Carrier Amplifier

The carrier amplifier is biased in a shallow Class-AB mode, representing a deliberate trade-off between linearity and efficiency. Unlike a deep Class-A bias, this quiescent current allows the device to handle the lower envelope of the signal with acceptable linearity while consuming less DC power.

• Quiescent Current: Typically set between 5-15% of the peak drain current.
• Conduction Angle: Operates for slightly more than 180 degrees of the RF cycle.
• Efficiency Profile: Achieves peak efficiency at the back-off point, not at saturation.
• Linearity: Provides the baseline linearity for the entire Doherty architecture before the peaking amplifier activates.

Unlike the peaking amplifier, which remains in cutoff during low-power operation, the carrier amplifier is always active. It is solely responsible for amplifying the signal envelope up to the transition point where the peaking amplifier turns on.

• Low-Power Region: Handles the entire signal below the transition threshold.
• Voltage Swing: Reaches its maximum voltage swing precisely at the transition point, defining the first efficiency peak.
• Thermal Stability: Experiences a more constant thermal profile than the peaking amplifier, though self-heating memory effects still require compensation.

The carrier amplifier is the primary beneficiary of the active load-pull effect generated by the Doherty combiner. As the peaking amplifier injects current, the impedance seen by the carrier is dynamically transformed from a high value (typically 2*Ropt) down to the optimal load (Ropt).

• Impedance Trajectory: Moves from 2*Ropt at low power to Ropt at peak power.
• Voltage Saturation: The load modulation forces the carrier into voltage saturation early, maintaining high efficiency.
• Current Linearity: The fundamental current of the carrier remains roughly linear with input drive, simplifying the behavioral model.

While generally more linear than a Class-C peaking stage, the carrier amplifier introduces significant soft compression and phase distortion that must be corrected by digital predistortion. The nonlinear capacitance (Cgs, Cgd) varies with input drive, causing AM-PM conversion.

• Soft Compression: Gradual gain reduction before hard saturation, characteristic of GaN HEMT devices.
• Input Impedance Modulation: The nonlinear input capacitance causes phase shift that varies with envelope power.
• Memory Effects: Thermal trapping and bias circuit interactions introduce short-term and long-term memory, requiring a memory polynomial or Volterra-based DPD model.

The carrier amplifier sees the output load through an impedance inverter, typically a quarter-wave transmission line. This network is critical for transforming the modulated load impedance and ensuring the carrier's efficiency peak aligns with the back-off power level.

• Quarter-Wave Transformer: Converts the high impedance at the combiner node to a low impedance at the carrier's drain.
• Phase Alignment: The electrical length of this path must be precisely calibrated to ensure constructive combining with the peaking branch.
• Bandwidth Limitation: The frequency-dependent nature of the quarter-wave inverter is a primary constraint on the bandwidth of the Doherty amplifier.

The carrier amplifier achieves its maximum drain efficiency not at the peak envelope power, but at the Output Back-Off (OBO) level corresponding to the transition point. This is the fundamental mechanism enabling high average efficiency for high-PAPR signals.

• First Efficiency Peak: Occurs typically at 6 dB OBO in a symmetric Doherty design.
• Voltage Waveform: At this point, the voltage waveform is a full half-sinusoid while the current is half of its peak, minimizing overlap loss.
• DPD Relevance: The predistorter must account for the rapid efficiency and gain slope change around this transition region.

The auxiliary amplifier in a Doherty configuration, typically biased in Class-C, that activates only during high signal envelope peaks. Its primary role is to inject additional current into the load network, enabling the active load-pull effect that modulates the impedance seen by the carrier amplifier. This complementary operation allows the carrier to maintain high efficiency at back-off while the peaking device handles peak power demands.

The dynamic impedance transformation mechanism central to Doherty operation. As the peaking amplifier turns on and injects current, the impedance inverter transforms the load presented to the carrier amplifier from a high value (typically 2Ropt) down to the optimal Ropt at saturation. This ensures the carrier operates near its maximum efficiency point across a wide range of output power back-off levels.

Amplitude-to-phase modulation distortion represents the nonlinear phase shift introduced by the carrier amplifier that varies with the instantaneous input envelope magnitude. In GaN HEMT-based designs, this phase distortion is particularly pronounced and interacts with the load modulation process, creating complex dynamic nonlinearities that must be compensated by digital predistortion to meet Error Vector Magnitude (EVM) requirements.

The power-added efficiency (PAE) of the carrier amplifier when operating at average power levels significantly below saturation. Modern signals with high Peak-to-Average Power Ratios (PAPR) force continuous operation at 6-10 dB back-off. The Doherty architecture's primary value proposition is maintaining high efficiency in this region through load modulation, directly reducing thermal dissipation and operational costs in base stations.

Dynamic nonlinear distortions where the carrier amplifier's current output depends on past signal values, not just the instantaneous input. These arise from:

• Self-heating effects: Channel temperature changes with dissipated power
• Trap effects: Slow charge trapping/de-trapping in GaN HEMT semiconductors
• Bias circuit dynamics: Low-frequency time constants in the supply network These long-term memory effects require sophisticated Volterra series or memory polynomial models for accurate linearization.

A two-port network, typically realized as a quarter-wave transmission line at the carrier amplifier's output, that performs the critical impedance transformation enabling load modulation. It presents the carrier with an inverted version of the common load impedance, converting the peaking amplifier's increasing current draw into a decreasing impedance seen by the carrier. This passive network is the defining architectural element of the Doherty topology.