Amplifier distortion exists in several forms. One form occurs when an amplifier is driven by a signal with a sufficiently large amplitude, thus causing the amplifier to approach its 1-dB compression (P1dB) point. Another type of distortion involves the scenario in which two signals—with frequencies that are different but still close to one another in value—are driving an amplifier, resulting in second- and third-order distortion. Second-order distortion and why it must be considered in the case of wideband amplifiers is the focus of Custom MMIC’s application note, “IP2 Measurements of Wideband Amplifiers.”
The application note begins by presenting a spectrum plot of two sinusoidal signals. While the two signals have distinct frequencies, the values of these frequencies are close to one another. Next, spectrum plots of an amplifier’s output when driven by these same two input tones are shown. The spectrum plots include the fundamental output tones, as well as second- and third-order distortion tones.
For a narrowband amplifier, the second-order tones are outside its bandwidth. Hence, second-order distortion can likely be ignored in the case of narrowband amplifiers. However, the second-order tones are within the bandwidth of a wideband amplifier, meaning that second-order distortion must be considered in such cases.
The app note continues with an explanation of how the second-order intercept (IP2) point is derived. Plots are shown of the output power level of the fundamental and second-order difference tones of Custom MMIC’s CMD192 amplifier when driven by two signals. The IP2 value is obtained by drawing straight lines through the fundamental and second-order responses, with the IP2 point being where the two lines intersect.
Several factors must be considered when attempting to perform IP2 measurements. For one, the levels of the fundamental and second-order tones must be within the linear region of the amplifier. The sum of the two input signals’ frequencies must also be within the amplifier’s bandwidth. Lastly, one must ensure that the spectrum analyzer being used for the measurement is not distorting the results.
Custom MMIC, 300 Apollo Dr., Chelmsford, MA 01824; (978) 467-4290.
