9 Linearity of Output Swing Specifications

To this point linear and non-linear regions of operation for output swing limitations have been documented. In reality, the limitations given for output swing in the data sheet tables are generally saturation limitations. That means that one of the output transistors is fully turned on and the other is fully off. For a triangle input signal, the output is clipped when the signal reaches the saturation limit (see Figure 9-1). However, it is important to understand that the amplifier does not abruptly transition from linear operation to completely non-linear (saturated). In fact, as the output signal approaches the saturation limit, the signal begins to become non-linear. Unfortunately, it is not always obvious according to data sheet specifications where the output begins to become non-linear or the extent of the non-linearity.

Figure 9-1 Linear Output Range versus the Saturation Output Swing Limit

One common approach used to estimate the range where the amplifier has excellent linearity is to check the test conditions for the open-loop gain specification (A_OL). Figure 9-1 illustrates the data sheet specification for output swing and A_OL. Notice that the test condition for A_OL indicates that A_OL is specified for output voltages of 100 mV from the supply rails unloaded, and 200 mV from the rail with a 2-kΩ load. The same data sheet illustrates that the saturation limit is 5 mV from the supply rail unloaded, and 15 mV with a 2-kΩ load. The main point is that somewhere between saturation and the A_OL test limit the open loop gain begins to drop off leading to non-linear operation.