Abstract: A bi-directional amplifier (BDA) comprises a first pair of amplifier transistors and a second pair of amplifier transistors, wherein the first pair of amplifier transistors are cross-coupled with the second pair of amplifier transistors, and wherein the first pair of amplifier transistors and the second pair of amplifier transistors each comprise a differential common-emitter (CE) pair (or common-source (CS) pair) with equal transistor size or different transistor size. The BDA further comprises a plurality of blocking capacitors to decouple the collector and the base biases of the first pair of amplifier transistors and the second pair of amplifier transistors. Alternatively or additionally, the BDA further comprises two input/output baluns, through which a common voltage bias is applied to the collectors of each of the differential CE pairs (or drains of CS pairs in some implementations). The baluns enable single-ended measurement and characterization.

Abstract: A frequency characteristic adjustment circuit is disclosed. The frequency characteristic adjustment circuit is disposed between an optical circuit element and a drive circuit for driving the optical circuit element. A capacitor is connected to an output of the drive circuit. A current supply circuit is controlled by a voltage generated by the drive circuit. The current supply circuit supplies a different current value depending on a voltage received from the drive circuit to the optical circuit element.

Abstract: A transceiver circuit and related radio frequency (RF) circuit are provided. An RF circuit is coupled to a transceiver circuit configured to generate an envelope tracking (ET) target voltage. The RF circuit includes a tracker circuit and a power amplifier circuit(s). The tracker circuit may have inherent frequency-dependent impedance that can interact with a load current of the amplifier circuit(s) to cause degradation in an ET modulated voltage, which can lead to spectral distortions in an RF offset spectrum. As such, a voltage compensation circuit is provided in the transceiver circuit and configured to add a voltage compensation term in the ET target voltage. By adding the voltage compensation term into the ET target voltage, it is possible to compensate for the degradation in the ET modulated voltage, thus helping to reduce the spectral distortions in the RF offset spectrum and improve linearity and efficiency of the amplifier circuit(s).

Abstract: In order to provide unbalanced ringing when an exchange line card provides a balanced ringing output the interface provides a ringing detector in circuit with a customer pair. On detecting balanced ringing signals from the line card, the interface takes over the customer line pair and provides unbalanced ringing signals to the customer and monitors for customer answer. By inserting the interface in circuit at the main distribution frame of an exchange, legacy customer premises equipment which is unable to respond to balanced signalling can be provided with an appropriate ringing signal without the necessity to provide differing line cards at the exchange.

Abstract: In one embodiment, the present invention includes an apparatus having a first pair of low voltage operational amplifiers to generate an output representative of an absolute value difference of first and second line voltages of a subscriber loop, and a third low voltage operational amplifier having an input coupled to the output of the first pair of low voltage operational amplifiers to filter the output and to provide a switch control signal for a switching regulator that provides a voltage used to generate the first and second line voltages. The apparatus may include additional circuits such as a limit circuit to limit the input to the third low voltage operational amplifier and one or more speedup circuits to reduce a filter time constant of the third low voltage operational amplifier during a ringing mode of the subscriber loop.

Abstract: Driver circuit for driving a transmit signal for a line comprising a signal amplifier which delivers a transmit signal, present at a signal input of the signal amplifier, amplified with a gain to a signal output, a line interface circuit which is connected between the signal output of the signal amplifier and the line, and comprising a hybrid circuit which is connected to the line interface circuit, wherein the hybrid circuit suppresses the transmit signal and couples a received signal received via the line to a signal input of the signal amplifier for the purpose of impedance synthesis.

Abstract: A field-mounted process device with multiple isolated connections includes a connection that can be an input or an output. The given input or output can couple to multiple sensors or actuators, respectively. The process device can be wholly powered through its communication I/O port. The process device includes a controller adapted to measure one or more characteristics of sensors coupled to an input connection and to control actuators coupled to an output connection. The controller can be further adapted to execute a user generated control algorithm relating process input information with process output commands.

Abstract: The present invention relates to a transceiver for bidirectional frequency division multiplexed transmission, a communication system including one or more transceivers. Optionally, the communication system is a communication system for a digital subscriber line. The transceiver comprises transmission means with a voltage source output or a current source output for transmitting data in a transmission frequency range, receiving means for receiving data in a receiving frequency range, and a coupling impedance for connecting the transmission means and the receiving means to a transmission medium. The magnitude of the coupling impedance in the transmission frequency range is smaller than the magnitude of the coupling impedance in the receiving frequency range if the transmission means has a voltage source output and is higher than the magnitude of the coupling impedance in the receiving frequency range if the transmission means has a current source output.

