## 2 Class-D PA for Polar-Mode Architectures
### Class-D PA Demonstrator Description  
The class-D PA was implemented based on the simulations and principles described in the previous subsection. This implementation is based on transmission line transformers [23; 24; 25], and the transistor used is the CGHV40090 PP GaN on silicon carbide (SiC) HEMT from the semiconductor manufacturer Wolfspeed that contains two cells closely located in one package, and, consequently, it is especially suitable for push-pull circuits (due to its low  
Figure 5: Simulation results of the class-D PA (\(I_{DQ}\) = 2 A and \(P_{IN}\) = 1 \(W\)): (**a**) output envelope voltage and drain efficiency as a function of supply voltage and (**b**) output power flatness and drain efficiency performance in the operating frequency band.  
transistor-to-transistor inductance). This device is primarily intended for operation at higher frequencies than VHF, but it can be successfully operated at the VHF band because it does not contain any built-in matching networks, as long as it is properly stabilized. Its output equivalent capacitance is relatively low (\(C_{DS}\) = 6 pF per transistor cell), which allows reasonable implementation of class D at the VHF band for the power levels and voltages involved in this circuit. This low-capacitance figure, combined with the low inductance provided by the hybrid coupler (T2), is the key factor that contributes to achieving a high cutoff frequency for the amplifier power supply injection port (\(v_{E}\)). The hybrid coupler used in this amplifier is made of a 10 cm section of 25 \(\Omega\) Polytetrafluoroethylene (PTFE) coaxial (not loaded with ferrite) cable. This hybrid and the output balun are connected by the following circuits:  
* A symmetrical L-C matching network (LCMN) that is responsible for impedance transformation, consisting of a 28 pF total capacitance ATC 100B capacitors arrangement and two 40 mm sections of 25 \(\Omega\) PTFE coaxial cable.
* Two (one per branch) low-Q resonators (LC1 and LC2) that provide both DC blocking and reactance compensation functions to ensure that wideband operation are made with a 28 \(\mu\)H Coilcraft inductor and a 68 pF ATC 100B capacitor each.  
The output balun, which is responsible for providing a 25 \(\Omega\) symmetrical impedance to each branch, consists of a 300 mm section of 50 \(\Omega\) PTFE RG316 coaxial cable coiled (7 turns) on a 61-material ferrite core from Fair Rite.  
The input circuit of this amplifier is made of a balun formed by a 300 mm section of RG178 50 \(\Omega\) PTFE coaxial cable coiled (8 turns) on a 61-material toroidal core from Fair Rite. This balun is loaded at its output by four paralleled 100 Ohm SMD 1/4 W thick film resistors (providing an approximately 25 \(\Omega\) load) that are located across the transistor gates. These resistors are also used to stabilize the amplifier and to inject the negative DC voltage gate bias required by the transistor.  
It is important to note that the power supply port (\(v_{E}\)) to the output RF port (\(v_{O}\)) isolation provided by this amplifier strongly depends on the effectiveness of the balanced-to-unbalanced performance and bandwidth of their baluns; consequently, they must be carefully designed and experimentally adjusted.  
This prototype is built on a 0.8 mm thick FR4 two-sided laminate material with 1 oz. gold-nickel-plated copper layers. It is possible to use this inexpensive material for the amplifier PCB while keeping power losses low because most of the amplifier power is conducted by the coaxial cable transmission line transformers and circuit capacitors. This PCB assembly was mounted on an aluminum heatsink fitted with an embedded copper heat-spreader located below the transistor that is attached by means of screws. It did not require forced ventilation to operate at room temperature at full output power.