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RF Electronics Chapter 3: Transformers and Hybrids Page 50 2022, C. J. Kikkert, James Cook University, ISBN 978-0-6486803-9-0. Chapter 3 Transformers and Hybrids Introduction At RF frequencies, Transformers are used to: 1 Invert signals by producing a 180-degree phase shift. 2 Change Impedances, to ensure that devices are matched correctly, thus ensuring that most of the power is transferred into or out of the devices. 3 Change balanced signals from a TV antenna to unbalanced signals for transmission using coaxial cables and connecting to a TV set. 4 Change unbalanced signals to balanced ones for use in mixers. 5 Provide DC isolation and permit DC and RF signals to be carried on the same coaxial cable for masthead amplifiers and other remote powered applications. RF Transformers differ from audio and 50 Hz mains transformers in that: 1 Ferrites are normally used for the magnetic material. 2 The windings may be an appreciable fraction of the wavelength. 3 Capacitive coupling between the primary and secondary windings and the self-capacitance of the windings must be taken into account. The capacitance between the windings, extends the frequency response, but at a 1:1 turns ratio. The self-capacitance of the windings causes resonances. 4 The transformers may need to operate over several decades of frequency. 5 At VHF frequencies and above, transmission lines may be used as transformers, without the use of ferrites. Such transformers operate over a relatively small frequency range. Hybrids are used to split or combine signals, while providing isolation between those signals. Figure 3.1 shows the commonly used symbol for a hybrid. The input signal is divided equally between Out1 and Out2. Out1 and Out2 are isolated from each other. This allows us to drive two amplifiers, thereby improving amplifier stability and increasing the output power handling capability. The power-handling capability of RF transistors operating up to 1 GHz can be as high as 100 Watt. Figure 3.2 shows how two such 100 Watt amplifiers, together with two hybrids can be used to produce a 200 Watt amplifier. This also gives some redundancy against failure of an amplifier. For an ideal hybrid, if one amplifier fails, the output will drop by 6 dB, with the power from the remaining amplifier being split between the antenna and the resistive load at the output hybrid, but the system still operates. For higher Figure 3.1. Hybrid symbol. RF Electronics: Design and Simulation 50 www.cadence.com/go/awr

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