Introduction:
Linear integrated circuits (ICs) are widely used in various electronic devices and systems. The two-transistor differential amplifier is a fundamental building block in linear ICs due to its high common-mode rejection ratio (CMMR). This characteristic makes it suitable for a wide range of applications.

Explanation:
The two-transistor differential amplifier consists of two transistors connected in a differential configuration, with their emitters tied together and connected to a current source. The input signals are applied to the bases of the transistors, and the amplified output is taken from the collectors.

High CMMR:
The CMMR of an amplifier is a measure of its ability to reject common-mode signals. Common-mode signals refer to signals that are present at both inputs of the amplifier. In many practical applications, common-mode signals can cause interference and degrade the performance of the amplifier. The two-transistor differential amplifier provides a high CMMR, which means that it can effectively reject common-mode signals and amplify the differential signals.

Benefits of High CMMR:
1. Improved Signal Quality: By rejecting common-mode signals, the two-transistor differential amplifier ensures that the amplified output primarily consists of the desired differential signal. This leads to improved signal quality.

2. Noise Rejection: Common-mode signals often contain noise, which can be introduced through various sources such as electromagnetic interference (EMI) and ground loops. The high CMMR of the differential amplifier helps in rejecting the noise present in the common-mode signals, resulting in a cleaner output.

3. Increased Dynamic Range: The high CMMR allows the differential amplifier to operate effectively even in the presence of large common-mode signals. This extends the dynamic range of the amplifier, making it suitable for a wide range of input signal levels.

Other Advantages:
While high CMMR is the primary reason for the widespread use of the two-transistor differential amplifier in linear ICs, it also offers other advantages:

1. High Voltage Gain: The differential amplifier provides high voltage gain, which is necessary for amplifying weak signals to a level suitable for further processing.

2. High Input Resistance: The input resistance of the differential amplifier is relatively high, which allows it to interface with various signal sources without significantly loading them.

3. Input Voltage Dependent Linear Transfer Characteristic: The transfer characteristic of the differential amplifier is linear and depends on the difference between the input voltages. This linearity is crucial for many applications that require accurate signal amplification.

Conclusion:
The two-transistor differential amplifier is a key component in linear integrated circuits due to its high CMMR, which allows it to reject common-mode signals and amplify differential signals effectively. Additionally, it offers other advantages such as high voltage gain, high input resistance, and a linear transfer characteristic. These characteristics make it suitable for a wide range of applications in electronic devices and systems.