Sisonic and ECM in Differential Circuits Date： 20.Jan.2005 Author： Bill Ryan Benefits of Differential Circuits Differential amplifiers are desirable to use in audio applications， especially those where signal levels are very low such as those from microphones. Classically， the benefits received from using differential amplification are listed below： 1. Increase of Common Mode Rejection Ratio （CMRR） 2. Signal to Noise （SNR） increase. 3. Rejection in Electromagnetic Interference （EMI） Signal Concepts Any voltage can be characterized by a potential difference between two terminals. It is important to note that all voltage signals can be characterized by this potential difference between two terminals – The configuration of the two terminals and how the signal is delivered from output to input allows the signal to be more generally described in one of three ways： Single-ended Single-ended signal is a signal that is delivered between a signal trace and a ground. One terminal for a single-ended connection is always at fixed potential （usually ground）。 Differential Differential signals are ones that travel through a signal pair. On the signal pair， neither of the terminals is ground. Common Mode Like differential signals， common mode signals travel between a signal pair. Common mode signals represent a special case of differential signal where the voltage potential on both signals is the same. Stated another way， a common mode signal is a differential signal where the single-ended component is the same. Differential Input Amplifier Below is a diagram of a basic differential input amplifier. The operation of the amplifier is as its name would imply： The output of the amplifier is the difference between the “In+” and “In-” voltages multiplied by the gain of amplifier. It is interesting to note that from the basic differential input formula， it is easy to see why differential input amplifiers provide common mode rejection （CMRR）： By amplifying only the difference between the input terminals， any signal appearing on both input terminals will not appear at the output