Power Amplifier- Memory-less Non Linear Modeling

The power amplifier’s nonlinearity broadens the input signal’s bandwidth. This is known as spectral regrowth
which is undesirable since it causes interference with adjacent channels. It also causes
distortions within the signal bandwidth, which affects the bit error rate at the receiver. Most recent
transmission schemes, such as wideband code division multiple access (WCDMA) or orthogonal
frequency division multiplexing (OFDM), are especially vulnerable to the nonlinear distortions due to
high fluctuations in their power levels. To ensure that linearity is maintained to a high degree, it is
necessary to examine the nature of amplifier distortion. Audio amplifier distortion is of concern for
many years. The conventional feedback techniques used at audio frequencies are not applicable to
many RF amplifiers due to following problems: 1. Stability at high bandwidth. 2. Cost for high gains in
RF stages. Memory less or Instantaneous Nonlinear Model assumes that the PA has no memory
effects, that is, it has no knowledge of past events and hence the present o/p signal is only a function
of present i/p signal. The model is further simplified by restricting the analysis to signal and distortion
contained within the first harmonic zone, such models are referred to as band pass memory-less
nonlinear models and are further simplified since they do not need to model even order nonlinearities.
A memory less nonlinear model can be modeled in polar (amplitude and phase) or Cartesian (I and Q)
form.