Once the
operating point is chosen, the input signal causes variations around this point which is then reflected
to the output. The closer the operating point is to the compression region and the larger the amplitude
variations that exist on the modulation scheme, the more saturation (or clipping) can occur. However,
we desire to operate in this region simply because we require these output power levels and the high
efficiency values. Moving the operating point away from the compression region pushes us further into
the linear region. This is accomplished by reducing the transmit power as well as the PA efficiency [13].
Spectral Regrowth Issues. We have spent some considerable amount of time presenting the ISI-free
conditions and their relationship to frequency emissions. When a modulation signal encounters a nonlinearity,
in this case the transmit PA, the signal becomes distorted and its occupied frequency BW
increases. The out-of-band spectral components that were once suppressed are now regenerated.
MODULATION THEORY 57
FIGURE 2.9 Transmit power amplifier nonlinear model.
FIGURE 2.10 Typical characteristics of transmit power amplifiers.
Input Amplitude, AIn
Output Amplitude,
AOut
& Output Phase,
fOut
Ideal Linear Function
AM-AM
Linear Region
AM-PM
Complex to
Amplitude &
Phase
AM-AM
Function
AM-PM
Function
Amplitude &
Phase to
Complex
Amplitude
Phase
PA Input
Signal
PA Output
Signal
58 CHAPTER TWO
This phenomenon is called spectral regrowth.
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