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Every telephone signal port used to transmit and receive voice or modulated data from a modem employs a hybrid (duplexer) circuit. Such circuits are used to separate the transmit and receive signals. They’re also needed to reject the local transmitting signal in the receiver.

In this basic hybrid circuit, amplifier A1 maintains a low output resistance compared to the line impedance R (Fig. 1). While this amplifier is used to buffer the transmitting signal from the line, op-amp A2 is used to buffer the receiving signal. The transmitted signal is connected through different paths to both the positive and the negative input of the receive buffer (A2). Assume R2 equals R3 and R1 is equivalent to the supposedly pure-resistive line impedance (R) at the primary of the transformer (T1). Perfect rejection of the transmitted signal would then take place at the receive buffer output (A2).

Capacitor C1 resonates with T1’s inductance in the middle of the frequency band (≈ 1.7 kHz). This cancels the reactive impedance of the transformer in this range. The line-impedance value, however, is an uncontrolled value and is far from being purely resistive. In a wider frequency range, therefore, the trans-hybrid rejection is quite limited and frequency-dependent.

A method borrowed from RF technology “stabilizes” a reactive impedance with the penalty of a gain loss. The technique involves inserting a resistive attenuator prior to the reactive impedance. In this case, the additional gain loss isn’t a problem. It can easily be restored in the transmitter’s and the receiver’s unidirectional signal paths.

This inserted symmetrical PI-type attenuator of 6 dB is calculated for a characteristic line impedance of 600 Ω (Fig. 2). As the attenuation increases, the impedance becomes more resistive, producing greater trans-hybrid rejection ratios. The design may involve excessive attenuations, however. If so, very high signal levels will be required at the output of A1 and input of A2 to produce the same signal levels on the line.