4.5: Loadpull
- Page ID
- 46069
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\(\newcommand{\avec}{\mathbf a}\) \(\newcommand{\bvec}{\mathbf b}\) \(\newcommand{\cvec}{\mathbf c}\) \(\newcommand{\dvec}{\mathbf d}\) \(\newcommand{\dtil}{\widetilde{\mathbf d}}\) \(\newcommand{\evec}{\mathbf e}\) \(\newcommand{\fvec}{\mathbf f}\) \(\newcommand{\nvec}{\mathbf n}\) \(\newcommand{\pvec}{\mathbf p}\) \(\newcommand{\qvec}{\mathbf q}\) \(\newcommand{\svec}{\mathbf s}\) \(\newcommand{\tvec}{\mathbf t}\) \(\newcommand{\uvec}{\mathbf u}\) \(\newcommand{\vvec}{\mathbf v}\) \(\newcommand{\wvec}{\mathbf w}\) \(\newcommand{\xvec}{\mathbf x}\) \(\newcommand{\yvec}{\mathbf y}\) \(\newcommand{\zvec}{\mathbf z}\) \(\newcommand{\rvec}{\mathbf r}\) \(\newcommand{\mvec}{\mathbf m}\) \(\newcommand{\zerovec}{\mathbf 0}\) \(\newcommand{\onevec}{\mathbf 1}\) \(\newcommand{\real}{\mathbb R}\) \(\newcommand{\twovec}[2]{\left[\begin{array}{r}#1 \\ #2 \end{array}\right]}\) \(\newcommand{\ctwovec}[2]{\left[\begin{array}{c}#1 \\ #2 \end{array}\right]}\) \(\newcommand{\threevec}[3]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \end{array}\right]}\) \(\newcommand{\cthreevec}[3]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \end{array}\right]}\) \(\newcommand{\fourvec}[4]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \\ #4 \end{array}\right]}\) \(\newcommand{\cfourvec}[4]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \\ #4 \end{array}\right]}\) \(\newcommand{\fivevec}[5]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \\ #4 \\ #5 \\ \end{array}\right]}\) \(\newcommand{\cfivevec}[5]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \\ #4 \\ #5 \\ \end{array}\right]}\) \(\newcommand{\mattwo}[4]{\left[\begin{array}{rr}#1 \amp #2 \\ #3 \amp #4 \\ \end{array}\right]}\) \(\newcommand{\laspan}[1]{\text{Span}\{#1\}}\) \(\newcommand{\bcal}{\cal B}\) \(\newcommand{\ccal}{\cal C}\) \(\newcommand{\scal}{\cal S}\) \(\newcommand{\wcal}{\cal W}\) \(\newcommand{\ecal}{\cal E}\) \(\newcommand{\coords}[2]{\left\{#1\right\}_{#2}}\) \(\newcommand{\gray}[1]{\color{gray}{#1}}\) \(\newcommand{\lgray}[1]{\color{lightgray}{#1}}\) \(\newcommand{\rank}{\operatorname{rank}}\) \(\newcommand{\row}{\text{Row}}\) \(\newcommand{\col}{\text{Col}}\) \(\renewcommand{\row}{\text{Row}}\) \(\newcommand{\nul}{\text{Nul}}\) \(\newcommand{\var}{\text{Var}}\) \(\newcommand{\corr}{\text{corr}}\) \(\newcommand{\len}[1]{\left|#1\right|}\) \(\newcommand{\bbar}{\overline{\bvec}}\) \(\newcommand{\bhat}{\widehat{\bvec}}\) \(\newcommand{\bperp}{\bvec^\perp}\) \(\newcommand{\xhat}{\widehat{\xvec}}\) \(\newcommand{\vhat}{\widehat{\vvec}}\) \(\newcommand{\uhat}{\widehat{\uvec}}\) \(\newcommand{\what}{\widehat{\wvec}}\) \(\newcommand{\Sighat}{\widehat{\Sigma}}\) \(\newcommand{\lt}{<}\) \(\newcommand{\gt}{>}\) \(\newcommand{\amp}{&}\) \(\definecolor{fillinmathshade}{gray}{0.9}\)Power amplifiers are designed beginning with an initial concept, detail designed using nonlinear circuit simulation tools, and finally optimized in the laboratory. However, computer-assisted design relies on models of components that can never capture all effects including, for example, thermal effects and parasitic coupling of components. A final experimental optimization for gain and efficiency while limiting distortion is nearly always required [36, 37, 38]. Once the design has been optimized in the laboratory, it is found that a design can be fixed for manufacturing. Minimal manual tuning of individual production units to peak performance is then usually all that is required. The most useful experimentally based design optimization approach uses the load-pull method. This technique uses computer-controlled variable input and output matching networks to search for the optimum input and output conditions. As well, performance with digitally modulated signals can be investigated, and this is very difficult to do in simulation.
A load-pull system is shown in Figure \(\PageIndex{1}\). The automated tuners realize variable input and output networks and are generally based on automated double-stub slabline tuners where the position of the stubs is also variable. The output impedance of the active device in a power amplifier is typically around a few ohms or less and it is often necessary to use an impedance transformer to scale the output impedance of the device up to the typically \(50\:\Omega\) system impedance of the automated tuners.
The load-pull system shown in Figure \(\PageIndex{1}\) is configured to measure the reflected power at the input of the amplifier (through the input
Figure \(\PageIndex{1}\): Load-pull system.
Figure \(\PageIndex{2}\): WiMAX amplifier block diagram showing the DC gate and drain currents.
directional coupler), the spectrum at the output of the amplifier (again through a directional coupler), and the output power (following a high-power attenuator). From the spectrum and power measurements, the gain, efficiency, output power, EVM, and spectral regrowth metrics are found. Many of these factors can be optimized [36] by systematically presenting impedances to the active device from a grid of possible input and output impedances. The tuners shown in Figure \(\PageIndex{1}\) provide controlled matching networks at the fundamental frequency. More elaborate systems can provide separate tuning at several harmonics. Microwave computer-aided design tools also support load-pull calculations for amplifiers.
The input automated tuner, the active device, the impedance transformer, and the output automated tuner become the amplifier. Once the optimum settings of the automated tuners are found for a range of frequencies, the design task then becomes realizing matching network with \(S\) parameters that correspond to the tuner settings.