Transformerless hybrid circuit having direct powered line-side amplifiers

Abstract: A telephone line-powered Data Access Arrangement (10) has amplifiers (15,27) therein directly powered from the telephone line without voltage regulators or filters. The amplifiers, such as the amplifier (15) driving an LED (40) in an optical coupler (14), draw a constant power supply current independent of the signal being amplified. Thus, amplified signals are not coupled back onto the telephone line.

Description: This invention relates to data access arrangements. For purposes here, the term Data Access Arrangement or DAA is used to refer to a transformerless hybrid and isolation means generally, although it is understood that more components may be necessary to form a complete DAA, such as a line switch and loop current control. In prior art DAAs, a hybrid converts the two-wire telephone line (bidirectional) path into a unilateral receive path and a unilateral transmit path. Signals arriving on the telephone line pass through the receive path over an optical isolator or coupler to a modem or the like. Similarly, signals from a modem or the like pass through the transmit path over another optical coupler and are impressed on the telephone line. The received signal strength presented to the hybrid from a mismatched telephone line (the telephone line being terminated at the DAA with an impedance different from the match impedance) is usually very weak, typically many tens of decibels below the signal strength of the transmitted signal from the DAA to the telephone line. For the DAA to adequately send the receive signal over the optical isolator or coupler, a line-powered amplifier amplifies the receive signal and performs part of the hybrid function. An additional line-powered amplifier further amplifies the received signal and drives an LED within the optical coupler. The amplifiers used in the prior art DAA are of conventional design, i.e., the power supply current varies with the signal being amplified. For the line-powered amplifier driving the LED in the optical isolator, variations in the power supply current to the amplifier has approximately the same magnitude as the signal to the LED. Thus, a filter and regulator is provided to filter out variations the power supply currents that can interfere with the received signal to the point of making the received signal useless. The foregoing also applies to the other telephone line-powered amplifiers in the DAA, where variations in power supply current consumption may interfere with the received signal. EP-A-0572175 discloses a hybrid circuit using opto-isolators for galvanic isolation between a line side and an application (e.g., a modem) side. Amplifiers are included which are powered from the telephone line via a resistor, a voltage regulating Zener diode and a filter capacitor. These components are necessary because the amplifiers need the filtering effect provided by the resistor/Zener diode/capacitor combination to isolate signals from the amplifiers from the telephone line signals. Thus, it is desirable to provide a DAA arrangement that allows for direct powering from the telephone line without filtering or voltage regulation. Further, it is desirable to provide a DAA arrangement with direct telephone line-powered amplifiers that do not significantly introduce undesired signals onto the telephone line. According to this invention there is provided a data access arrangement as claimed in claim 1. The invention will now be described with reference to the accompanying drawings, in which: FIG. 1 is a simplified schematic drawing of an exemplary DAA embodying the invention: and FIG.2 is a simplified schematic diagram of an exemplary line-side amplifier. Detailed Description As discussed below in more detail and in accordance with the exemplary embodiment shown in FIG. 1, the data access arrangement (DAA) 10 has an application portion 12 and a telephone line-side portion 13 galvanically isolated from the application portion 12 by at least one optical coupler 14 with an LED 40. The line-side portion 13 has a hybrid circuit 16 coupling to the telephone line (coupled to TIP and RING) and a line-side amplifier 15, responsive to the hybrid 16, for driving the LED 40 with signals received from the telephone line. The line-side amplifier 15 is directly powered from the telephone line and draws a substantially constant power supply current from the telephone line. In more detail, the DAA 10 in FIG. 1 has an application portion 12 and a telephone line-side portion 13. Both portions 12, 13 have a receive path 17 and a transmit path 18. The receive path 17 and the transmit path 18 are each unilateral paths from and to the hybrid 16. Hybrid 16 couples to the two-wire telephone line (a bidirectional path) and acts as a two-wire to four-wire converter to couple the bidirectional telephone line path to the unilateral receive path 17 and the unilateral transmit path 18. Signals arriving on the telephone line pass through the receive path 17 to the modem circuitry 20. Similarly, signals from a modem 20 pass through the transmit path 18 and are impressed on the telephone line for transmission. Operation of the hybrid 16 is similar to the Output Driver and Near-end Echo Canceling Subcircuits in EP-A-0572175. The only substantial difference between what is disclosed here and what is disclosed in EP-A-0572175 is the feedback path provided by resistor 52 for canceling the transmitted signal from the received signal. This provides for superior cancellation of the transmitted signal than in the hybrid disclosed in EP-A-0572175. Galvanic isolation between the portions 12, 13 is accomplished by optical isolation. Optical isolation in the receive path 17 is provided by optical coupler 14, driver amplifier 15 and photodetector amplifier 22. Similarly, in the transmit path 18, optical isolation is provided by optical coupler 25, driver amplifier 26, and photodiode amplifier 27. The operation of the above is similar to that described in connection with FIG. 3 of EP-A-0572175. An exemplary optical coupler having an LED light source and two photodiodes is a Siemens IL-300 optical coupler. Unique to this implementation is bias circuit 19 which provides substantially invariant bias voltages and currents to the various circuits in the DAA 10. The amplifiers on the application portion 12 (here amplifiers 22 and 26) are powered from, for example, an exemplary modem circuit 20. In contrast, the amplifiers on the line-side portion 13 (here amplifiers 15, 27 and those in the hybrid 16) are powered directly from the telephone line. No voltage regulation or passive filtering is provided or needed, although it is understood that regulation or filtering may be provided if desired. To allow for the direct powering of the amplifiers by telephone line current, the amplifiers should not draw a supply current that varies with the signal amplified. In FIG. 2 an exemplary amplifier 100 which has a substantially constant power supply current consumption is shown. The amplifier has a differential input stage 101 and an output stage 110. The input stage 101 has a high power supply noise immunity and is shown being powered from the V+ power supply bus (which is connected to the telephone line in FIG. 1) by constant current sources 102. The output stage 110 is powered from the V+ supply bus by constant current sources 112. Advantageously, the constant current sources 102, 112 makes the power supply current consumption of the amplifier 100 substantially independent of the signal being amplified thereby. Moreover, the amplifier 100 operates totally in the class-A mode which provides for very linear operation and low distortion, unlike prior art amplifiers which typically have class-AB or -B output stages with cross-over distortion. Using the exemplary amplifier 100 as the LED 40 driver in the receive path 17 (FIG. 1), varying signal current to the LED 40 does not get applied to the telephone line current because of the current sources 112 providing a fixed current. Current through the LED 40 varies with the desired signal in response to current shunted away from the LED 40 by output transistor 115. Thus, the amplified receive signal presented to LED 40 does not appreciably affect signals on the telephone line. Moreover, if all of the line-side portion 13 amplifiers are of the same or similar design as that shown in FIG. 2, then virtually no signals amplified by the line-side portion 13 amplifiers will affect the desired signals on the telephone line. Amplifiers in the application portion 12 of the DAA 10 may also be of the type shown in FIG. 2. However, more conventional amplifiers may be used there because of the usual availability of a regulated and filtered power supply voltage E+. Except for the optical couplers 14, 25, all of the circuitry shown in the line-side portion 13 of the DAA 10 has been implemented on a single chip. While a modem circuit is shown as the application for the disclosed DAA 10 (FIG. 1), it is understood that other uses may be found for the disclosed invention, such as facsimile machines, phone patches, answering machines, etc.

