Backgrounds and Motivations

The fifth generation (5G) wireless communication has been the focus of interests in recent years. Compared to the current fourth generation long-term evolution (4G-LTE) configuration, 5G promises at least a tenfold increase in data rate, reaching the multi-Gb/s levels [1]-[2]. Higher carrier frequency bands, e.g., 28 GHz, 38 GHz, 60 GHz, etc., stood out as potential candidates as the required available bandwidth increase beyond 1 GHz. Under the same fractional bandwidth (FBW), a wider available bandwidth comes with having a higher carrier frequency. Furthermore, higher frequency bands lead to other benefits such as smaller physical footprints for the antenna and circuit components. The high free space loss and extra loss due to the oxygen absorbance peak at the 60-GHz band limit its applications to short-range, multi-Gb/s, device-to-device communications.

The 38-GHz band is a valid candidate for 5G communication receiver applications.

As often the first component after the receiving antenna, the gain and noise figure performances of the LNA dominate the signal-to-noise ratio (SNR) of the overall receiver. III-V compound semiconductor technologies are often used in LNA designs for the high gain and low noise performances. GaAs-based HEMT has been a stable and mature technology for microwave applications with relatively low cost for the performance it provides. LNA designs using GaAs-based HEMT processes have been reported for satellite communications and radio astronomy applications [3]-[8], in which high sensitivity and good noise performance of the receiver are required. In chapter 2, a Q-band LNA with source degeneration and RC-feedback techniques for a balanced gain, low noise, and wideband performance in 0.15-μm GaAs pHEMT will be introduced.

In recent years, the demand for high data rates in wireless communications has brought increasing interests towards millimeter-wave frequencies. The 60-GHz band has stood out thanks to its wide unlicensed band. A wide available bandwidth means that multi-Gb/s communication is

achievable even under the most basic, spectrally inefficient modulation schemes such as on-off keying (OOK). This result in a transceiver system with low complexity and DC power consumption, which is well suited for the short-range, multi-Gb/s communication applications targeted at mobile devices [21]-[28].

Switching-amplifier configuration of an OOK modulator operates by switching the output amplifier on/off in accordance with the baseband data signal [21]-[23], [32]-[33]. Modulation and amplification are performed by the same circuit, which only consumes DC power at on-states. This result in a transmitter of low DC power consumption and complexity. Critical performances of switching-amplifier configuration modulators are output power, gain, on-off isolation, and maximum data rate. Cascode circuit is commonly used for the switching-amplifier configuration.

Modulation is performed by switching the gate bias of the common-gate device. However, due to leakage of the devices, cascode-based modulators often suffers from low on-off isolation. For OOK transmitters, higher SNR at the receiver end can be achieved by either increasing the output power, or minimizing leakage at off-state while maintaining gain at on-state, i.e., increasing on-off isolation.

With the former often leads to high DC power consumption, improvement in on-off isolation is commonly pursued [22]-[28], [32]-[36]. In chapter 3, a 60-GHz cascode-based OOK modulator design with transformer feedback technique in 90-nm CMOS will be introduced. The technique improves the output power, gain performances at on-state, and isolation performance at off-state.

Wireless communications in millimeter-wave frequencies gain many interests in recent years due to the large available bandwidth and therefore higher data rates [39]-[47], [50]. Wireless systems designed at W-band benefit from having smaller circuit footprints and antenna sizes, with the short wavelength also provides potentials for imaging applications in area such as bio-medical research. Advanced nanoscale technologies such as 28, 40, and 65-nm CMOS processes have the advantage of higher gain and fmax/fT, which are especially critical in amplifier designs at millimeter-wave frequencies. However, shorter gate-lengths lead to lower breakdown voltages,

which in turn limit the supply voltage. This poses challenges to PA designs in achieving high levels of output power. Power combining is widely used for increasing the output power under low supply voltages [47]-[50]. Due to the short wavelength at millimeter-wave frequencies, the phase difference between combining paths becomes highly sensitive to any physical asymmetry of the power combining structure. Phase difference between combining paths degrades the combined power, and therefore the amplifier efficiency.

Binary power combining uses transmission lines for the purposes of both in-phase power combining and impedance matching. A wideband performance can often be achieved with careful design of the transmission lines [44]-[46], [50]. However, a large transformation ratio between the optimal impedance of the devices and the load impedance leads to longer transmission lines, and therefore higher insertion losses. Furthermore, transmission lines often take up large layout footprint, even at millimeter-wave frequencies. Transformer-based power combining has stood out in millimeter-wave PA designs recently [39], [48]-[49]. Transformers also serves the purposes of both in-phase power combining and impedance matching, but in a much more compact layout footprint than transmission lines. Since a large impedance transformation ratio does not necessarily translate to a large insertion loss of the transformer, the technique is especially suited for multi-way, high output power PA designs. In chapter 3, a W-band PA with a transformer-based, four-way, radial-symmetric power combining structure adopted at the output for low insertion loss and matching imbalances in 65-nm CMOS will be introduced.

1.2 Literature Surveys