13 Feb 2017
Adapting the Doherty architecture to improve RF power amplifier performance
Jawad H Qureshi, Walter Sneijers, John Gajadharsing of Ampleon look at ways of improving Doherty amplifier performance.
The Doherty architecture has been in use for decades to improve the efficiency of RF power amplifiers (PA), and it is now commonly employed in high-power mobile base-stations. The architecture does have drawbacks, though, and one of them is that its bandwidth can be limited. Recent modifications to the basic Doherty PA, DPA, architecture have overcome this limitation, making it more useful for applications such as broadcast TV transmitters.
The improvement in the efficiency and bandwidth of the modified Doherty architecture comes with a chain of consequences for its use. It is more complex to integrate into circulator-less transmitters with a harmonic reflective filter, because it is a single-ended architecture and lacks second-harmonic traps. The modified architecture also needs a Doherty power combiner designed to work at the base impedance of the PA devices, which is difficult to do at the required power levels due to the size of the transmission lines needed.
Most high-power wideband class-AB PAs deal with these issues by using a push-pull topology, but this is difficult to implement for ultra-wideband DPAs because it requires a wideband balun with an electrical length of between 20 and 30 degrees at impedances down to 2Ω. One way to overcome this issue is to combine the function of a quarter-wave transmission line with the balun. Although the performance of the resultant DPA is impressive, it cannot amplify the full UHF band due to the difficulty of making a good wideband balun.
We’re trying to address this chain of consequences and their related trade-offs by using an odd-mode wideband Doherty structure that integrates the balun and the wideband Doherty combiner, so providing the necessary bandwidth and second-harmonic signal suppression. Our prototype offers more than 40% average efficiency across most of the UHF broadcast frequency range (from 470MHz to 810MHz) at an average output power of 220W, and a peak output of more than 1.4kW. The PA is also compatible with digital pre-distortion techniques.
Designing an odd-mode Doherty combiner
Here’s a simplified schematic of the wideband combiner (Figure 1).
Figure 1: Schematic of the odd-mode Doherty combiner (Source: Ampleon)
Most of the circuit is built using broadside-coupled transmission lines, with an ideal wideband balun at the output. Broadside-coupled lines can be designed for almost any impedance, and offer very high common-mode impedances if the ground plane is far from them, compared to the thickness of the substrate. Broadside-coupled lines also offer much lower characteristic impedances than similarly sized micro-strip transmission lines, due to the higher capacitances between the transmission-line conductors.
Circuit analysis can be simplified if evaluated separately for odd (differential) and even (common) mode conditions. If the inputs of the main and peak transistor pairs are driven by differential signals, the odd-mode analysis of the circuit represents its response to the fundamental signals. In contrast, the even-mode analysis represents its response to second-harmonic signals.
For the odd-mode analysis, the transistor inputs are excited differentially and the circuit is analyzed for fundamental frequencies from 470MHz to 810MHz. Under these conditions, all the broadside-coupled transmission lines act like micro-strips and the circuit becomes like two Doherty amplifiers, excited differentially and combined using a balun (see Figure 2). This results in a very wideband performance in both back-off and full power conditions.
Figure 2: The equivalent circuit of the odd-mode Doherty (Source: Ampleon)
To analyze the circuit for second harmonic currents, the input signals to the main and peaking transistor pairs are assumed to be in phase. The circuit is then analyzed for second harmonic frequencies from 900MHz to 1900MHz.
Under these conditions all the broadside-coupled transmission lines, in combination with the output balun, present open-circuit conditions and the circuit is simplified to that shown in Figure 3, with its associated frequency response plot.
Figure 3: Even-mode equivalent circuit of odd-mode Doherty and its response to second-harmonic frequencies (Source: Ampleon)
Output balun for Doherty amplifier
Our modified Doherty circuit’s operation relies upon a very wideband planar balun that can transform the circuit’s output impedance to 50Ω. For our 1.4kW wideband UHF DPA prototype, the required fractional bandwidth is close to 55% and the required impedance transformation ratio is more than 20. To meet these specifications, we exploited the transmission-line characteristics of a commonly used planar balun to create a wideband multi-section impedance transformer, as shown in Figure 4.
Figure 4: Output impedance transformer and balun (Source: Ampleon)
This structure acts as a balun and impedance transformer, with a transformation ratio of 20, over more than 55% of the fractional bandwidth. The structure’s input reflection is less than -25dB and its loss is less than 0.5dB.
Building a prototype
To demonstrate the odd-mode Doherty architecture, we built a prototype for UHF broadcast TV applications using Ampleon’s BLF888A/B LDMOS devices.
The use of broadside-coupled transmission lines complicates the layout, so we used a multi-layer PCB, as seen in Figure 5, with the circuitry on its inner layers shaded. The PCB is mounted on a base-plate with air cavities, to provide high common-mode impedances to the coupled lines.
Figure 5: The layout of the odd-mode Doherty prototype (Source: Ampleon)
Measuring the prototype Doherty amplifier
We measured the performance of the prototype using a commercial DVB-T exciter and the results are shown in Figure 6.
Figure 6: The measured average efficiency, gain and output PAR at 220W output power (Source: Ampleon)
The input signal of the DPA was a DVB-T signal with a peak to average ratio (PAR) of 9.5dB. The measurements were performed at an output power of 220W, so that the output PAR was always more than 8dB. The prototype was more than 40% efficient across most of the band.
The same DVB-T exciter was used to pre-distort the output of the DPA, showing that the prototype can be used with pre-distortion to achieve the necessary adjacent channel leakage power ratio, a measure of the amplifier’s ability to restrict its output to the intended channel.
The Doherty architecture combines at least two PAs, each tuned to be most efficient at a different power level, to achieve good efficiency at the wide range of instantaneous power levels demanded by complex modulation schemes such as DVB-T.
Such architectures can be very efficient, but this is usually at the cost of having to manage complex trade-offs elsewhere in the design, such as in the combining circuitry or in the circuitry that couples the output of the PAs to the load.
In this differential Doherty architecture, we’ve produced good wideband efficiency, as well as controlling the wideband harmonics that would otherwise make it more difficult to apply the design to transmitters that don’t use circulators.
This approach could be applied to asymmetric wideband DPAs, and to three-way DPAs. It can also be thought of as two ultra-wideband DPAs operating on the top and bottom of the same substrate in differential mode, thereby packing twice as much power into the same area while reducing the volume of the transmitter.
Page 1 of 1
About the author
Jawad H Qureshi is a Principal RF Engineer with Ampleon, Nijmegen, The Netherlands. Jawad has an BSc in Electrical Engineering from the University of Engineering & Technology, Taxila, India, a Masters in Microwave Engineering and Phd in Microwave Circuit Design from Delft University of Technology.
Walter Sneijers is a Principal Applications Engineer with Ampleon, Nijmegen, The Netherlands. Walter has a BSc in Electronics and Communication Engineering from Breda University, The Netherlands.
John Gajadharsing - Senior Director, Architecture and Application at Ampleon, Nijmegen, The Netherlands.
Created in 2015, Ampleon is shaped by 50 years of RF power leadership. Recently being spun-off from NXP Semiconductors, the company is set-out to exploit the full potential of data and energy transfer in RF. Ampleon has more than 1,250 employees worldwide, dedicated to creating optimal value for customers. Its innovative, yet consistent portfolio offers products and solutions for a wide range of applications, such as cellular base stations, radio/TV/broadcasting, radar, air traffic control, cooking, lighting, industrial lasers and medical.
Most popular articles in RF topics
Share this page
Want more like this? Register for our newsletter