Characteristics of Practical Op Amps

A difference amplifier containing a common and a difference-mode signal ideally amplifies only the difference-mode signal. However, in practical circuits, the output voltage depends on both differential and common-mode gains.

The ratio of differential gain to the common-mode gain is defined as the common mode rejection ratio. It quantifies the ability of op amps to reject common-mode signals.

Consider a sound system where an op amp picks up music and electrical noise. A high CMRR allows it to effectively ignore the noise and amplify only the music.

The gain-bandwidth product of an op amp is the product of its open-loop voltage gain and the frequency at which it is measured. It is a constant for any given op amp.

A unity gain buffer, or voltage follower, is a specific op amp configuration that provides unit voltage gain. It has low output impedance and high input impedance, making it ideal for signal isolation.

In an audio system, a voltage follower prevents the audio signal from a high-impedance source from being effectively shorted to ground by a low-impedance speaker.