Power Amplifiers (PAs) are commonplace in Radio Frequency (RF) systems extensively used in applications consisting of over-the-air wireless communications. Wireless communications is a broad market and PAs fill a wide array of those applications. Due to the broad range of applications, PA design techniques must be chosen carefully for each application-specific scenario.
Single-ended PAs have limitations that restrict their practicality in many applications, especially where higher output power capability is required. For this reason, the hybrid coupled PA technique is an important power amplifier topology to understand for both PA designers and users. A traditional hybrid coupled amplifier topology is shown in Figure 1, where two amplifiers are operated in parallel utilizing 3-dB, 90° hybrid couplers. These types of amplifiers are called 90° hybrid coupled or quadrature-coupled amplifiers. The amplifiers are operated in quadrature, meaning they operate 90° out of phase in respect to the voltage component of the RF signal. Quadrature hybrid coupled PAs offer technical advantages over single-ended PAs, making them ideal for many wireless RF communications systems.
What is a Quadrature Hybrid Coupled Amplifier?
The 90° hybrid coupled amplifier technique utilizes 90° hybrid couplers and two amplifiers in a power-combining configuration for a 3-dB increase in output power capability. Hybrid couplers are 4-port passive devices that are configurable as signal splitters or signal combiners. A detailed block diagram is provided in Figure 2, which depicts the signal amplitudes and phases throughout the RF lineup.
The 3-dB, 90° hybrid coupler, as shown at the input (left) of Figure 2, can be used to split an input signal into two equal amplitude paths where the signal voltages are separated in phase by 90°. The fourth port is the isolation port that terminates Voltage Standing Wave Ratio (VSWR) reflections into a 50-ohm load resistor, maintaining a high isolation between each amplifier. The mismatch reflections from each amplifier are not seen by the other amplifier, thereby providing a 50-ohm match at the input port of the hybrid coupler. Mismatch reflections are both terminated into the isolation port and cancelled at the input port due to a 180° phase mismatch at the input.
After the signal is split and each signal is amplified by the amplifiers, a second 3-dB, 90° hybrid coupler is used as a signal combiner at the output. The output hybrid coupler re-combines the equal amplitude and 90° out-of-phase signals such that they combine in phase to produce a 3-dB increase in signal amplitude at the output port; for example, if each amplifier is operating at a saturated output power of +40 dBm (10 Watts) as shown in Figure 2, the two +40-dBm signals re-combine in phase to produce +43 dBm (20 Watts) of output power at the output port of the hybrid coupler.
A +3-dB increase in output power is realized with hybrid coupled amplifiers that would otherwise only be achievable by using higher-output-power devices when using a single-ended amplifier design. The 90° hybrid coupled amplifier and single-ended amplifiers both offer many tradeoffs to be carefully considered by the designer or user when selecting a PA technique for a specific application.
Table 1 provides a comparison of 90° hybrid coupled amplifiers and single-ended amplifiers. Noteworthy advantages of 90° hybrid PAs are the improved return loss and VSWR protection provided by the phase cancellation of reflected signals at the input port of the input coupler and the output port of the output coupler. A diagram is provided in Figure 3 that shows how the signal travels through the input coupler, providing improved return loss at the input port.
The input signal passes through the 90° hybrid coupler such that Path 1 is 0° phase-shifted and Path 2 is 90° phase-shifted. Mismatch reflections at the amplifier inputs are reflected back to the input coupler where the reflections from each amplifier become 180° out of phase and cancel at the input port. The reflected signal from Path 1 is 0° phase-shifted at the input port of the hybrid coupler, whereas the Path 2 reflected signal is 180° phase-shifted at the input port. At this point, Path 1 and Path 2 signals are 180° out of phase and will cancel out. This phase-cancelling benefit allows any prior circuitry in the RF chain — such as driver amplifiers, transceivers, and radios — to be well isolated from the reflections from Path 1 and Path 2 while being well matched into the 50-ohm termination of the 90° hybrid coupler. The result of this is an improved return loss. This minimizes large swings in gain ripple caused by poor impedance matching between the amplifier and any preceding RF circuitry.
At the isolation port, the reflections from the Path 1 and Path 2 amplifiers combine in phase but are isolated from each amplifier stage as the combined reflection is terminated into a 50-ohm load connected to the isolation port. This provides a high isolation between each amplifier, preventing each amplifier from seeing reflections from the other amplifier that would otherwise create additional ripple. The amplifier outputs are protected from high-VSWR conditions such as open and short when a 90° hybrid coupled amplifier technique is implemented. Similar to the input reflections, the output VSWR reflections are cancelled at the output port of the output hybrid coupler. The output reflections and phase cancelling results are illustrated in Figure 4.
Path 1 and Path 2 signals are 90° out of phase as they are amplified by each amplifier and re-combine at the hybrid coupler output port at a 90° phase shift from the originating input signal. Output reflections due to mismatch or open and short conditions reflect back through the hybrid coupler towards the PAs, undergoing an additional 90° phase change. As the output signal of 90° passes back through the hybrid coupler, Path 1 reflections are phase-shifted a total of 180° and Path 2 reflections remain at a total phase shift of 90° when compared to the original input signal. These reflections will pass back through the hybrid coupler once more as they move towards the output port and antenna, once again undergoing a 90° phase shift.
