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Explain the Common-mode and Differential-mode Signals

      

Explain the Common-mode and Differential-mode Signals

  

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Wilfred
- The importance of a differential amplifier lies in the fact that the outputs are proportional to the difference between the two input signals. Thus the circuit can be used to amplify the difference between the two input signals or amplify only one input signal simply by grounding the other input.
- The input signals to a DA are defined as:

(i) Common-mode signals: When the input signals to a DA are in phase and exactly equal in amplitude, they are called common-mode signals as shown in Fig. 7.7. The common-mode signals are rejected (not amplified) by the differential amplifier. It is because a differential amplifier amplifies the difference between the two signals (v1 – v2) and for common-mode signals, this difference is zero. Note that for common-mode operations, v1 = v2.

(ii) Differential-mode signals When the input signals to a DA are 180° out of phase and exactly equal in amplitude, they are called differential-mode signals as shown in Fig. 7.9.
The differential-mode signals are amplified by the differential amplifier. It is because the difference in the signals is twice the value of each signal. For differential-mode signals, v1 = –v2.
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- Therefore a differential amplifier will amplify the differential-mode signals while it will reject the common- mode signals.
Wilfykil answered the question on August 15, 2019 at 13:38

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