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RFIC PA Development for Communication and Radar Systems

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RFIC PA Development for Communication and Radar Systems 3 One of the most common methods to characterize a PA's usable maximum output power is with the 1dB compression point metric. The 1dB compression point, shown in Figure 2, describes the operating point at which a PA delivers gain that is exactly 1dB less than the gain it would otherwise deliver in its linear operating region. For example, if a PA delivers 18dB of gain in its linear region of operation, the 1dB compression point is defined as the output power at which the PA delivers exactly 17dB of gain. Figure 2: Input versus output power in a typical PA Efficiency is a measure of how well a device converts one energy source into another. Energy that doesn't get converted results in unwanted heat. In addition to reducing the amount of heat generated by the amplifier, power amplifier efficiency is a critical performance metric for preserving the battery in a mobile device and thereby extending its operating runtime. Efficiency is also important for high-power equipment, where the cost of the electric power over the lifetime of the equipment and the cost of the cooling systems can be significant compared to the purchase price of the equipment. There are several definitions used for describing RF amplifier efficiency, the most commonly used include drain efficiency and PAE. Drain efficiency, in the case of a field-effect transistor (FET), transistor, is the ratio of RF power delivered to the load versus DC drain bias (V d *I d ), (Equation 4). This measure of efficiency does not take into account the required drive power, which may be substantial for a PA. The PAE takes the RF power that is added to the device at its input into account and is expressed as: Maximum efficiency of a microwave device is a function of frequency, temperature, input drive level, load impedance, bias point, device geometry, and intrinsic device characteristics. Designing for optimum efficiency depends on these factors and usually involves some tradeoffs, which will be discussed later. A plot of amplifier gain, output power (dBm), and PAE as a function of swept-input power for a fixed frequency is shown in Figure 3. Figure 3: Amplifier gain, output power (dBm), and PAE as a function of swept-input power An amplifier must have enough bandwidth to suit the range of frequencies it is intended to amplify; too narrow a bandwidth will result in the loss of some signal frequencies. The future 5G-cellular standard includes the transmission of signals in the microwave range at 28GHz or 39GHz with bandwidths of several hundred MHz. In addition, 5G and LTE bands operating below 6GHz will also utilize carrier aggregation to enhance data rates, resulting in the need to support wide bandwidths at those frequencies as well.

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