Abstract: A data access arrangement (DAA) circuit for an item of customer premise equipment (CPE, i.e., telephone terminal equipment) utilizes RF coupling across an isolation barrier between two sides of the DAA circuit.

Abstract: According to the invention, means for reducing the interference caused by a voltage or a current in common mode, comprise adders (S3; S4) in each path for adding a first counter-reaction voltage (V1) to the voltage of the relevant path, a bridge (R1, R2) and an inverter (I3) for provision of said first counter-reaction voltage which is equal to half the sum of the voltages (VA, VB), respectively supplied to the inputs (A; B), with an opposing sign. According to the invention, the effect of a delay introduced by the inverter (I3) may be reduced, whereby said stage further comprises means (S5; S6) for adding in addition to the input voltage for each path, a second counter-reaction voltage (Va1; Vb1), and means (R1, R2, I1, S1; I2, S2) for provision of a second counter-reaction voltage (Va1; Vb1) which is a function of the input voltage (VA; VB) at the input corresponding to said path, with an opposing sign and with a delay identical to that generated by the inverter (I3).

Abstract: A line interface couples a data transceiver to a transmission line via a transformer, the data transceiver transmitting signals in a first frequency range and receiving signals in a second frequency range. The line interface includes an input port for receiving an input signal voltage from an analog front end (AFE) chip, an output port, a line driver for amplifying the input signal voltage and supplying a transmit signal to the output port, a line port for sending the transmit signal and receiving a receive signal, termination resistors coupled between the output port and the line port, a receive signal port for supplying the receive signal to the AFE chip, a receive amplifier formed on the AFE chip coupled to the receive signal port, and a bridge network resistively coupling the line port and the output port to the receive signal port, the bridge network having a low-pass filter characteristic.

Abstract: Echo cancelling in a communication line system, particularly an ISDN or XDSL system, is performed with a tunable hybrid having tunable passive elements. The tunable hybrid is integrated in the analog front-end portion of the communication line system. The values of the tunable passive elements are digitally controlled by a microprocessor.

Abstract: A digital direct access arrangement (DAA) circuitry may be used to terminate the telephone connections at the user's end to provide a communication path for signals to and from the phone lines. Briefly described, a means for providing a proper hookswitch transition for a variety of international phone standards is provided. The invention may also be utilized with means for transmitting and receiving a signal across a capacitive isolation barrier. More particularly, a DAA circuitry may be utilized which satisfies many or all hookswitch transition standards without the use of additional discrete devices. The hookswitch transition standards may be satisfied by ramping down the current flowing through the hookswitch prior to transitioning the hookswitch state. In this manner the hookswitch current change as a function of time (di/dt) may be decreased. Thus, the current through the hookswitch may be actively controlled prior to switching the hookswitch from an off-hook condition to an on-hook condition.

Abstract: In at least one embodiment, a current mode differential active hybrid circuit having an H-bridge output driver with a first output resistor coupled across a transmission medium output. Drive circuitry is coupled to the output H-bridge and constructed to supply data modulated signals to the output H-bridge. A second H-bridge is coupled to the drive circuitry in parallel with the output H-bridge to allow a second output across a second output resistor.

Abstract: A CMOS implementation for a DC holding circuit in direct access arrangement (DAA) circuitry is disclosed that provides desirable inductive behavior while minimizing power dissipation required by the CMOS integrated circuit, particularly at high loop currents. The DC holding circuitry disclosed may include MOS transistors located on a CMOS integrated circuit and an off-chip power dissipating resistor that acts to dissipate power external to the CMOS integrated circuit. The CMOS implementation disclosed also allows a path for drawing DC current to power other CMOS circuits (e.g. ADCs and DACs) in the CMOS integrated circuit.

Abstract: A CODEC and a SLIC assembly perform current-sensing-voltage synthesis impedance matching and DC feed control functions. Signal processing that does not require high voltage, such as impedance matching and DC feed control, is performed in the digital domain by the CODEC while the SLIC assembly includes high voltage circuitry. This configuration is useful for Voice over Internet Protocol (VOIP) applications with a short subscriber line loop or other long loop applications. The SLIC includes high voltage operational amplifiers (op amps) to drive ring and tip signals. Bipolar transistors are also provided as bias compensating diodes for bias point stabilization over dynamic operating conditions such as temperature. The high voltage op amps include a composite MOSFET-bipolar complimentary symmetry driver stage that offers the bias control and stability of a bipolar device topology and drive capabilities of a power MOSFET device.

