New-Tech Europe | July 2018

range, and power rating, other more detailed device data such as Cds, Cgs, and transformation ratio were also carefully considered. Optimal Load Impedance Extraction Once a device was selected and a nonlinear model obtained, the optimal source and load impedances were determined. The required load impedances necessary to achieve maximum power, maximum efficiency, gain, or an acceptable trade-off between these performance metrics are frequency dependent and vary substantially over the operating bandwidth of a broadband design. To determine the correct load impedance, a combination of load- pull plotting at the fundamental and harmonic frequencies, waveform engineering, or circuit design techniques based on shaping the transistor voltage and current waveforms, were performed in Microwave Office. It should be noted that using waveform engineering in determining any optimal impedance relies on having access to the intrinsic device nodes, in other words, across the intrinsic current generator of the device plane rather than at the package reference plane. Assuming the nonlinear model provides these nodes, then a waveform engineering approach enables the visual observation of voltage and current swing, clipping, and class of operation of the amplifier. For this example, the load pull simulation was run at Vds = +28V, Idq = 90 mA across the operating band and the optimal power and efficiency impedances were extracted with the mid-band results shown in Figure 1. A target load region based on the overlap between the Pmax -1 dB and drain efficiency max (effmax) – 5 percent contours was defined. Clearly, the larger this target area is, the easier the matching problem becomes. In

the complexity of the task increases beyond what is required for an average performing power amplifier design. However, in the case of a broadband amplifier, particularly one with high performance specifications, the realized network is required to control the impedance variation over a far larger fractional bandwidth. After defining optimal impedances and target areas, the load network was developed using a simplified real-frequency technique (SRFT) [3] to design the ideal lumped element network and then convert to distributed stepped impedance format [4] before performing electromagnetic (EM) simulation on the network. In this example, the EM results agreed closely with the predictions of circuit-based modelling, but for less conventional matching topologies this might not be the case. In general, EM simulation is seen as an important step in reducing uncertainty in the design flow. Onedesign technique is to represent the conjugate of the optimal impedance as that of a two-terminal generator (port 1), after which the matching network design can be viewed as a problem of reducing the mismatch loss that exists between this complex-valued load and a 50Ω termination over the amplifier’s operating bandwidth. This mismatch Figure 1: Fundamental load pull analysis from within Microwave Office showing contours at constant compression with Pmax ≥4 1dBm and Effmax ≥70 percent for min, mid, and max frequency points over the operating bandwidth. The boundary region is defined as the intersection of Pmax – 1 dB and Effmax – 5 percent, Zo reference = 50Ω

this case Pmax occurred on a tightly- packed, clockwise rotating locus over the operating bandwidth, which was helpful in the case of the broadband amplifier. Load pull was performed at the fundamental frequency due to the broadband nature of the RFPA and consequent difficulties in achieving the optimal harmonic terminations [1] without using TX zeros in the network [2]. Load pull at the second harmonic was also performed and a region of high efficiency identified [1] that could be controlled in the network synthesis. Network Synthesis Narrowband RFPAs have the advantage of showing little variation of the optimal load impedance over their operating bandwidth and hence the task of network design is somewhat less complex. This is not to say that a low fractional bandwidth match is always trivial. Indeed an investigation of source and load impedances will reveal that for very high performance, the network fundamental impedance must often be precisely controlled to a single gamma point with significant sub-optimal performance penalties if the network locus ‘misses’ its target load impedance. Morever, precise control of harmonic termination impedances for F and F-1 classes and

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