## 3 High-Efficiency Wideband Envelope Amplifier
### Wideband Linear Envelope Amplifier  
To provide a WLEA with the required bandwidth for this application, an idea was proposed that should provide high slew rates and bandwidths, while being able to deliver a high output current. This is the use of RF HEMT GaN devices joined to operational amplifiers with high operating voltages and bandwidths in a DC-coupled common-drain arrangement. Specifically, this design uses the Texas Instruments THS3091 operational amplifier to drive four Macom NPT1004D GaN HEMTs connected in parallel. The THS3091 combines high-speed (6000 V/\(\upmu\)s) and wideband operation (305 MHz), with high output current drive capability (310 mA) from up to 32 V power supply. This allows the WLEA to operate in the supply voltage range required for the designed class-D VHF PA (0 V to 28 V), and its high slew rate and wideband operation together with its output current capability make it a reasonable choice for driving the equivalent gate  
Figure 8: Simplified diagram of the proposed EA circuit. capacitance (\(C_{GS}\)) of the four paralleled GaN HEMTs. The 8-pin SOIC package with a thermal pad, in which this device is packaged, improves heat dissipation, which makes possible its operation at high frequencies when driving capacitive loads. However, the main problem when using RF transistors in a common-drain linear power stage is their packaging, which is typically designed to provide heat dissipation through their source terminal, complicating the mechanical arrangement that satisfies both the common-drain scheme and the maintenance of a low-resistance thermal path. Accordingly, RF GaN transistors packaged in SOIC-8 have been preferred because of their small size and assembly flexibility. The chosen NPT1004D HEMTs can operate from DC to 4 GHz and have no integrated matching networks, making them ideal for this application. To remove the heat dissipated by these devices, the thermal pad intended for this purpose was not used, since it corresponds to the source of the transistor, which, in this case, is connected directly to the output of the EA, and doing so would complicate its mechanical assembly. For this reason, it was decided to capture the heat from the surface of the transistor package by means of a nonelectrically conductive thermal grease padding. This difficulty in removing heat from these devices has to do with the reason for using four transistors in parallel, which is to share the heat dissipated, allowing the transistors to operate within their temperature range. Simulations have been very helpful in determining the number of devices to parallel. Figure 9 shows the assembly diagram followed to achieve this. According to this assembly method, the WLEA prototype is built on a 0.8 mm double-sided FR4 printed circuit board to which an aluminum heatsink is attached. Some measurements were taken on this prototype to determine the performance this amplifier can provide for amplifying wideband envelope signals. To emulate the load that the class-D PA presents at its power port, a 6.8 \(\Omega\) power resistor was used in parallel with a 470 pF ATC capacitor. Considering the results obtained in Figure 10, this WLEA has proven to be stable for a 28 V amplitude and 200 ns pulse, simultaneously providing a high slew rate and power bandwidth. Figure 10: Measured performance of the linear GaN HEMT based WLEA for a paralleled 6.8 \(\Omega\) and 470 pF load: (**a**) output slew rate for a 200 ns pulse and (**b**) gain and phase frequency response. The red and blue circles and arrows indicate the y-axis to which the represented data belongs. Figure 9: Assembly diagram for heat-sinking SOIC-8 GaN HEMTs through their packages. Figure 11 shows the required WLEA dynamic performance, where the slew rate of the envelope (\(v_{E}\)) for a 2C-CH VDL-T signal, reaching a peak voltage of \(v_{Emax}\) = 28 V for carrier spacings of 5 MHz and 10 MHz, can be observed.