Balanced Line Receiver: Concept, Function, and Key Parameters

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A Balanced Line Receiver is also frequently called a Differential Receiver, this component serves as the receiving unit in a balanced transmission system. Its defining characteristic is having two inputs with opposite phases. The component also functions as a differential amplifier because it performs a differentiation (subtraction) process between its two inputs. The result of this differentiation can be amplified or attenuated, depending on the application's needs.


Basic Topology and Practical Implementation

The basic topology of a balanced line receiver is shown in Figure 1. This circuit is built from an operational amplifier (op-amp) with four resistors (R1, R2, R3, R4) that are chosen or manufactured with high precision. Generally, R1 = R2 and R3 = R4, and these values must be identical or as close as possible. To achieve a high level of precision, these components are typically implemented within a single integrated circuit (IC).


Fig.1 - Basic Differential Amplifier Diagram

For practical convenience, the four resistors are often made equal ($R_1 = R_2 = R_3 = R_4$), or $R_1$ and $R_2$ are made to be twice the value of $R_3$ and $R_4$. A classic example of this implementation is the INA134 and INA137 ICs (see Figure 2).
  • In the INA134, since $R_1 = R_3$, the output Vout will be 1×( $V_{IP} - V_{IN}$ ).
  • In the INA137, because $R_1$ and $R_2$ are double the value of $R_3$ and $R_4$).

Critical Parameter: CMRR (Common Mode Rejection Ratio)

A very important parameter when selecting a balanced line receiver (or any other type of differential amplifier) is the CMRR ( Common Mode Rejection Ratio).

As the name suggests, CMRR is a differential amplifier's ability to suppress or reject a signal that is identical and appears simultaneously on both inputs (positive and negative). The higher the CMRR value, the better the performance.

For example, a balanced line receiver like the INA134 has a CMRR of about 90 dB. This means that a common-mode signal will appear at the output at a level of approximately $10^{-(90/20)}$ times the level of the common-mode signal at its two inputs.



Applications in Various Industries

Although the INA134, INA137, and similar components are widely used in the audio industry, as generic differential amplifiers, they are also utilized for various functions in the instrumentation industry and beyond.

This short note is a continuation of the discussion on Balanced Line Transmission. I hope you find it useful.

TABIK. !


(This post is parallel to the status on the FaceBookGroup The Art of Electronics with the same Topic)


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