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RF Electronics Chapter 3: Transformers and Hybrids Page 51 2022, C. J. Kikkert, James Cook University, ISBN 978-0-6486803-9-0. power output requirements, many of these modules can be combined in a similar manner. As a result, solid-state amplifier modules with output powers of several kW can be made. Figure 3.2. Parallel operation of amplifiers. At VHF frequencies the hybrids are normally made using transmission lines. At lower frequencies, the hybrids can be made using transformers. Most telephone handsets, operating at audio frequencies, have either a transformer hybrid or an active hybrid using an Application Specific Integrated Circuit, (ASIC). Wideband Transformers For a conventional ferrite transformer, the upper length of the winding is limited to approximately 10% of the wavelength. At 1 GHz the wavelength is 300 mm and the maximum winding length is thus 30 mm. If the windings are like a transmission line, as used in TV baluns shown in figures 3.7 and 3.8, this limit does not apply. The corresponding lower limit [1] is: ��� � � � ����� � �� ��� Eqn. 3.1 Where R l is load impedance in Ohm, f min is the lowest operating frequency of the transformer in MHz and l min is the length of the winding in metres. For a Neosid F14 material with a r of 220, operating at 1 MHz in a 50 system, the minimum winding length is thus 113 mm. This minimum winding length should be allowed for in the design of transformers made using ferrites. Figure 3.3. Simplified RF transformer model. Transformers can be designed using the above equations, however by considering the simple conventional transformer model [2] shown in Figure 3.3, containing a Leakage inductance Ls and a Magnetising inductance Lm, the optimum number of turns required for RF transformers can be easily determined. To simplify the calculations assume that the transformer has a 1:1 turns-ratio, so that the load R L is also the load reflected into the primary. For other turns ratios, the load impedance reflected into the primary, is the impedance used for calculations. RF Electronics: Design and Simulation 51 www.cadence.com/go/awr

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