At the output port, the 180° Path 1 reflections are phase-shifted 90° such that Path 1 reflections are 270° total phase-shifted at the output port. Path 2 reflections remain unchanged at 90° passing through the 0° port of the hybrid coupler. At the output port, the Path 1 270° phase-shifted signal and the Path 2 90° phase-shifted signal cancel each other out as they are 180° out of phase. A great 50-ohms match is achieved at the output port for this reason. In cases where output VSWR are important, a hybrid coupled PA maintains a near 50-ohm VSWR without the need for large matching networks to transition the PA output impedance to 50 ohms. At the isolated port, the signals of Path 1 and Path 2 are in phase at 180° but are terminated into the 50-ohm load. These benefits allow very high output VSWR conditions to be tolerated including open and short load conditions without the inclusion of VSWR protection circuitry such as isolators, circulators, and VSWR detection.
Due to the 50-ohm match seen at the input port of the input coupler and the output port of the output coupler, the PA can be matched specifically for parameters such as maximum efficiency, output power, gain, and linearity. These parameters typically require tradeoffs with return loss and gain ripple but the inherent return loss improvement of hybrid couplers minimizes the need to balance these parameters. The input and output VSWR of the PA match can be intentionally poor to achieve a desired performance in efficiency, output power, gain, and linearity. As discussed previously, the saturated output power is increased by +3 dB in a hybrid coupled configuration. This characteristic yields further benefits by improving the P1dB and OIP3 by +3 dB compared to the single amplifier performance.
A hybrid coupled amplifier is a quick solution when improved output power, P1dB, OIP3, or linearity are required; for example, four commercial off-the-shelf (COTS) modules were hybrid coupled to achieve up to 200 Watts of RF output power. This significantly reduced the development time by reducing the need for large engineering design efforts for a custom higher-power solution that can be costly and timely. The amplifier design approach and measured output power results are presented in Figures 5 and 6, respectively.
All COTS items were utilized in this design so that no new engineering development time was spent designing a new PA. The costs to design a single amplifier to meet these requirements can be expensive, with longer development time due to the additional up-front testing and troubleshooting. A faster turnaround time from concept to production is easily realized with a hybrid coupled amplifier.
Improved thermal spreading occurs in hybrid coupled PAs. This benefits designers and end users by reducing heatsink requirements and maximizing individual device Mean Time Between Failure (MTBF). The amplifiers in a hybrid coupled design are spaced far enough apart that the thermal load at any given point is reduced. This increases the rate that heat can be removed with the heatsink, as heat is more evenly distributed. The size and weight of the design increases as amplifiers are added in a hybrid coupled approach. Design size begins to increase with hybrid coupled PAs due to the necessary inclusion of hybrid couplers, load resistors, additional PAs, and supporting bias circuitry.
A single-ended PA offers the smallest size but typically while sacrificing other parameters such as VSWR protection. The overall performance of a hybrid coupled PA is limited by the hybrid couplers. The bandwidth of a hybrid coupler will limit the overall achievable bandwidth of the PA. A hybrid coupler with poor amplitude balance, which is the difference in amplitude of Path 1 and Path 2 after the ideal 3-dB split, can cause unequal amplitude splitting that can cause each amplifier to be driven more or less into compression than the other amplifier.
Similarly, poor phase balance — the difference in phase at Path 1 and Path 2 compared to the ideal 90° phase shift — can cause poor phase combining and phase cancellation. This can reduce the overall output power if the signals do not combine in phase at the output port as well as minimizing the VSWR protection capability of a hybrid coupled PA. Furthermore, hybrid couplers are not lossless devices when realized and therefore, slightly reduce the output power by the loss of the hybrid coupler. Hybrid coupler losses are minimal — typically less than 0.5 dB — but should still be carefully considered when implementing, especially at high output power.
When considering an amplifier device or module, the designer and user must understand the tradeoffs between single-ended amplifiers and hybrid coupled amplifiers. To meet the ongoing demand for PA solutions, the PA design must be carefully considered. These decisions are often not straightforward, as requirements can begin to contest with each other; for instance, when size and VSWR protection are both requirements, the designer or user may want to adopt a single-ended amplifier to achieve size requirements. This would require the addition of an isolator or circulator to satisfy the requirement of VSWR protection. In these cases, a detailed analysis may be beneficial to determine if the inclusion of the isolator or circulator is smaller in size than the hybrid approach, which inherently provides VSWR protection.
Both amplifier techniques offer clear advantages for specific applications. The hybrid coupled amplifier technique is more quickly adaptable, especially for increased RF output power compared to single-ended amplifiers. The hybrid approach can utilize existing COTS amplifiers and hybrid couplers to quickly add amplifiers in hybrid configuration. When small size or low cost are critical design constraints, the single-ended amplifier is often the best technical solution. The 90° hybrid coupled amplifier technique is important to consider, especially considering the many advantages it provides.
This article was written by Justin Wells, RF Design Engineer at NuWaves Engineering, Middletown, OH. For more information, click here .