Q-factor Control Research Articles

Quality factor control of mechanical resonators using variable phononic bandgap on periodic microstructures

The quality factor (Q-factor) is an important parameter for mechanical resonant sensors, and the optimal values depend on its application. Therefore, Q-factor control is essential for microelectromechanical systems (MEMS). Conventional methods have some restrictions, such as additional and complicated equipment or nanoscale dimensions; thus, structural methods are one of the reasonable solutions for simplifying the system. In this study, we demonstrate Q-factor control using a variable phononic bandgap by changing the length of the periodic microstructure. For this, silicon microstructure is used because it has both periodicity and a spring structure. The bandgap change is experimentally confirmed by measuring the Q-factors of mechanical resonators with different resonant frequencies. The bandgap range varies depending on the extended structure length, followed by a change in the Q-factor value. In addition, the effects of the periodic structure on the Q-factor enhancement and the influence of stress on the structural length were evaluated. Although microstructures can improve the Q-factors irrespective of periodicity; the result of the periodic microstructure is found to be efficient. The proposed method is feasible as the novel Q-factor control technique has good compatibility with conventional MEMS.

Graphene perfect absorber with loss adaptive Q-factor control function enabled by quasi-bound states in the continuum

Numerous device structures have been proposed for perfect absorption in monolayer graphene under single-sided illumination, all of which requires the critical coupling condition, i.e., the balance between the loss of graphene and the leakage rate of the device. However, due to the difficulty of the precise control of the quality of synthesized graphene and unwanted doping in graphene transferred to the substrate, the loss of graphene is rather unpredictable, so that the perfect absorption is quite difficult to achieve in practice. To solve this problem, we designed a novel perfect absorber structure with a loss adaptive leakage rate control function enabled by the quasi-bound states in the continuum (BIC) and numerically demonstrated its performance. Our designed device is based on a slab-waveguide grating supporting both the quasi-BIC and the guided-mode resonance (GMR); the quasi-BIC with an adjustable leakage rate controlled by an incident angle is responsible for absorption, while the GMR works as an internal mirror. Since the proposed device scheme can have an arbitrarily small leakage rate, it can be used to implement a perfect absorber for any kind of ultrathin absorbing media. Due to the simple structure avoiding an external reflector, the device is easy to fabricate.

Hybrid cascode compensation with Q-factor control module for three-stage OTAs driving ultra-large load capacitors

PurposeThe purpose of this study is to discuss the effect of hybrid cascode compensation with quality factor (Q-factor) control module for the three-stage amplifiers driving ultra-large load capacitors. Compared to the present frequency compensation solutions, it extends the amplifier bandwidth by establishing an extra AC feedback pathway besides the primary pathway through the Miller capacitor, increasing the loop gain at the gain–bandwidth product (GBW) frequency by pushing to the higher frequencies the nondominant poles.Design/methodology/approachA Q-factor control block is used to improve the damping factor of the compensation loop with no power or area overhead, thereby reducing the frequency peaking and the undesired oscillation in the time response for small load capacitors. The Q-factor control module is realized by a tiny-size on-chip capacitor, and provides an extra feedback loop to feed the damping current back to the input stage. A left-half-plane (LHP) zero is also introduced to further improve the stability.FindingsA prototype of the proposed amplifier is simulated in 180-nm CMOS with a quiescent current of 24-µA from 1.80-V voltage supply. It achieves a 3.98-MHz gain–bandwidth product for 500-pF load capacitor, while the overall compensation capacitor is limited to 0.5-pF and the DC gain is extended beyond 100-dB.Originality/valueThe proposed amplifier is absolutely stable for the load capacitors ranging between 80-pF and 100-nF.

Control of quality factor in laterally coupled vertical cavities

In this work, the authors demonstrate the control of quality factor (Q-factor) is laterally coupled vertical cavities. A 3 μm-side-length square cavity directly connected to a 1.5 μm-width, 10 μm-length Fabry–Perot (FP) cavity has been fabricated and measured. Dynamic tuning of the Q-factor has been successfully observed by bringing the square cavity and FP cavity into resonance. The vertical-cavity-based Q-factor control scheme provides a new choice for the modulation of light and holds potentials for applications associated with optical information processing.

Design and Analysis of Three-Stage Amplifier for Driving pF-to-nF Capacitive Load Based on Local Q-Factor Control and Cascode Miller Compensation Techniques

This paper presents a new frequency compensation approach for three-stage amplifiers driving a pF-to-nF capacitive load. Thanks to the cascode Miller compensation, the non-dominant complex pole frequency is extended effectively, and the physical size of the compensation capacitors is also reduced. A local Q-factor control (LQC) loop is introduced to alter the Q-factor adaptively when loading capacitance CL varies significantly. This LQC loop decides how much damping current should be injected into the corresponding parasitic node to control the Q-factor of the complex-pole pair, which affects the frequency peak at the gain plot and the settling time of the proposed amplifier in the closed-loop step response. Additionally, a left-half-plane (LHP) zero is created to increase the phase margin and a feed-forward transconductance stage is paralleled to improve the slew rate (SR). Simulated in 0.13-µm CMOS technology, the amplifier is verified to handle a 4-pF-to-1.5-nF (375× drivability) capacitive load with at least 0.88-MHz gain-bandwidth (GBW) product and 42.3° phase margin (PM), while consuming 24.0-µW quiescent power at 1.0-V nominal supply voltage.

Q-factor control of a microcantilever by mechanical sideband excitation

We demonstrate the coupling between the fundamental and second flexural modes of a microcantilever. A mechanical analogue of cavity-optomechanics is then employed, where the mechanical cavity is formed by the second vibrational mode of the same cantilever, coupled to the fundamental mode via the geometric nonlinearity. By exciting the cantilever at the sum and difference frequencies between fundamental and second flexural modes, the motion of the fundamental mode of the cantilever is damped and amplified. This concept makes it possible to enhance or suppress the Q-factor over a wide range.