Differential mode, common mode signal, differential amplifier circuit

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Differential mode, common mode signal, differential amplifier circuit [Copy link]

For example, if an ADC has two analog inputs, and the AD conversion result depends on the voltage difference between the two inputs, then we say that this ADC has differential input, and the two analog inputs are called differential inputs. However, the voltages applied to the differential inputs are not always equal in magnitude and opposite in direction, and in many cases they are of the same sign. (Note: not necessarily one positive and one negative) We call their difference the differential input, and their common value (i.e., the average value) the common input.

Differential is the name of a circuit form....

Differential mode is the definition of a signal....(relatively speaking, there is a common mode..)

Differential =======Differential

Answer: Differential signal: AC signals of equal magnitude and opposite direction, common signal: equal magnitude and same direction. In differential amplifier circuits, common signal and differential signal are often mentioned. In differential amplifier circuits, common signal will not be amplified. It can be understood as the current model caused by the temperature drift of the transistor. In order to visualize the temperature drift, common signal is proposed. Differential signal is the input signal, that is, Ui, which is the object of amplification.

In differential amplifier circuit, there are two input terminals. When signals of equal magnitude and opposite phase are input to these two terminals respectively (this is a useful signal), the amplifier can produce a large amplification factor. We call this signal a differential signal, and the amplification factor at this time is called differential amplification factor. If equal-sized and phase-matched signals are input at the two input ends, (this is actually a signal generated by the temperature change of the previous stage, which is a harmful thing), we call this signal a common-mode signal, and the amplification factor at this time is called a common-mode amplification factor. Due to the structural characteristics of the differential amplifier circuit, the circuit has a strong negative feedback to the common-mode signal, so the common-mode amplification factor is very small. (Generally less than 1) The calculation formula is divided into single-ended output and double-ended output, so there are four common-mode signals and differential-mode signals, which refer to the input signals of the differential amplifier when the double-ended input is used.

Common-mode signal: When the double-ended input is used, the two signals are the same. Differential mode signal: When the input is double-ended, the phase difference between the two signals is 180 degrees. Any two signals can be decomposed into a common mode signal and a differential mode signal.

Assume that the two input signals are: A and B.

m and n are the common-mode signal component and differential-mode signal component of the input signals A and B respectively. The input signals A and B can be expressed as: A=m+n;B=mn. The input signals A and B can be regarded as the synthesis of a common mode signal m and a differential mode signal n. Where m=(A+B)/2;n=(AB)/2.

The differential amplifier takes the difference between the two signals as the output signal. The output signal is AB. Compared with the common mode signal and the differential mode signal in the original two signals, it can be found that:

The common mode signal m=(A+B)/2 disappears, while the differential mode signal n=(AB)/2 is amplified twice.

This is the working principle of the differential mode amplifier.