AM-AM Distortion

The most common form of AM-AM distortion where amplifier gain decreases as input power increases beyond the linear region. Characterized by the 1 dB compression point (P1dB), where gain drops by 1 dB from its small-signal value. In saturation, further input increases produce negligible output changes, causing severe clipping distortion and spectral regrowth.

A less common nonlinearity where gain increases with input amplitude before eventual compression. Often observed in Class AB and Class B amplifiers at moderate drive levels due to changing conduction angles. Gain expansion can partially cancel subsequent compression stages, a technique exploited in Doherty amplifier design for efficiency enhancement.

The static nonlinear transfer characteristic mapping instantaneous input amplitude to output amplitude. Key representations include:

• Rapp model: Smooth saturation for solid-state PAs
• Saleh model: Two-parameter formula with adjustable sharpness
• Ghorbani model: Enhanced accuracy for modern GaN devices
• Polynomial model: Odd-order power series capturing soft nonlinearity

AM-AM and AM-PM distortion are coupled phenomena in physical amplifiers. As the input envelope drives the transistor into compression, the device's parasitic capacitances and transit times change, simultaneously causing both amplitude distortion and phase shift. Accurate behavioral models must capture both effects using complex-valued coefficients in complex baseband Volterra formulations.

AM-AM distortion degrades communication performance through multiple mechanisms:

• EVM degradation: Constellation points compress inward, increasing error vector magnitude
• Spectral regrowth: Nonlinearity generates out-of-band emissions, raising ACLR
• BER increase: Distorted symbol amplitudes reduce noise margin at the receiver
• Constellation warping: Higher-order QAM signals suffer non-uniform compression across amplitude levels

AM-AM distortion is measured using vector network analyzers or vector signal analyzers with modulated or continuous-wave stimuli. Common techniques:

• Single-tone power sweep: Maps output vs. input power at fixed frequency
• Two-tone test: Reveals compression behavior under multi-carrier loading
• Modulated signal analysis: Captures dynamic compression with realistic waveforms
• Hot S-parameter measurements: Characterizes nonlinear behavior under large-signal drive

The nonlinear relationship between the input signal's amplitude and the output signal's phase shift. While AM-AM distorts magnitude, AM-PM introduces unwanted phase modulation that rotates the constellation diagram.

• Caused by the input capacitance variation of the transistor with signal level
• Critically degrades Error Vector Magnitude (EVM) in high-order QAM
• Must be modeled and corrected simultaneously with AM-AM for effective linearization
• Becomes dominant in GaN HEMT and Doherty amplifier architectures

The dependence of a power amplifier's current output on past input values, causing the AM-AM characteristic to become a dynamic surface rather than a static curve.

• Thermal memory: Die temperature changes with signal envelope, altering gain over milliseconds
• Electrical memory: Bias network impedance and trapping effects vary with envelope frequency
• Short-term memory (nanoseconds) vs. long-term memory (microseconds to milliseconds)
• Transforms the simple AM-AM curve into a hysteresis loop on the gain plane

The reduction in amplifier gain as the input power increases, representing the most visible manifestation of AM-AM distortion.

• 1 dB compression point (P1dB): Input power where gain drops by 1 dB from linear
• Saturation: Region where output power no longer increases with input
• Caused by transistor clipping against supply rails and reduced transconductance
• Defines the transition from linear to nonlinear operation on the AM-AM curve

Nonlinear distortion products generated at sum and difference frequencies when a multi-tone signal passes through a power amplifier.

• Third-order intermodulation (IM3) products fall in-band and adjacent channels
• Directly caused by the odd-order AM-AM nonlinearity (cubic term)
• Measured by Third-Order Intercept Point (IP3) and ACLR/ACPR
• The primary reason AM-AM distortion must be corrected in multi-carrier systems