Abstract: A current sensing circuit (50) for use in an ADSL interface circuit senses an input current (IL) flowing through a resistor (R1) and provides an image output signal at an output which is an image of the input current.

Abstract: A balance network directional coupler (BNDC) allows for effective coupling to sources of transmitted signals, communication lines, and devices that process received signals. The BNDC includes a transmission port, first and second amplifiers, a reception port, a communication line port, a transformer system, and a balance network. The second amplifier has a second gain of a polarity opposite the polarity of the polarity of the first gain. The transformer system has first and second windings each having first and second terminals, the first and second terminals of the first winding being electrically coupled to the communication line port, the first terminal of the second winding being electrically coupled to the reception port, and the second terminal of the second winding being electrically coupled to an output terminal of the second amplifier.

Abstract: In an electronic inductor circuit, an operational amplifier drives the base of the electronic inductor transistor, and receives negative feedback from the emitter of the transistor. The transistor and operational amplifier combine to form a voltage-controlled current source (VCCS) with respect to the loop current. A voltage divider connected across the rectified Tip and Ring voltage (or another node of the circuit at an equivalent voltage) provides a DC reference to the positive input of the operational amplifier, so that the line current automatically increases with an increase in line voltage. A capacitor couples the transmit signal driver to the positive input of the operational amplifier. This electronic inductor circuit can be driven using a low voltage supply and provides sufficient linearity for high-speed modem applications.

Abstract: A surge reset circuit used to automatically reset a surge protection circuit within a telephone line interface circuit. A sensor (e.g., a zenering device) advantageously generates a voltage level representing current draining out of the transient voltage suppression portion of the surge protection circuit after a transient energy surge. If the current is draining, which may indicate a potential lockup situation, feedback circuitry generates a reset feedback signal that is provided to a cutoff device. The cutoff device, in response to receiving the feedback signal, disables a current limiter portion of the surge protection circuit to reduce the current draining from the transient voltage suppression portion. As the current level is reduced, the transient voltage suppression portion will reset when the current drops below a threshold value or is substantially eliminated. As a result, the feedback circuitry removes the reset feedback signal from the cutoff device to re-enable the current limiter.

Abstract: The invention relates to a MOS transistors substitutive circuit having a transformer/data interface function, in particular for ISDN networks, comprising first (11a) and second (11b) power supply/transmitter blocks, the first power supply/transmitter block (11a) being connected between a voltage reference (V) and a first data interface (RX), and the second power supply/transmitter block (11b) being connected between a ground potential reference (GND) and a second data interface (TX), both power supply/transmitter blocks being connected to a supply voltage reference (VDD). The MOS transistors substitutive circuit according to the invention comprises first (12) and second (12′) MOS transistor pairs connected to the voltage reference (V), the MOS transistors being diode configured and held in their saturation range, so as to have a high A.C. impedance and virtually zero D.C. impedance, thereby minimizing power dissipation through the substitutive circuit.

Abstract: A splitter separates telephony traffic (POTS) from digital subscriber line (ADSL) traffic occupying a higher frequency band. The splitter comprises a low-pass filter for passing the telephony traffic, the low-pass filter being operable to vary its filtering response between a first low-pass response for use during telephony speech traffic, and a second, more restrictive, low-pass response for use during at least part of the time that telephony signalling traffic is present. During speech traffic a filter is used which provides a good impedance match with a telephony terminal or linecard so as to minimize detrimental effects on sidetone and echo performance which may impair a user's speech quality. The filter response can be varied according to a detected property of the telephony traffic, such as amplitude or rate of change of amplitude of the traffic.

Abstract: A communication apparatus includes circuitry operable to facilitate communication over a digital subscriber line. The apparatus also includes a filter operable to be coupled between the circuitry and the digital subscriber line. The filter has a corner frequency and is operable to attenuate transients associated with the circuitry when the apparatus is in use. The filter is tuned to an impedance of the circuitry.

