Effect of baseband impedance on FET intermodulation

Abstract

The intermodulation performance of an 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, as previous studies have shown. In this paper, an FET is analyzed to gain an understanding, useful to the circuit designer, of the contributing mechanisms, and to enable the prediction of bias points and the design of networks that can minimize or maximize these effects. Compact formulas are given to facilitate this. An amplifier was tested, showing good agreement between the theoretical and measured results.

Study of the influence of bias and matching networks on the distortion and memory of FET-based power amplifiers

“…The dips of Figure 9 are now peaks of IMD, more noticeable as input power increases. This result is fully consistent with the work reported in [2,3], concerning the analysis of IMD by using series expansions up to third order of the device nonlinearities. Besides, it was demonstrated that the variation of the IMD with the drain impedance could be inverted in sign, and even neglected, with an accurate bias point selection.…”

“…Nevertheless, no imbalance can be detected between lower and upper IMD components. This result is fully coherent with those stated before [2,3,11,12], because imaginary parts of impedance at fundamental and second harmonic bands are close to zero. …”

“…These amplifiers are demanded for tight linearity and efficiency requirements, and a careful design of this kind of circuits is essential for the performance of the whole transmitter. This can be achieved by means of the understanding of the different mechanisms giving rise to nonlinearities and memory in these circuits [1][2][3].…”

Waveform engineering applied to linear-efficient PA design

“…Continuous Wave (CW) waveform measurement and engineering has established itself as a highly effective design solution for the development of power amplifiers [7][8] [9]. However, modern modulation schemes such as CDMA and OFDM demand characterisation over extended instantaneous bandwidths of up to 100 MHz.…”

Behavioral modeling of power amplifiers for wireless applications

“…6). Bandwidth dependent memory effects [17] are determined through proper selection of the values of the corresponding LPFs' parameters listed in Table I. For wider two-tone spacings, and with the appropriate selection of the low pass filters' parameters (as discussed in previous paragraph), the model can still capture bandwidth dependent memory effects.…”