This approach limits the increase in voltage output for the main amplifier. It also limits clipping or increased distortion at higher voltage levels. Matching is required at the input of both the main and peaking amplifiers. The output matching should be designed using sophisticated load-pull techniques or highly refined models. For Doherty design cases, in which the peak amplifier is larger than the main amplifier, additional signal power should be delivered to the peak amplifier.
This approach improves efficiency. Commercially available modules, which drive Doherty PAs, enable control of the amplitude and phase of each amplifier in the configuration. Beyond Asymmetric Doherty typologies, there are other multi-amplifier typologies that can increase efficiency beyond the capability of a single-amplifier architecture.
Using this stack topology, half of the drain voltage is dropped across the top device with a DC-elevated source and gate voltage. Care must be taken to choke the RF in the shared bias between the upper and lower devices, as they are out of phase when quadrature-balanced.
Input and output impedance matching is another critical stage in the design of a PA, as the bandwidth, frequency, and load impedances all dictate the specification requirements for the matching circuitry. GaN devices benefit from greater efficiency in these stages, which is partially due to the decreased current through the matching circuitry.
This, in turn, reduces current and resistive losses through the conductors of the matching elements. However, adapting matching circuitry for GaN devices does become more complicated as a function of the effects of increased voltage swing, bandwidth, temperature, and low impedance on the output of a GaN PA. Load-pull and source-pull techniques are a common method for evaluating the impedance performance parameters of a PA design.
They also enable an effective description of what impedance matching specifications may be required in real-world conditions. We also use nonlinear simulation tools for matching synthesis, depending on the accuracy and robustness of the nonlinear device model.
Advanced Power Amplifier Techniques to Enable Highly Efficient RF Transmitter Design
The behavior of transistors and fundamental circuit elements changes with temperature, packaging, and the materials used. As a result, the fabrication of a device come into play significantly. Virtually all device characteristics—breakdown voltages, leakage currents, transconductance, gain, PAE, etc. As PAs shrink in size and power efficiency, thermal performance is becoming a more critical feature.
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