Abstract: A communication system of the present invention utilizes ring detection circuitry on both sides of an isolation barrier. More particularly, the ring detection circuitry may include ring burst circuitry on the phone line side of the isolation barrier and ringer timing circuits on the powered side of the isolation barrier. The digital burst peak signal may be transmitted through the isolation barrier to the ringer timing circuits. By splitting the ring detection circuitry so that the ringer timing circuits are placed on the powered side of the isolation barriers, a significant reduction in the power usage on the phone line side of the barrier related to the ring detection function may occur. The outputs of the ringing timing circuits may be provided to circuits on either side of the isolation barrier. Thus, the ring detection function may be accomplished in a system utilizing an efficient bidirectional capacitive barrier while still minimizing power usage on the line side of the barrier.

Abstract: An interface for coupling a modem port with a transmission line includes a hybrid converter having a variable gain amplifier coupling a transmit path of a first differential polarity and a received path of a second differential polarity.

Abstract: A method and apparatus are provided for reducing the amplitude of signals that often are inadvertently coupled to a telephone line from an isolation barrier system, or that cause undesirable interference within the circuitry of the isolation barrier system. Such signals have undesirable peaks in the frequency domain (spectral peaks) due to the existence of periodicities in the data signal that is transmitted across the isolation barrier. Such spectral peaks are reduced by this invention, which comprises randomizing or scrambling the data signal before it crosses the isolation barrier, and descrambling the data after it has crossed the isolation barrier. In one embodiment, the data signal is scrambled by combining it with a random bit stream (or a pseudo random bit stream), which effectively “whitens” the resulting scrambled signal, thus removing spectral peaks that exist in the original data signal.

Abstract: A CMOS implementation for a DC holding circuit in direct access arrangement (DAA) circuitry is disclosed that provides desirable inductive behavior while minimizing power dissipation required by the CMOS integrated circuit, particularly at high loop currents. The DC holding circuitry disclosed may include MOS transistors located on a CMOS integrated circuit and an off-chip power dissipating resistor that acts to dissipate power external to the CMOS integrated circuit. The CMOS implementation disclosed also allows a path for drawing DC current to power other CMOS circuits (e.g. ADCs and DACs) in the CMOS integrated circuit.

Abstract: A CMOS implementation for a DC holding circuit in direct access arrangement (DAA) circuitry is disclosed that provides desirable inductive behavior while minimizing power dissipation required by the CMOS integrated circuit, particularly at high loop currents. The DC holding circuitry disclosed may include MOS transistors located on a CMOS integrated circuit and an off-chip power dissipating resistor that acts to dissipate power external to the CMOS integrated circuit. The CMOS implementation disclosed also allows a path for drawing DC current to power other CMOS circuits (e.g., ADCs and DACs) in the CMOS integrated circuit.

Abstract: Large signal swings are obtained for the transmitted signal of a telephone line modem using differential drivers. Compensation for the complex nature of line impedance is provided without affecting the response of the receive circuits. Common-mode noise is effectively cancelled due to a fully balanced topology for the transmit and receive portions of the hybrid circuit.

Abstract: Isolation and interconnection of a telephone network of tip and ring lines and the analog ports of a modem are achieved with a transhybrid that incorporates optically-coupled isolation stages. Optical isolators directly connect to the tip and ring lines to provide optimal isolation and unidirectional transfer of signals.

Abstract: A circuit for providing an interface between a telephone set and a modem having voice processing capabilities that allows digitization of the voice to occur and provides an improved capability for the detection of a signal indicating a transfer from voice mode to data mode. A typical telephone interface consists of a `2 wire system` on which both the incoming and outgoing signals are carried simultaneously. These two signals need to be separated in order to digitize the outgoing signal. This is done using a hybrid transformer network that attempts to subtract the incoming signal from the outgoing signal. To switch from voice mode to data mode, it is necessary to `listen` for the data tones while a voice call is in progress. These tones are sent by a remote modem as a signal for the local modem to switch into data mode. These tones may be received at a very low level which can prevent the tone detection circuitry from properly functioning.

Abstract: The balancing bridge circuit is provided between a four-wire circuit and a two-wire circuit, to provide a thin-type balancing bridge circuit having satisfactory transmission characteristics and a MODEM and an exchange and a repeater of a telephone circuit having this balancing bridge circuit built therein. The transmitting terminals of the four-wire circuit and the connection points of the balancing bridge circuit are connected together through the photo-coupler. The receiving terminals of the four-wire circuit and the connection points of the balancing bridge circuit are connected together through the photo-coupler. The terminals of the two-wire circuit are connected to the connection points of the balancing bridge circuit. By the presence of the compact photo-couplers, it is possible to ground the transmitting terminal and the receiving terminal together. Common-mode noise reaching the terminals is not transmitted to the receiving terminals.

