Phase Alignment

The fundamental requirement that the carrier and peaking amplifier branches present identical phase delay from the input splitter to the output combiner reference plane. This ensures the peaking amplifier's current injection arrives in-phase with the carrier's output at the Doherty combiner node. Mismatch causes destructive interference, reducing combined output power and forcing the carrier amplifier to dissipate excess energy as heat. Precision is typically maintained within ±5 degrees of phase error across the operating bandwidth.

The input network, often a Wilkinson divider or hybrid coupler, must deliver signals to the carrier and peaking paths with a precise phase offset. In a symmetric Doherty, this is typically a 90-degree offset to compensate for the impedance inverter in the output path. Any deviation from this nominal phase relationship at the input directly translates to a phase error at the combiner, undermining the active load-pull mechanism. Active phase adjustment circuits are often integrated to calibrate out splitter imbalances.

Phase alignment is inherently frequency-dependent due to the dispersive nature of quarter-wave transformers and device parasitics. While alignment can be perfect at a single center frequency, group delay variations across the modulation bandwidth cause phase dispersion. This limits the instantaneous bandwidth of the Doherty amplifier. Broadband designs employ Klopfenstein tapers or multi-section matching networks to flatten the phase response and maintain constructive combining over wider channels.

Phase alignment is not a static condition; it interacts dynamically with the amplifier's inherent AM-PM distortion. As the peaking amplifier turns on and its input impedance changes, it can pull the phase of the preceding stages. This dynamic phase modulation adds to the static misalignment, creating a power-dependent combining error. Advanced digital predistortion models must capture this interaction to linearize the composite output effectively.

The physical layout of the Doherty combiner network must enforce geometric symmetry between the two paths. Asymmetric microstrip or stripline routing introduces unequal phase lengths. Even a 1 mm difference in a 30 GHz mmWave design can represent a significant fraction of a wavelength, causing severe phase error. Electromagnetic simulation of the entire output network is mandatory to verify phase balance before fabrication.

Phase alignment is verified and calibrated using a Vector Network Analyzer (VNA) to measure the transmission phase (S21) of each branch independently. The difference in unwrapped phase between the carrier and peaking paths is computed. Phase shifter components or transmission line lengths are then trimmed to null this difference. This calibration must be performed at the amplifier's nominal operating power level to account for hot S22 effects.