Claims: A data access arrangement having an application portion (12) and a telephone line-side portion (13) galvanically isolated from the application portion by at least one optical coupler (14) with an LED (40), both portions having amplifiers therein, the line-side portion having: a hybrid (16) coupling to the telephone line; and a line-side amplifier (15), responsive to the hybrid, for driving the LED with signals received from the telephone line; and the arrangement is CHARACTERIZED IN THAT the line-side amplifier driving the LED is directly powered from the telephone line and draws a substantially constant power supply current from the telephone line. An arrangement as claimed in claim 1, wherein the hybrid includes amplifiers, and all of the amplifiers in the telephone line-side portion are directly powered from the telephone line and draw a substantially constant power supply current from the telephone line. An arrangement as claimed in claim 2, wherein each of the line-side amplifiers has a differential input stage (101) with differential inputs, an output stage connected to an output, and a power supply terminal, the output stage having a fixed current source (112) connected between the power supply terminal and the output; and a variable current sink (115), responsive to the input stage, connected to the output. An arrangement as claimed in claim 3, wherein the input stage is coupled to the power supply terminal by at least one fixed current source (102), such that the current consumption of the input stage is substantially invariant. An arrangement as claimed in claim 4, wherein the optical coupler has therein a photodiode responsive to the LED and disposed across the differential inputs of the line-side amplifier driving the LED. An arrangement as claimed in claim 5, wherein the hybrid forms a receive path (17) and a transmit path (18), and the optical coupler is disposed in the receive path of the arrangement. An arrangement as claimed in claim 6, including an additional optical coupler 25 disposed in the transmit path of the arrangement. An arrangement as claimed in claim 7 having a modem (20) coupled to the transmit and receive paths of the arrangement.

Inventors: CWYNAR DONALD THOMAS, LATURELL DONALD RAYMOND, CWYNAR, DONALD THOMAS, LATURELL, DONALD RAYMOND
Applicants: AT & T CORP, AT&T CORP.
Application year: 1995
Main classification: H04B1
IPC codes: H04M19, H04M1, H04M11, H04L27, H03F3, H04L25, H04B1