Abstract: A circuit and method for sensing in current mode when a telephone has been answered in a telephone system in which an answer is indicated when a dc component of a signal across a resistor exceeds a predetermined reference current. Currents sensed at the terminals of the resistor are combined to provide a third current that includes the dc component, without attenuation of the dc component. A filter separates the dc component from the third current so that the dc component can be compared to the reference current.

Abstract: A method and apparatus for providing a telephone hybrid function is provided. The present invention maximizes dynamic range while providing a receive signal without interference from a transmit signal. The present invention also provides single-ended to differential conversion. The present invention may be practiced with an integrated circuit powered by a telephone line and coupled to the telephone line without the need for a high voltage isolation barrier between the integrated circuit and the telephone line.

Abstract: A channel unit network having a two-wire port for interconnecting a two-wire, bidirectional signal transmitting means with a four-wire digital transmitting means in a communication system to create a precision bidirectional simulated two-wire cable pair with an extended operation range over a transmission medium includes a signal processing circuit, tip and ring switch mode driver, a negative inductor circuit, a negative capacitor circuit, and a 200 Hz impedance circuit. The negative conductor circuit and the negative capacitor circuit are used to generate respective negative inductance and negative capacitance at 24 Hz in order to maintain loop stability. The 200 Hz impedance circuit is used to generate a positive impedance at a frequency of approximately 200 Hz so as to prevent overloading of external telephone communication testing circuitry.

Abstract: A galvanic isolation device having a light-emitting diode and a photo-transistor. A control circuit, which is coupled to the light-emitting diode and which controls the light-emitting diode, is provided. A signal generator circuit, which is coupled to the photo-transistor, is also provided. The signal generator circuit includes a fixed-gain amplifier circuit that is coupled to a current path formed by a collector electrode and an emitter electrode of the photo-transistor. A biasing circuit, which is coupled to a base electrode of the photo-transistor, biases the photo-transistor into a saturated state.

Abstract: Isolation and interconnection of a telephone network of tip and ring lines and the analog ports of a modem are achieved with a transhybrid that incorporates optically-coupled isolation stages. Optical isolators connect to the tip and ring lines to provide optimal isolation and unidirectional transfer of signals.

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

Abstract: The invention relates to a method and circuit for implementing an impedance associated with a monolithically integrated telephone subscriber circuit connected to a telephone line having a pair of terminals. The circuit consists of a resistor connected serially to one terminal of the telephone line, and a series of current mirror circuits. The current mirror circuits are connected in a closed loop configuration to the one terminal of the telephone line. The current mirror circuits divide, by a predetermined factor, the value of the resistor when a DC or very low frequency signal is input to the telephone circuit.

Abstract: An electronic hybrid useful in telephone line-powered Data Access Arrangements. The hybrid has a transmit amplifier with an output driving the base of a bipolar transistor. The emitter of the transistor is connected to a sense resistor which is, in turn, connected to one wire of a telephone line. The collector of the transistor connects to another wire of the telephone line. A summing node couples together signals on the telephone line and signals across the sense resistor such that the remaining signal on the summing node is substantially the signal received from the telephone line without the signal to be transmitted.

Abstract: A circuit and method for isolating a subscriber line interface circuit ("SLIC") from the ringer generator in a telephone system. The circuit uses a gain blocking device to provide a signal gain when the ringer generator is not being utilized and to provide a high impedance when a ringer signal is present. The high impedance may be provided through an amplifier circuit which is tri-stated when the amplifier is not energized.

Abstract: A line feed circuit employing a low-inductance transformer, and providing a major portion of the terminating impedance as line feed resistors connected between a telephone line and the transformer primary. A low impedance output amplifier drives a low resistance transformer secondary, thus reducing the need for large transformer inductances. By connecting the line feed resistors between the transformer and the telephone line, high voltage surges are absorbed by the resistors, and thus little is dissipated in the transformer. The transformer can be made physically small, because of the reduced inductance, reduced winding insulation, and reduced power requirements. The resistance forward type of line feed circuit lends itself well to simple implementation with flux cancellation and impedance synthesis circuits.

