Design Articles

Most operational amplifier data sheets define large signal as any output signal that swings greater than 2 VPP for ±5-V amplifiers and 5 VPP for ±15-V amplifiers. However, the performance that is specified as large signal depends on the intended application as well as the internal architecture of the specific device. For example, a low-voltage, +5-V, fully-differential, voltage-feedback architecture analog-to-digital converter (ADC) driver amplifier such as the THS4521 specifies a 2-VPP large-signal bandwidth because 2 VPP is a common ADC analog input range. On the other hand, a high-voltage, +28-V, current-feedback architecture, high output power line driver device such as the THS6204 specifies large-signal performance for 4-VPP to 20-VPP output.

In fact, the output voltage swing of an amplifier can affect the bandwidth by more than 50% at 20-VPP output compared to a small-signal (200 mVPP) bandwidth as a direct result of slew rate limitation or loading. For a detailed discussion on the parameters affected by the output voltage swing, refer to the application note Large-Signal Specifications for High-Voltage Line Drivers (SBOA126), available for download from the TI website. As a first-order rule, distortion performance is also degraded by 6 dB for second-harmonic distortion and 12 dB for third-harmonic distortion every time the signal doubles.