Broadband Doherty

The post-matching Doherty architecture places individual matching networks after the carrier and peaking transistors but before the combiner. This decouples the impedance matching from the load modulation network, significantly expanding the fractional bandwidth over which proper load modulation is maintained. Unlike conventional designs where the impedance inverter limits bandwidth, post-matching allows the combiner to operate over a wider frequency range while preserving the active load-pull effect.

Conventional quarter-wave impedance inverters are inherently narrowband. Broadband Doherty designs replace these with multi-section transmission line transformers or Klopfenstein tapers that provide near-constant impedance transformation across an octave or more. These wideband transformers maintain the critical 90-degree phase shift and impedance inversion required for load modulation without the frequency sensitivity of single-section lines, enabling consistent back-off efficiency across the entire operating band.

The defining characteristic of a broadband Doherty is the maintenance of consistent load modulation across frequency. In narrowband designs, the impedance presented to the carrier amplifier at back-off deviates from the optimal value as frequency shifts, degrading efficiency. Broadband architectures employ techniques such as:

• Absorptive harmonic terminations that remain effective over wide bandwidths
• Compensation networks that flatten the frequency response of the combiner
• Asymmetric power splitting that adjusts the carrier-to-peaking drive ratio with frequency This ensures the carrier amplifier sees the correct modulated impedance for high-efficiency operation regardless of operating frequency within the band.

Maintaining precise phase alignment between the carrier and peaking paths across a wide frequency range is a critical challenge. Broadband Doherty designs incorporate phase-compensating networks and delay lines that equalize the electrical lengths of both branches. Without this, the outputs would not combine constructively at the combiner, causing:

• Reduced total output power
• Degraded power-added efficiency
• Increased AM-PM distortion requiring more aggressive digital predistortion Advanced designs use lumped-element phase shifters that maintain the required 90-degree relative phase over the full band.

Gallium Nitride High Electron Mobility Transistors are the preferred active devices for broadband Doherty amplifiers due to their inherent wideband characteristics:

• Low parasitic capacitances that simplify broadband matching
• High output impedance that reduces the transformation ratio burden on the combiner
• Low knee voltage enabling high efficiency across wide bandwidths
• Reduced thermal memory effects compared to LDMOS, simplifying linearization The combination of GaN HEMT technology with post-matching architectures enables single-amplifier designs covering multiple 5G frequency bands simultaneously.

Broadband Doherty amplifiers exhibit frequency-dependent nonlinear behavior that complicates digital predistortion. The memory effects, AM-AM, and AM-PM distortion profiles change across the operating band, requiring:

• Wideband DPD models with sufficient bandwidth to capture all distortion products
• Multi-dimensional look-up tables indexed by both instantaneous power and frequency
• Adaptive coefficient tracking that updates predistorter parameters as the operating channel changes
• High-speed feedback receivers with bandwidth exceeding 5x the signal bandwidth The linearization system must compensate for both the inherent PA nonlinearity and the frequency-dependent variations in the Doherty combiner's load modulation behavior.

A topology where individual matching networks are placed after the carrier and peaking transistors, before the combiner. This decouples the impedance matching from the load modulation network, significantly simplifying the impedance inverter design and extending the achievable fractional bandwidth beyond conventional Doherty limits. The post-matching network absorbs parasitic device capacitances, enabling consistent load modulation across a wide continuous frequency range.

The critical two-port network that transforms the load impedance to its inverse value for the carrier amplifier. In broadband designs, the traditional single-section quarter-wave transformer is replaced with multi-section or tapered transmission line structures. These maintain a consistent 90-degree phase shift and impedance transformation ratio over a wide bandwidth, ensuring that the active load-pull effect remains effective and the efficiency peak stays centered at the target output back-off.

The intentional presentation of controlled impedances at the second and third harmonic frequencies to the transistor's intrinsic current source. In broadband Doherty designs, Class-F or Class-J harmonic terminations shape the voltage and current waveforms to minimize overlap and reduce dissipated power. Wideband harmonic control networks must present a short-circuit at the second harmonic and an open-circuit at the third harmonic across the entire operating band, a challenge addressed through multi-resonance matching structures.

Broadband Doherty amplifiers exhibit frequency-dependent AM-AM and AM-PM distortion along with complex memory effects that vary across the operating band. A single narrowband DPD model is insufficient. Wideband linearization requires:

• Generalized Memory Polynomial (GMP) models with multi-dimensional coefficient arrays
• Band-segmented DPD where separate predistorters are trained for sub-bands
• Real-time adaptation to track changes in self-heating and trap effects across frequency This ensures consistent ACLR and EVM compliance across the entire broadband signal.

A design where the peaking amplifier has a larger transistor periphery than the carrier amplifier, extending the high-efficiency output back-off range beyond 6 dB. This is critical for modern signals with high peak-to-average power ratios. In broadband implementations, the asymmetric current ratio must be maintained across frequency, requiring careful design of the input power splitter and phase alignment networks to ensure the correct drive ratio and phase alignment at the combiner over the entire band.

Gallium Nitride High Electron Mobility Transistors are the preferred technology for broadband Doherty amplifiers due to their high power density, low parasitic capacitance, and high operating voltage. Key advantages include:

• Lower knee voltage enabling wider voltage swing and higher efficiency
• Reduced soft compression characteristics that simplify linearization
• Superior thermal conductivity managing self-heating effects
• High output impedance simplifying broadband matching network design The intrinsic wideband capability of GaN devices directly translates to broader Doherty bandwidth.