Opportunity
Modern wireless communication systems, such as those employing high-order modulation formats with multiple sub-channels, are characterized by signals with a high peak-to-average power ratio (PAPR). This presents a significant challenge for power amplifiers (PAs) in base stations and transmitters, which must not only deliver high output power and efficiency at saturation but also maintain high efficiency at output power back-off (OBO) levels to conserve energy. While Doherty Power Amplifiers (DPAs) have been widely adopted due to their simplicity and ruggedness in handling high PAPR signals, their operational bandwidth is fundamentally limited by traditional design components like the drain-source capacitance, offset lines, and impedance transformers. To achieve broader bandwidths, conventional solutions often employ post-matching networks (PMNs). However, these PMNs, typically constructed from multi-section stepped-impedance lines or shunt stubs, inevitably increase the circuit's physical size and introduce higher insertion losses, which are particularly detrimental at high frequencies or in on-chip implementations. The industry's evolution towards denser, distributed small-cell base stations creates a pressing need for DPAs that are simultaneously broadband and physically compact, without the performance trade-offs associated with bulky PMNs.
Technology
This patent discloses a novel Doherty Power Amplifier architecture that achieves broadband performance without a post-matching network, resulting in a significantly more compact design. The core innovation lies in the use of specialized dual-mode impedance transformers (ITs) in both the carrier (main) and peaking (auxiliary) amplifier paths. Each dual-mode IT is engineered to convert the complex load-pull impedance of its respective transistor to the desired real load impedance at the amplifier's output simultaneously at two distinct frequencies. This dual-frequency operation inherently creates a broader usable bandwidth. The dual-mode IT structure can be implemented using transmission lines equivalent to an LC circuit featuring a first inductor, a third inductor, and a shunt LC tank. A key design feature is the integration of a short drain bias line, intentionally made shorter than a quarter-wavelength, which increases the impedance conversion ratio and improves bandwidth by reducing fluctuations in the transformed impedance. Furthermore, the architecture cleverly utilizes the existing circuit components to enable performance-enhancing harmonic injection without adding extra circuitry. Specifically, the shunt stubs from the dual-mode ITs in the carrier and peaking paths are placed adjacently to form a coupled-line bandpass filter. This filter acts as a bridge, selectively passing the second harmonic while blocking the fundamental frequency, facilitating an out-of-phase second harmonic injection between the two amplifier paths. This self-generated harmonic injection reshapes the voltage and current waveforms, particularly at saturation and higher operating frequencies, leading to improved output power and efficiency.
Advantages
- Eliminates the need for a bulky post-matching network (PMN), leading to a drastic reduction in circuit size and area.
- Achieves broad operational bandwidth (e.g., 2.1-3.1 GHz as demonstrated) comparable to traditional PMN-based DPAs.
- Maintains high efficiency both at saturation and at output power back-off (OBO) across the target bandwidth.
- Integrates a self-generated, out-of-phase second harmonic injection mechanism using existing circuit elements, enhancing high-frequency performance (output power and efficiency) without adding complex external circuitry.
- Reduces insertion loss by omitting the lossy PMN.
- Lowers overall fabrication cost due to the smaller circuit footprint and reduced component count.
- The dual-mode IT design, with its equivalent LC representation, facilitates easier adaptation for on-chip (monolithic microwave integrated circuit - MMIC) implementations.
- Well-suited for modern wireless infrastructure, particularly for compact, multi-standard small-cell and femto-cell base stations where size and cost are critical.
Applications
- Cellular base station transmitters, especially for 4G LTE and 5G networks.
- Small-cell, micro-cell, and femto-cell access points in dense urban or indoor environments.
- Broadband wireless communication systems requiring efficient amplification of high-PAPR signals.
- Microwave backhaul links.
- Satellite communication terminals.
- Software-defined radio (SDR) platforms.
- On-chip power amplifier designs for integrated wireless transceivers.
