Mostek Corporation

Michael B Terry

James R Hellums

David N Larson

Jeffrey R Ireland

Low voltage, low power RC oscillator circuit

A compensation circuit (10) controls the gain of transmit and receive amplifiers (47), (49) as a function of a residual input current (I.sub.res). Circuit (10) includes a constant current source (24) which is connected to produce mirrored constant currents in transistors (26) and (28). The residual current is passed through a resistor (R.sub.loop) to produce a reference voltage. The constant current from the transistor (26) is divided with the first part of the current passing through the resistor (R.sub.loop) and the second part of the current passing through a resistor (50) and a transistor (54). The transistor (54) is connected in a mirror configuration with a transistor (58). When the residual current increases, the current mirrored to transistor (58) decreases. A transistor (32) is connected in parallel with the transistor (58) to receive the remaining current from the transistor (28) which is not drawn by the transistor (58). The transistor (32) serves as the master side of a mirror circuit having slave transistors (38) and (42). The current mirrored to the transistors (38), (42) is drawn from the amplifiers (47), (49) wherein the current through the amplifiers is proportional to the gain thereof. Thus, the gain of the amplifiers is inversely proportional to the residual current thereby compensating the amplifier gain for the length of the telephone line.

Loop length compensation circuit

A sidetone circuit (44) is connected to the terminals of a two-line telephone system. The sidetone circuit (44) receives inputs from a DTMF source (62) and a microphone (72). These inputs are selectively passed through a circuit (82) to produce a modulating signal which controls a current source (46) and a current (54). The current source (46) is connected between the telephone line terminals. The input audio signal from the microphone (72) modulates the current source (46) to impress a voltage upon the telephone line. The current source (54) is connected between a first of the telephone lines and a balance node (60). A resistor (58) is connected between the balance node and the second of the telephone lines. An incoming audio signal over the telephone line is coupled to the balance node (60) for summing with an inverted audio signal. The input audio signal from the microphone (72) is further coupled to the balance node (60). The inverted subscriber audio signal serves to attenuate the noninverted subscriber audio signal. The resulting signal at the balance node (60) is coupled through a capacitor (120) to an amplifier (88) for driving an output speaker (104). As a result of this circuit configuration the incoming audio signal has a minimum attenuation while a maximum amplitude signal is applied to the telephone line and a controlled sidetone signal is returned to the subscriber's speaker (104).

Sidetone circuit

A low loop current switch latch circuit (10) monitors a residual current which is derived from a telephone line and is proportional to the current drawn by a telephone subscriber circuit. Bipolar transistors (20, 26) are connected to produce a proportional current to the residual current. A current mirror circuit includes a master transistor (30) and a slave transistor (32). The proportional current is drawn through the master transistor (30). The current produced by the slave transistor (32) is provided to a node (33). A constant power source (36) provides a constant current to a node (39). A pair of cross-coupled transistors (40, 46) are connected to the nodes (33, 39) such that the cross-coupled transistors are set to first and second states as a function of the current derived from the slave transistor (32). A bias current transistor (52) is activated by the cross-coupled transistors (40, 46) to draw a preset biased current from an amplifier (60), thereby changing the amplifier from a first gain to a second gain condition. The circuit (10) therefor sets the gain of the amplifier (60) through switching action utilizing hysteresis, the switching being a function of the amplitude of the residual current passed through the transistor (20).

Low loop current switch latch circuit

A current supplementation circuit is designed to operate in conjunction with a two-terminal telephone line for supplying additional current to the output of a voltage regulator (42). A nonregulated voltage is received at first and second terminals (18, 20) with the nonregulated voltage comprising both DC and AC signals. An essentially regulated voltage is produced at a terminal (12) at the output of the voltage regulator (42). An essentially constant current is drawn from the first terminal (41) and provided to the second terminal (20) to provide an indication that the subscriber circuit is active. This constant current comprises first and second partial currents. The first partial current is passed through a control transistor (98) through a parasitic collector to the second terminal (14). The second partial current is passed through the control transistor (98) to the regulated voltage terminal (12) when the voltage at this terminal drops. The current supplementation circuit serves to bypass the voltage regulator (42) and supply additional current to the voltage regulated node when the voltage at that node drops below its intended level.

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Denton, TX, United States of America

Michael B Terry