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RF Electronics Chapter 9: Impedance Matching of Power Amplifiers Page 332 2022, C. J. Kikkert, James Cook University, ISBN 978-0-6486803-9-0. Example 9.3: Broadband Amplifier Using Device Models Typical UHF TV transmitters cover a wide bandwidth from 470 – 790 MHz [7, 8]. To minimise the transmitter running cost, the power consumption needs to be minimised and the power amplifier needs to be very efficient in terms of the amount of DC power required to produce the required RF power. The Power Added Efficiency (PAE) is the ratio of the effective RF output power (P out – P in ) over the DC supply power and is a measure of how efficient the amplifier is at converting DC power into RF energy. The power dissipated in the amplifier is (1 – PAE) and that heat needs to be removed to keep the amplifier operating reliably. A high PAE is thus desirable. Just like audio amplifiers, for low RF frequencies, a Class A amplifiers can have a PAE up to 50% if transformer coupling is used. Class AB (push-pull) amplifiers can have a PAE up to 78%. An ideal Class C amplifier has a PAE of 100%. If maximum power transfer impedance matching is used, then the maximum PAE is half of those values. Equation 9.2 shows that maximum power matching is not always implemented in power amplifiers. At higher RF frequencies, the device losses tend to increase and lower PAE values result. Class C amplifier biasing is suitable for FM transmitters, which operate at a constant power. Modern modulation methods, like OFDM, used for broadcasting or cellular radio have a large output peak power to average power ratio and require a linear voltage (or power) gain that can be obtained using Class A or Class AB operation. These amplifiers can use a single transistor or two transistors in a push pull operation. For a Class A or Class AB amplifier the PAE is proportional to the output power up to a power level when saturation occurs. When the amplifier saturates, it generates harmonics that must be filtered out before transmission. Saturation also causes Intermodulation Distortion (IMD), which generally cannot be removed by filtering. Amplifier design is this a balance between power linearity, PEA, harmonics, saturation, bandwidth and amplifier stability. To estimate the PAE, amplifier saturation and IMD, a non-linear, power dependent model of the active device used is required. Most RF power device manufacturers now publish such nonlinear models for their devices. Figure 9.46. Basic Class AB power amplifier. The amplifier in this example uses Class AB push pull operation, since that result in a lower harmonic level. The device used is a CGH40090PP, 4 GHz, 28V, 60W, GaN on SiC device [9]. That device contains 2 FET's in the one device, providing a thermal match RF Electronics: Design and Simulation 332 www.cadence.com/go/awr

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