AM-AM Distortion

The most common form of AM-AM distortion where the amplifier's gain decreases as the input signal approaches the saturation region. As the input amplitude increases, the output amplitude fails to increase proportionally, causing the gain curve to flatten or roll off.

• 1 dB Compression Point (P1dB): The output power level where the gain drops by 1 dB from its ideal linear value, a critical specification for amplifier linearity
• Saturation Power (Psat): The maximum output power the amplifier can deliver, beyond which no further increase in input power produces additional output
• Soft vs. Hard Compression: Soft compression exhibits a gradual gain reduction, while hard compression shows an abrupt transition to saturation, typical of different transistor technologies like GaN vs. LDMOS

A less common but significant form of AM-AM distortion where the amplifier's gain actually increases with input amplitude before eventually compressing. This occurs in certain amplifier classes and transistor types.

• Class C Amplifiers: Exhibit pronounced gain expansion due to the conduction angle increasing with drive level, causing the transistor to conduct for a larger portion of the RF cycle
• Bipolar Junction Transistors (BJTs): Can show gain expansion at moderate power levels before the onset of compression, unlike FET-based amplifiers which typically compress monotonically
• Crossover Distortion: In Class AB push-pull amplifiers, the transition between the two active devices can create a region of reduced gain at low amplitudes, appearing as expansion when the amplifier enters its linear region

The mathematical relationship mapping input amplitude to output amplitude, typically visualized as a non-linear curve deviating from the ideal 45-degree line. This function is the core target of DPD linearization.

• Rapp Model: A widely-used behavioral model for solid-state power amplifiers that captures the AM-AM characteristic with a smooth saturation curve using only a few parameters
• Saleh Model: Originally developed for traveling-wave tube amplifiers (TWTAs), this two-parameter model describes both AM-AM and AM-PM conversion with closed-form expressions
• Polynomial Models: Represent the AM-AM characteristic as an odd-order polynomial series, where the coefficients capture the non-linear gain terms at each harmonic order

AM-AM non-linearity generates intermodulation products when multiple frequency components are present in the input signal. These unwanted spectral components appear at sum and difference frequencies of the original tones.

• Third-Order Intercept Point (IP3): A figure of merit extrapolating the theoretical point where the third-order intermodulation products would equal the fundamental tones, used to characterize amplifier linearity
• Two-Tone Test: A classic measurement technique where two closely-spaced sinusoidal tones are applied to the amplifier, and the resulting intermodulation products are measured to quantify non-linearity
• Spectral Regrowth: In modulated signals, intermodulation causes the signal bandwidth to broaden, spilling power into adjacent channels and violating regulatory emission masks

The AM-AM characteristic is heavily influenced by the amplifier's DC bias point and class of operation. Shifting the bias changes the conduction angle and fundamentally alters the shape of the non-linearity.

• Class A: Operates with 360-degree conduction, offering the most linear AM-AM characteristic but the lowest efficiency, typically below 50%
• Class AB: A compromise between linearity and efficiency with conduction angles between 180 and 360 degrees, exhibiting mild compression near saturation
• Back-Off Operation: Amplifiers are intentionally operated at power levels well below their P1dB to maintain linearity, trading efficiency for signal fidelity. A 10 dB back-off is common for signals with high PAPR

Accurate characterization of AM-AM distortion requires specialized test equipment and techniques to capture the instantaneous envelope behavior of the amplifier under realistic modulated signal conditions.

• Vector Network Analyzer (VNA): Provides swept-power measurements to extract the static AM-AM curve, but cannot capture memory effects or dynamic behavior
• Vector Signal Analyzer (VSA): Captures the full complex envelope of modulated signals, enabling extraction of AM-AM characteristics under realistic wideband stimulus conditions
• Complex Baseband Equivalent: The AM-AM characteristic is typically measured and modeled at complex baseband, where the input and output envelopes are represented as complex-valued signals, separating amplitude and phase effects

The companion impairment to AM-AM distortion, where the phase shift introduced by a power amplifier varies as a function of the instantaneous input amplitude. While AM-AM describes gain compression, AM-PM describes phase rotation. Both must be corrected simultaneously by a modern DPD system to achieve full linearization, as uncorrected AM-PM causes constellation rotation and degrades Error Vector Magnitude (EVM).

The dependence of a power amplifier's current output on past input values, caused by thermal dynamics, biasing networks, and trapping effects in the transistor. Memory effects transform a static AM-AM curve into a dynamic, history-dependent surface. A DPD model that only corrects static non-linearity will fail when memory effects are significant, requiring architectures like the Generalized Memory Polynomial (GMP) or recurrent neural networks.

A regulatory metric measuring the amount of transmitted power that spills into adjacent frequency channels due to spectral regrowth caused by power amplifier non-linearity. AM-AM distortion is a primary contributor to ACLR. Regulatory bodies like the FCC and 3GPP set strict ACLR limits, and DPD is the primary technique to achieve compliance while operating the amplifier near its compression point for maximum Power-Added Efficiency (PAE).

The primary countermeasure to AM-AM distortion. DPD applies an inverse model of the power amplifier's non-linearity to the input signal in the digital domain. When the predistorted signal passes through the amplifier, the cascaded response is linear. Key DPD architectures include:

• Look-Up Table (LUT) DPD: Indexes complex gain corrections by instantaneous amplitude.
• Generalized Memory Polynomial (GMP): Captures both static non-linearity and memory effects.
• Neural Network DPD: Uses deep learning to model complex inverse behavior.

A high-efficiency amplifier architecture combining a main amplifier and a peaking amplifier with an impedance inverting network. The Doherty design achieves excellent PAE but exhibits severe and complex AM-AM distortion, including a characteristic inflection point where the peaking amplifier turns on. This makes Doherty PAs a prime candidate for advanced, neural network-based DPD solutions that can model the sharp non-linearity transition.

A comprehensive metric quantifying the deviation of a transmitted signal's constellation points from their ideal locations. EVM captures the aggregate effect of all impairments, including AM-AM distortion, AM-PM distortion, I/Q imbalance, and phase noise. For high-order modulation schemes like 1024-QAM, even small amounts of AM-AM-induced compression can render the signal undecodable, making EVM the ultimate measure of DPD effectiveness.