New-Tech Europe | July 2018

can, however, be evaluated at the 50Ω side (port 2) of the network as shown in Figure 2a. As a passive network, the output matching circuit had an operating power gain < 1, equal to its efficiency as determined by internal dissipative loss only. The necessarily smaller transducer gain was the product of this efficiency with the effect of loss due to reflection at the input. These quantities are presented as percentage efficiencies in Figure 2b. The transducer gain was evaluated for a generator whose impedance is the conjugate of the target load impedance to be seen by the device drain. Although the output was matched for compressed power and efficiency, not small reflection at the drain, the second factor was found to agree closely with the predicted reduction in compressed power due to imperfect realization of the target load impedance. Thus, the plotted transducer gain was a good measure of the overall quality of the output matching achieved. A further analysis (Figure 2b) of the load network using transducer power gain (GT) as a measure of load network mismatch loss between the transistor and the purely real 50Ω termination was also considered. An efficiency figure for the load network was calculated as 96.6 percent at 2800 MHz, with close correlation to the value calculated from the return loss at the same frequency. For comparative purposes the operational power gain (GP), which considers purely ohmic loss in the network, was also calculated to have an efficiency of 97.7 percent. Although this dissipation loss does not directly include reflection losses, its value does depend on the termination impedances as these affect the distribution of current and voltage within the network, and hence the copper and dielectric losses respectively.

Figure 2a: Load network loss and match as a function of frequency of the realized distributed load network.

Figure 2b: Transducer power gain (GT) as a function of frequency to express load network efficiency of the realized distributed load network. Operational power gain (GP) is shown for comparison.

Source Network Control of the source impedance variation over operating bandwidth was achieved through the use of a bandpass filter network, which also has the advantage of reducing low frequency gain, where the transistor’s inherent gain is very high. This particular source impedance matching network is also responsible for assisting with the amplifier’s low frequency stability. The impedance transformation ratio of about 15:1 needs a more elaborate network. In general, although not used here, matching networks with a deliberate

Achieving a broadband optimal match using this transistor was relatively straightforward for several reasons. Firstly, the transformation ratio is relatively low (about 2:1) over the operating bandwidth; secondly, the load impedance for optimal Pmax points were tightly packed, and thirdly, the optimal impedance varied with increasing frequency on a clockwise rotating locus. As commented above, the fairly low transformation ratio was a useful criterion favoring selection of this GaN device in a broadband RFPA application.

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