Harmonic Distortion in Operational Amplifiers

Last edited: 2024-10-30 13:24:43

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Signal distortion is unwanted changes in the output signal relative to the input signal. Some of these distortions can be harmonic, meaning that disturbances occur at multiples of the fundamental frequency.

Total Harmonic Distortion

Distortion can be measured in various ways, but a common measurement is total harmonic distortion (THD). THD is defined as

THD=V22+V32+...+Vn2V1, \text{THD} = \frac{\sqrt{V_2^2+V_3^2+ ... +V_n^2}}{V_1},

where VnV_n is the n-th harmonic amplitude in the frequency domain. V1V_1 is called the fundamental frequency. Generally, only the first five or six harmonic amplitudes are significant for THD, and often the first three are sufficient in practice without greatly impacting the margin of error.

Slew Rate of Operational Amplifier

Harmonic distortion manifests in various forms. One example is the slew rate of an operational amplifier. The slew rate denotes the maximum rate of change of the output signal, dictated by the specified slew rate value of the operational amplifier. When this rate of change is exceeded, the waveform is replaced by a linear ramp that adheres to the maximum slew rate until it eventually intersects with the original waveform again, as illustrated in the figure above. This phenomenon can lead to significant alterations in signal fidelity, particularly in high-frequency applications, where the ability to accurately track rapid changes in the input signal is critical for maintaining overall performance.

Clipping in Operational Amplifier

Another form of harmonic distortion is clipping. Clipping can be categorized into soft clipping and hard clipping.

Hard Clipping

Hard Clipping

Hard clipping means that the output signal can only reach a certain amplitude, resulting in the waveform appearing "chopped off." For a negatively feedbacked operational amplifier, this amplitude depends, among other factors, on the power supply voltage. The output voltage cannot exceed the operational amplifier's supply voltage. Another way to achieve hard clipping is by using diodes between the output voltage and ground, as the output voltage in that case cannot exceed the threshold voltage

Soft Clipping

Soft Clipping

Soft clipping, on the other hand, means that the peaks of a waveform experience a reduced increase, resulting in a more "rounded" appearance. Soft clipping in an op-amp occurs when the output signal approaches the supply voltage limits but does not abruptly cut off; instead, it gradually compresses the signal as it nears those limits. This typically happens when the input signal exceeds the linear operating range of the op-amp, causing the output to start saturating.

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