AM-AM Distortion

As the input drive level increases, the amplifier's incremental gain deviates from its small-signal value, eventually reaching saturation where further input increases produce negligible output changes. This gain compression is quantified by the 1-dB compression point (P1dB)—the output power where gain drops by 1 dB from linear.

• Soft compression: Gradual gain roll-off typical in GaN HEMT devices
• Hard compression: Abrupt saturation characteristic of LDMOS technologies
• Impact: Reduces effective signal dynamic range and introduces constellation distortion

The AM-AM characteristic is the static nonlinear transfer curve mapping instantaneous input amplitude to instantaneous output amplitude. For an ideal linear amplifier, this is a straight line through the origin. Real amplifiers exhibit a compressive nonlinearity that can be modeled as:

• Rapp model: Captures smooth saturation behavior in solid-state PAs
• Saleh model: Originally developed for traveling-wave tube amplifiers
• Polynomial model: Uses odd-order terms to represent symmetric nonlinearity
• Cann model: Extends Rapp with additional parameters for improved accuracy

AM-AM distortion in the time domain directly produces spectral regrowth in the frequency domain—the expansion of the transmitted signal's bandwidth into adjacent channels. This is the primary mechanism degrading Adjacent Channel Leakage Ratio (ACLR).

• Third-order nonlinearity: Produces spectral components at 3× the original bandwidth
• Fifth-order nonlinearity: Extends regrowth to 5× bandwidth, affecting alternate channels
• Regulatory impact: Excess ACLR violates 3GPP and FCC spectral emission masks
• DPD target: Digital predistortion must invert this nonlinearity to suppress regrowth

While AM-AM distortion affects the magnitude of the output signal, AM-PM distortion introduces an input-amplitude-dependent phase shift. These two mechanisms together form the complete nonlinear behavioral model of a power amplifier.

• AM-AM: Output amplitude compression at high drive levels
• AM-PM: Phase rotation that varies with envelope magnitude
• Combined effect: Both degrade Error Vector Magnitude (EVM) and constellation fidelity
• DPD complexity: Full vector predistortion must correct both AM-AM and AM-PM simultaneously

Memoryless AM-AM distortion assumes the output depends only on the instantaneous input envelope—valid for narrowband signals. Quasi-memoryless models extend this by including AM-PM. However, wideband signals excite memory effects where the current output depends on past envelope values.

• Memoryless: Static transfer curve, sufficient for signals with bandwidth < 5 MHz
• Short-term memory: Electrical memory from bias networks and matching circuits
• Long-term memory: Thermal and trapping effects with time constants from microseconds to milliseconds
• Modeling requirement: Memory polynomial or Volterra series needed for >20 MHz bandwidths

In a Doherty power amplifier, the AM-AM characteristic exhibits a distinctive dual-region behavior due to load modulation. Below the back-off transition point, only the carrier amplifier operates with high gain. Above the transition, the peaking amplifier activates, altering the load impedance seen by the carrier.

• Low-power region: Carrier-only operation with linear AM-AM response
• Transition region: Nonlinearity spike as peaking amplifier turns on
• High-power region: Combined operation with compressed but efficient AM-AM
• DPD challenge: The piecewise nonlinearity requires more complex predistortion models than single-ended amplifiers

The complementary nonlinear mechanism to AM-AM distortion where the phase shift introduced by the amplifier varies as a function of the instantaneous input envelope magnitude. While AM-AM describes amplitude compression, AM-PM captures phase rotation that degrades modulation accuracy.

• Measured in degrees of phase shift versus input power
• Critically impacts Error Vector Magnitude (EVM) in high-order QAM
• Must be compensated simultaneously with AM-AM in DPD
• Originates from voltage-dependent parasitic capacitances in the transistor

The physical mechanism underlying AM-AM distortion where the amplifier's incremental gain decreases as the input drive level increases. Quantified by the 1-dB compression point (P1dB) — the output power at which gain drops by 1 dB from its small-signal value.

• Marks the transition from linear to nonlinear operation
• Caused by transistor saturation and clipping near the supply rails
• Soft compression in GaN HEMT devices is more linearizable than hard compression
• The shape of the compression curve determines DPD model complexity

Dynamic nonlinear distortions where the amplifier's current output depends on past signal envelope values, not just the instantaneous input. These effects cause the AM-AM characteristic to become frequency-dependent and hysteretic.

• Thermal memory: Slow gain changes from self-heating with millisecond time constants
• Electrical memory: Bias network impedance variations at envelope frequencies
• Trapping effects: Charge capture/release in semiconductor materials (gate lag, drain lag)
• Requires memory polynomial or Volterra-based DPD models rather than static LUTs

The fundamental design conflict where biasing a power amplifier for high linearity inherently reduces its DC-to-RF conversion efficiency. This trade-off is the primary motivation for using digital predistortion to correct AM-AM distortion.

• Class-A amplifiers: Excellent linearity, maximum 50% theoretical efficiency
• Class-AB amplifiers: Moderate linearity, 50-78.5% theoretical efficiency
• Class-C amplifiers: Poor linearity, high efficiency approaching 100%
• DPD enables operating in efficient but nonlinear bias regions while meeting spectral mask requirements

The primary regulatory compliance metric directly degraded by AM-AM distortion. ACLR measures the ratio of transmitted power within the assigned channel to power leaking into adjacent frequency channels due to spectral regrowth from amplifier nonlinearity.

• Typical 3GPP requirement: -45 dBc for LTE/NR base stations
• AM-AM distortion creates odd-order intermodulation products that spread the spectrum
• DPD must suppress ACLR by 15-25 dB to meet specifications
• Measured with modulated test signals, not single-tone CW

The measurement process of mapping the amplifier's instantaneous output amplitude versus input amplitude to create the static nonlinearity profile used for DPD model training. This characteristic is typically extracted from complex baseband IQ data.

• Requires synchronized capture of input and output waveforms
• Extracted using least-squares fitting or moving-average binning of measured samples
• The characteristic is the basis for Look-Up Table (LUT) predistorter indexing
• Must account for time alignment to avoid smearing the AM-AM curve