Abstract: A method and apparatus for providing a telephone line interface having selectable termination characteristics, such as impedance parameters, is provided using AC and DC transconductance loops. The present invention may be used with modem apparatus to provide regulatory compliance of line termination characteristics in multiple countries having different, even conflicting, standards or regulations. The preferred embodiment of the present invention uses a programmable switched capacitor filter to drive a transconductance driver that drives the telephone line. An AC transconductance loop controls AC transconductance characteristics, while a DC transconductance loop controls DC transconductance characteristics.

Abstract: An isolation circuit for a telephone line interface includes a pair of capacitors that differentially couple analog transmit and receive channels in full duplex communication across an isolation barrier. Each transmit or receive signal is processed to generate respective paired complementary signals. A switching network is clock driven to differentially charge the paired capacitors in incremental fashion with the paired complementary signals. On the opposite side of the barrier, a clock driven switching network receives the differential charges on the paired capacitors, generating paired complementary signals that are processed to form an analog information signal. Clock signals, off-hook and ringing signals are transmitted in similar fashion. The invention can use two pairs of capacitors that differentially couple analog transmit and receive channels in a user device across an isolation barrier to a line pair for full duplex communication.

Abstract: An opto-coupling device lies between an analog input circuit, such as a telephone line, and a processing apparatus, such as a modem. The device has a high impedance circuit; this is, its impedance is high with respect to the input impedance of the analog input circuit. This high impedance circuit has a voltage/current conversion circuit. The variations of the current flowing through the conversion circuit are directly induced by the variations in the voltage of the analog input signal. The high impedance circuit is in series with a constant current source, and is connected to the analog input circuit. The inputs of the opto-coupling device are in parallel with the voltage/current conversion circuit.

Abstract: A hybrid circuit applies a data sending signal and an inverted data sending signal to the ends of a primary coil, so that the primary coil receives voltages based on the sum of the voltages of the two signals. As a result, the gains of internal amplifiers can be small, thereby maintaining low level voltages in the hybrid circuit. In addition, because the hybrid circuit is balanced, grounding is unnecessary and noise is kept at a low level.

Abstract: An improved solid state isolation device that uses opto-isolators to provide an electrical barrier between a telephone network and user telephone equipment includes a load and current control circuit, and an adjustable termination circuit to eliminate or reduce signal distortion and insensitivity to low signal levels resulting from low loop current, and lack of complex impedance compensation/termination for diverse telephone system parameters. An improved trans-hybrid return loss circuit reduces internal losses; while loop termination losses are reduced by the use of Shottky diodes in the line coupler circuit. A high pass filter improves common mode rejection ratios in the transmit and receive paths, and adjustable compensation is provided to account for manufacturing variances in the opto-isolators. An additional connection is also provided to adapt the opto-isolator circuit for use with 3-wire ring circuitry.

Abstract: A battery feed circuit supplies a feed current to a telephone set via a 2-wire telecommunication line formed of a TIP line and a RING line.

Abstract: This amplifier biasing circuit is used in a telecommunication ring trip circuit to control the bias of a sense amplifier (SA) having input terminals (I1, I2) coupled to output terminals (A, B) of a Herter bridge (R0/R5) which is also coupled to a telecommunication line (L) and to a ringing signal source (RC). The bias circuit is a negative feedback loop coupled between terminals I1 and I1, I2 of SA and regulates the voltage at the latter terminals to a reference value when the input voltage on terminal I1 exceeds a predetermined value in one direction, by connecting a variable bias impedance (R6, PNP1, R7, PNP2) between the amplifier input terminals and VBAT.

Abstract: An improved solid state isolation device that uses opto-isolators to provide an electrical barrier between a telephone network and user telephone equipment includes a load and current control circuit, and an adjustable termination circuit to eliminate or reduce signal distortion and insensitivity to low signal levels resulting from low loop current, and lack of complex impedance compensation/termination for diverse telephone system parameters. An improved trans-hybrid return loss circuit reduces internal losses; while loop termination losses are reduced by the use of Shottky diodes in the line coupler circuit. A high pass filter improves common mode rejection ratios in the transmit and receive paths, and adjustable compensation is provided to account for manufacturing variances in the opto-isolators. An additional connection is also provided to adapt the opto-isolator circuit for use with 3-wire ring circuitry.