Transfer characteristic

We sometimes have affine relationships in terms of $v_{out}$ and $v_{in}$. If we plot, where $v_o$ is the $y$-axis, then this IO plot is called a transfer characteristic.

The plot implies it continues to $\pm \infty$, but this never occurs in practice. The voltage in a circuit will always be within the values of the supply rail. An exception is a charge pump.

For amplifiers

Circuits, including amplifiers, commonly have two supply rails, $+V_{CC}$ and $-V_{CC}$, and a ground rail.

We say that the behaviour is linear within the supply range. At the peaks, it saturates as the output voltage approaches the supply rail values.