Md. Ziaur Rahman Khan

A modified a priori SNR for speech enhancement using spectral subtraction rules

The letter addresses the problem of single-channel speech enhancement using a spectral subtraction method. The proposed approach is directed toward finding a self-adaptive averaging factor to estimate the a priori SNR. Performance of the modified averaging factor is evaluated using conventional spectral-subtraction algorithms in which the averaging factor is otherwise kept as a constant. Improved results are obtained in terms of speech quality measures for various types of noise and at different SNR levels when the proposed time-frequency varying averaging factor is adapted in the conventional subtraction rules.

Identification of noisy AR systems using damped sinusoidal model of autocorrelation function

This letter presents a novel method for minimum-phase autoregressive (AR) system identification at a very low SNR using damped sinusoidal model representation of the autocorrelation function of the noise-free AR signal with guaranteed stability. The new model parameters are estimated solely from the given noisy observations. Then AR parameters are obtained directly from the estimates of the damped sinusoidal model parameters. The simulation results show that the proposed method can estimate the AR system parameters with high accuracy even at an SNR as low as -5dB.

BLDC motor controller for Regenerative Braking

Design of a controller for Regenerative Braking in a Brushless DC (BLDC) Motor is presented. The patterns of the switching pulses of the BLDC motor are described in terms of Boolean equations. The controller circuit is designed using the developed Boolean equations. This circuit plays a major role in motoring and regenerative action on BLDC motors. Necessary simulation is performed in PSIM. A cost analysis for the construction of controller circuit is also included here. This circuit can be useful for electric vehicles.

Identification of autoregressive signals in colored noise using damped sinusoidal model

This work addresses a new method for autoregressive (AR) parameter estimation from colored noise-corrupted observations using a damped sinusoidal model for the autocorrelation function of the noise-free signal. The damped sinusoidal model parameters are first estimated using a least-squares based method from the given noisy observations. The AR parameters are then directly obtained from the damped sinusoidal model parameters. The performance of the proposed scheme is evaluated using numerical examples.

Determination of optical band gap of powder-form nanomaterials with improved accuracy

Abstract. Accurate determination of a material’s optical band gap lies in the precise measurement of its absorption coefficients, either from its absorbance via the Beer–Lambert law or diffuse reflectance spectrum via the Kubelka–Munk function. Absorption coefficients of powder-form nanomaterials calculated from absorbance spectrum do not match those calculated from diffuse reflectance spectrum, implying the inaccuracy of the traditional optical band gap measurement method for such samples. We have modified the Beer–Lambert law and the Kubelka–Munk function with proper approximations for powder-form nanomaterials. Applying the modified method for powder-form nanomaterial samples, both absorbance and diffuse reflectance spectra yield exactly the same absorption coefficients and therefore accurately determine the optical band gap.

Empirical expression for base transit time in bipolar transistors

A new and compact formula for the base transit time, τ b , of a modern high speed npn bipolar transistor with exponential base doping profile is derived considering doping dependence of mobility, bandgap narrowing effect, high injection effect and carrier velocity saturation at the base edge of the collector–base junction. The collector current density, J c , and minority carrier stored charge, Q b , in the base are separately expressed as a function of the injected electron density n o in the base in order to find an empirical expression for τ b . The modelling of J c , Q b and τ b is essential for the design of high-speed bipolar transistor. The expressions are applicable for arbitrary injection before the onset of the Kirk effect and they are simple and straight forward to give insight into device operation. The base transit time calculated analytically is compared with simulation results in order to demonstrate the validity of the assumptions made in deriving the expression. The closed form expressions for collector current density and base transit time offer a physical insight into device operation and are a useful tool in device design and optimization.

Sol–gel synthesis of DyCrO3 and 10% Fe-doped DyCrO3 nanoparticles with enhanced photocatalytic hydrogen production abilities

DyCrO3 and 10% Fe-doped DyCrO3 nanoparticles have been synthesized using a sol–gel method to investigate their performance in photocatalytic hydrogen production from water. The synthesized nanoparticles have been characterized by performing X-ray diffraction, energy dispersive X-ray spectroscopy and UV-visible spectrophotometric measurements. In addition, field emission scanning electron microscopy has been performed to observe their size and shape. The Fe-doped DyCrO3 nanoparticles show a significantly smaller band gap of 2.45 eV compared to the band gap of 2.82 eV shown by the DyCrO3 nanoparticles. The Fe-doped DyCrO3 nanoparticles show better photocatalytic activity in the degradation of rhodamine B (RhB) compared to the photocatalytic activity shown by both the DyCrO3 and Degussa P25 titania nanoparticles. The recycling and reuse of Fe-doped DyCrO3 four times for the photo-degradation of RhB shows that Fe-doped DyCrO3 is a stable and reusable photocatalyst. To evaluate the extent of the photocatalytic hydrogen production ability of the synthesized nanoparticles, a theoretical model has been developed to determine their “absorptance”, a measure of the ability to absorb photons. Finally, 10% Fe-doped DyCrO3 proves itself to be an efficient photocatalyst as it demonstrated three times greater hydrogen production than Degussa P25.

Comparison of lead acid and Li-ion battery in solar home system of Bangladesh

Different technical issues regarding the use of Li-ion and lead acid battery in solar photovoltaic system is presented in this study. Economic comparison for using these two types of batteries in solar home system (SHS) of Bangladesh is also presented in terms of net present cost (NPC). Li-ion batteries, especially LiFeSO4 batteries, are technically more advantageous for use in intermittent solar photovoltaic system than conventional lead acid battery. Fast charging rate, high energy density, extended cycle life, low maintenance are advantages of Li-ion batteries compared to lead acid batteries. At present main disadvantage of Li-ion batteries is its high capital cost. From HOMER (an micro-grid simulation software) it is found that instead of high cycle life and maximum depth of discharge (DOD) of Li-ion batteries than lead acid ones, NPC of SHS with Li-ion battery is higher than that of system with lead acid battery if project lifetime is less than 5 years. For project lifetime greater than 5 years, NPC of Li-ion based system is comparable to lead acid system. Moreover, for small size of systems using Li-ion batteries in SHS is economical than lead acid batteries. As system size increases, use of Li-ion battery in SHS becomes costlier than lead acid battery. Environmental effect of two considered types of batteries is also discussed here.

Microcontroller based control system for electric vehicle

This paper presents microcontroller based electric vehicle control system, especially for rickshaw. Switching pulses are generated by microcontroller for an inverter which drives the brushless DC motor in electric rickshaw. Pulses for controlling both motoring and regenerative braking actions are generated following the operating principles of brushless DC motor in these two modes. The proposed control system is very flexible and the results show that, the control system will be very effective if it is implemented in hardware.

Two-dimensional model of Dual Metal double gate tunnel FETs

Dual Metal double gate tunnel field-effect transistor (DMDG TFET) is a potential candidate for the next generation device fabrication. In this work a 2-D analytical model of the surface potential, electric field of DMDG TFET is developed by solving quasi-two-dimensional Poisson equation. These expressions can be numerically integrated to find the drain current. This model incorporates the effects of drain voltage, gate work function, gate length, gate dielectric thickness and silicon film thickness. The effectiveness of the proposed model is confirmed by a comparison with 2-D numerical simulations.