Differential gain and common mode gain are two important measures of an amplifier's performance in amplifying differential and common mode signals.

Differential gain:

The differential gain is the gain of an amplifier for a differential input signal, which is the difference between two input voltages. In other words, it is the amplification factor of the signal that appears across the two input terminals of the amplifier. Differential gain is typically expressed in decibels (dB) or as a ratio of output voltage to input voltage.

2.Common mode gain:

The common mode gain is the gain of an amplifier for a common mode input signal, which is a signal that is applied to both input terminals of the amplifier in-phase. In other words, it is the amplification factor of the signal that appears at both input terminals of the amplifier. Common mode gain is typically expressed as a ratio of output voltage to input voltage.

Ideally, an amplifier should have a high differential gain and a low common mode gain. A high differential gain means that the amplifier is capable of amplifying small differential signals, while a low common mode gain means that the amplifier does not amplify signals that are common to both inputs.

A low common mode gain is important because common mode signals are often caused by noise, interference, or ground loops, which can degrade the performance of the amplifier. Therefore, an amplifier with a low common mode gain is less susceptible to noise and interference, which makes it more reliable and accurate.

In this mode, the same type of signal is given to inputs. Both signals appear in the same phase in the resistance . They are adding so that gain is diminished. This is also known as degeneracy mode. Degeneracy mode is a technique used to reduce the common mode voltage gain in a differential amplifier. The technique involves introducing a small amount of resistance in series with the emitter of each transistor in the differential pair. This additional resistance reduces the transconductance of the transistors, which in turn reduces the gain of the amplifier. The circuit is shown below.

Since both inputs are same.

This is the common mode voltage gain in common mode of differential amplifier. The value of the additional resistance is typically chosen such that the ratio of the collector resistor to twice the emitter resistance (Rc/2Re) is equal to the common mode voltage gain of the amplifier. In other words, the degeneration factor (Rc/2Re) is chosen to match the desired common mode voltage gain of the amplifier.

A high common mode voltage gain can cause unwanted amplification of noise or interference, which can degrade the performance of the amplifier. Therefore, it is important to design differential amplifiers with low common mode voltage gain to minimize the effects of noise and interference. Techniques such as common mode rejection ratio (CMRR) and shielding can be used to reduce the common mode voltage gain in differential amplifiers.

The common mode voltage gain of a differential amplifier is generally considered to be an undesirable characteristic, as it amplifies signals that are common to both inputs, such as noise or interference. However, in some cases, the common mode voltage gain can be intentionally used for certain applications, such as:

Measurement of common mode noise: The common mode voltage gain of a differential amplifier can be used to measure the level of common mode noise or interference present in a signal. By applying a known common mode voltage and measuring the resulting output, the level of common mode noise can be determined.
Signal processing: In some applications, the common mode voltage gain can be used for signal processing, such as filtering or modulation. For example, a common mode voltage can be applied to a differential amplifier to modulate the signal on the input terminals, which can then be demodulated at the output.
Amplification of small common mode signals: In some cases, a small common mode signal may need to be amplified, and the common mode voltage gain of a differential amplifier can be used for this purpose. By using degeneracy techniques to control the common mode voltage gain, the differential amplifier can be optimized for amplification of small common mode signals.

This note is a part of the Physics Repository.

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