Distortion and Memory Effects
Impedances external to transistor
The intermodulation performance of a FET in the common-source configuration is dependent on the impedance presented to its gate and drain terminals, not only at fundamental, but also at harmonic and baseband, frequencies. At baseband frequencies, these terminating impedances are usually determined by the bias networks, which may have varying impedance over the frequencies involved. This can give rise to asymmetry in two-tone intermodulation levels, and changing intermodulation levels with tone spacing.Phenomena internal to transistor
Self heating gives microwave FETs a temperature response to power
dissipation. This is significant at high-frequency
signal rates, even though the apparent time constant of heating is a
few kilohertz. Self heating causes variation in intermodulation with
frequency spacing and the extent of this variation is linked to
bias.
Charge trapping effects in High Electron Mobility Transistors (HEMTs) are linked with anomalous intermodulation behavior, known as memory effects. This behavior can be clearly observed as changes in intermodulation levels with tone-spacing, and two-tone asymmetry. A Volterra-series analysis of an HEMT with trapping predicts the distortion accurately, and allows an understanding of the mechanisms involved.