individual current sources.

Abstract: A circuit for indicating the status of a telephone line at the CPE end, where the status is either on-hook, ringing, or off-hook is disclosed. The telephone line is the normal two-wire telephone line. First, a transconductance amplifier converts a floating voltage difference between the two-wire telephone line into a current. The current is proportional to the voltage difference. Next, a current to voltage converter converts the current from the transconductance amplifier to a single ended ground referenced voltage. The single ended voltage is proportional to the current from the transconductance amplifier, thus the single ended voltage is proportional to the voltage difference across the two-wire telephone line. A voltage controlled switch is turned ON when the single ended voltage is greater than a predefined value indicating the on-hook status. When the single ended voltage is below the predefined value, the voltage switch is turned OFF indicating the off-hook status.

Abstract: An enhanced subscriber line interface circuit for a digital switching system which includes a line configuration and protection circuit connection tot he subscriber line and to a thick-film hybrid module. The thick-film hybrid module comprises a high voltage interface circuit and a PCM conversion circuit. The high voltage interface circuit provides power to the subscriber line, converts received analog voice signals transmitted from the subscriber instrument into differential voltage voice signals and detects the subscriber lines status. The PCM conversion circuit is connected to the high voltage interface circuit and is disposed to convert received differential voice signals into PCM digital signals.

Abstract: A hybrid balance and combination codec filter circuit used in a subscriber line interface circuit over which a connection is established between a digital switching network and a subscriber instrument. The hybrid balance and combination codec filter circuit in combination includes a transmit section that converts differential voltage audio signals representing voice transmissions transmitted from the subscriber instrument into Pulse Code Modulated (PCM) encoded digital data, for transmission to the digital switching network. A receive section converts PCM encoded digital data representing voice signals switched through the digital switching network into differential voltage audio signals for transmission to the subscriber instrument. An included hybrid balance and echo cancellation section connected between the transmit section and the receive section reshapes the audio band signals output by the receive section.

Abstract: An enhanced high voltage line interface circuit for a digital switching system over which a connection is established between a digital switching system and a subscriber instrument via a subscriber loop. The high voltage line interface circuit includes in combination circuits that provide power to the subscriber loop and convert received analog voice signals transmitted from the subscriber instrument into differential voltage voice signals for subsequent conversion to digital data for use by the digital switching system. Additionally, differential voltage voice signals from the digital switching system are converted to analog voice signals for transmission along the subscriber loop to the subscriber instrument. The high voltage line interface circuit further includes circuits that detect and monitor the subscriber loop status and develop digital logic output signals that are used to report the status of the loop to a digital switching system controller.

Abstract: A thermal protection circuit for an integrated circuit subscriber line interface is disclosed. The thermal protection circuit monitors the temperature of the integrated circuit and develops an output voltage signal that shuts-down the integrated circuit when a specific temperature is exceeded. The thermal protection circuit includes first and second current networks arranged to each develop first and a second currents respectively, of substantially the same value. The second current develops a reference voltage which is applied to an output transistor. When the base-emitter junction voltage of the output transistor is less than or equal to the reference voltage, the transistor begins to operate. This increases the current flowing through the first current network which is replicated in the second current network. The increased current flowing in the second current network increases the reference voltage, thereby, fully operating the output transistor and developing the output voltage signal.

Abstract: A circuit for synthesizing an impedance across the tip and ring leads of a telephone line circuit is disclosed. The circuit includes tip drive and ring drive amplifiers connected to the tip and ring leads respectively, of a subscriber loop. The tip drive and ring drive amplifier circuits are arranged to convert feed voltage from a central office battery to feed current to drive the subscriber loop. A common-mode amplifier circuit connected between the tip and the ring leads senses the voltage dropped across the subscriber loop and outputs a control voltage to the tip and ring drive amplifiers, offsetting the feed currents applied to the subscriber loop. A differential amplifier circuit also connected to the tip and ring leads of the subscriber loop, detects a voltage difference between the tip lead and the ring lead and converts the detected differential voltage into a single ended output voltage of a specific gain.