Rugged GaN low-noise amplifier MMIC to limit the output power under extreme conditions
Fig. 1: AlGaN/GaN output power limiting LNA MMIC chip.
Fig. 2: Measured output power as a function of input power of the output power limiting LNA MMIC (solid lines) and of the LNA MMIC (dashed lines) at 5 GHz and 6 GHz.
Fig. 3: Measured small-signal gain (S21) and noise figure (NF) of the output power limiting LNA MMIC (solid lines) and of the LNA MMIC (dashed lines) at Vds = 16 V and Ids = 50 mA.
Nowadays, gallium nitride (GaN) technology is well established in transceiver/receiver module front-end applications. The technology scores with high output powers in the transmit path combined with low noise, high linearity and ruggedness in the receive path.
Rugged GaN low-noise amplifiers (LNAs) commonly withstand pulsed input overdrive powers up to 43 dBm. Therefore, an input protection limiter is no longer required, allowing high-performance transceiver implementations on a single chip. For virtually all GaN LNAs, ruggedness is achieved by applying the gate bias voltage through a high ohmic resistor. However, this concept alone may not be sufficient to protect the subsequent stages of the receiver path, e.g., when realized in complementary metal-oxide-semiconductor (CMOS). In order to operate reliably under overdrive conditions, a GaN LNA would be required to provide attenuation in excess of around 20 dB, which is rarely addressed in literature.
The Brandenburg University of Technology Cottbus-Senftenberg (BTU) and the Ferdinand-Braun-Institut (FBH) have presented an adaptive drain DC supply. It switches the drain voltages to ground and an output attenuator becomes effective in case of overdrive condition [1]. This output power limited LNA concept was realized in a hybrid implementation, which leads to a reduced bandwidth of the small-signal gain.
The concept of an output attenuator, limiting the output power of a rugged GaN LNA in case of overdrive conditions is is implemented as a single-chip solution to improve bandwidth and reduce size [2]. The core of the output power limiting LNA MMIC is a rugged LNA MMIC based on AlGaN/GaN and an output attenuator realized by a GaN HEMT in common-gate configuration, see Fig. 1. This LNA MMIC requires no external control voltages and is completely independent due to the fact that only the existing gate supply voltage of the first amplifier stage switches the output attenuator.
The measurement results obtained for this output power limited circuit are compared to an LNA MMIC featuring the LNA alone, as shown in Fig. 2. The single chip implementation reveals that the concept is capable of achieving an attenuation up to 29 dB under overdrive, leaving noise figure and small-signal gain virtually unchanged, see Fig. 2 and 3, respectively.
Publications
[1] E. Kaule, C. Andrei, S. Gerlich, R. Doerner, and M. Rudolph, “Limiting the Output Power of Rugged GaN LNAs,” in 2019 49th European Microwave Conference (EuMC), Oct. 2019, pp. 794–796.
[2] E. Kaule, C. Andrei, and M. Rudolph, “Output Power Limited Rugged GaN LNA MMIC,” in 2021 16th European Microwave Integrated Circuits Conference (EuMIC), April 2022, pp. 133–135.