The unique properties of high electron mobility GaN transistors (GaN-HEMTs) − high power density and high operating voltage despite small dimensions − make them excellent devices not only for RF power amplifiers. They are also very suitable as switches in fast DC/DC converters and modulators for supply modulation (SM) techniques. These include envelope tracking (ET), where the RF power amplifier (PA) supply voltage is varied according to its instantaneous signal envelope. However, the excellent charge transport properties lie in the two-dimensional electron gas (2DEG) established in the heterojunction between the AlGaN and the GaN layers. This 2DEG is only present in n-type GaN.
This makes modulator design for switch-based SM systems more difficult: the modulator with the fast switching GaN-HEMT is positioned on the high-voltage side, meaning that its source potential is continuously varying with the signal envelope. Hence, a galvanically isolated switch driver is required in the modulator, adding considerable parasitic capacitance to the switch. This reduces the possible switching frequency and thereby the achievable system bandwidth. In modern 5G telecommunication systems, this is detrimental since the bandwidth is very large and exceeds that of previous system generations by a factor of 10. A recently found workaround to overcome this shortcoming is to move the modulator to the low-voltage side and position the RF PA between the modulator and the high-voltage side.
This approach simplifies the modulator design since the source of the switching transistor is connected to system ground, allowing the gate to be switched towards this fixed ground potential. In addition, it makes the galvanic isolation of the switch driver obsolete, reduces parasitic capacitances, and thereby enables faster switching and larger modulation bandwidth.
A novel patented invention developed in the RF Power Lab at FBH is a packaged floating-ground RF power GaN-HEMT, facilitating RF PA designs for floating operation. To achieve this, a capacitor to bypass the RF to system ground is placed inside the transistor package close to the transistor chip. A separate external connection is added where the DC and LF ground potential is applied. This novel device allows straightforward floating-ground RF PA designs but also opens a whole new field of GaN-based circuits that benefit from the reverse operation. The main challenge is to achieve stability in the frequency region between the modulated LF band and the RF band where the PA system is operating. Devices using this topology are available as discrete packaged devices and as MMICs. Recent work has shown the feasibility to use these discrete devices in actual power amplifier designs. Focus has also been on improving and optimizing the in-package circuitry. The work shows very promising results for future reverse-type designs for space applications and 5G.
To position the RF PA on the high-voltage side means that its source potential is ‘floating’, i.e., it needs to be modulated in the DC and low-frequency (LF) domains, while system ground remains the RF ground. Hence, a ground separation for the different frequency domains is required at the transistor source. To assure broadband operation this separation has to be established as close as possible to the transistor with as little impact as possible on the RF performance.
