The permanent magnet synchronous machines have been attracting more and more attention due to the advantages of high torque density, outstanding efficiency and maturing technologies. Under the urges of mandatory energy efficiency requirements, they are considered as the most potential candidates to replace the comparatively low-efficient induction machines which dominate the industrial market. However, most of the high performance permanent magnet machines are based on high cost rare-earth materials. Thus, there will be huge demands for low-cost high-performance permanent magnet machines. Ferrite magnet is inexpensive and abundant in supply, and is considered as the most promising alternative to achieve the goal of low cost and high performance. In consideration of the low magnetic energy, this thesis explored the recent developments and possible ideas of ferrite machines, and proposed a novel multi-layer spoke-type interior permanent magnet configuration combining the advantages of flux focusing technique and multi-layer structure. With comparable material cost to induction machines, the proposed ferrite magnet design could deliver 27% higher power with 2-4% higher efficiency with exactly the same frame size. Based on the data base of International Energy Agency (IEA), electricity consumed by electric machines reached 7.1PWh in 2006 [1]. Considering that induction machines take up 90% of the overall industrial installation, the potential energy savings is enormous. This thesis contributes in five key aspects towards the investigation and design of low-cost high-performance ferrite permanent magnet machines. Firstly, accurate analytical models for the multi-layer configurations were developed with the consideration of spatial harmonics, and provided effective yet simple way for preliminary design. Secondly, the influence of key design parameters on performance of the multi-layer ferrite machines were comprehensively investigated, and optimal design could be carried out based on the insightful knowledge revealed. Thirdly, systematic investigation of the demagnetization mechanism was carried out, focusing on the three key factors: armature MMF, intrinsic coercivity and working temperature. Anti-demagnetization designs were presented accordingly to reduce the risk of performance degradation and guarantee the safe operation under various loading conditions. Then, comparative study was carried out with a commercial induction machine for verification of the superior performance of the proposed ferrite machine. Without loss of generality, the two machines had identical stator cores, same rotor diameter and stacking length. Under the operating condition of same stator copper loss, the results confirmed the superior performance of the ferrite machine in terms of torque density, power factor and efficiency. Lastly, mechanical design was discussed to reduce the cost of mass production, and the experimental effort on the prototype machine validates the advantageous performance as well as the analytical and FEA predictions.

All multimedia devices now incorporate video CODECs that comply with international video coding standards such as H.264 / MPEG4-AVC and the new High Efficiency Video Coding Standard (HEVC) otherwise known as H.265. Although the standard CODECs have been designed to include algorithms with optimal efficiency, large number of coding parameters can be used to fine tune their operation, within known constraints of for e.g., available computational power, bandwidth, consumer QoS requirements, etc. With large number of such parameters involved, determining which parameters will play a significant role in providing optimal quality of service within given constraints is a further challenge that needs to be met. Further how to select the values of the significant parameters so that the CODEC performs optimally under the given constraints is a further important question to be answered. This thesis proposes a framework that uses machine learning algorithms to model the performance of a video CODEC based on the significant coding parameters. Means of modelling both the Encoder and Decoder performance is proposed. We define objective functions that can be used to model the performance related properties of a CODEC, i.e., video quality, bit-rate and CPU time. We show that these objective functions can be practically utilised in video Encoder/Decoder designs, in particular in their performance optimisation within given operational and practical constraints. A Multi-objective Optimisation framework based on Genetic Algorithms is thus proposed to optimise the performance of a video codec. The framework is designed to jointly minimize the CPU Time, Bit-rate and to maximize the quality of the compressed video stream. The thesis presents the use of this framework in the performance modelling and multi-objective optimisation of the most widely used video coding standard in practice at present, H.264 and the latest video coding standard, H.265/HEVC. When a communication network is used to transmit video, performance related parameters of the communication channel will impact the end-to-end performance of the video CODEC. Network delays and packet loss will impact the quality of the video that is received at the decoder via the communication channel, i.e., even if a video CODEC is optimally configured network conditions will make the experience sub-optimal. Given the above the thesis proposes a design, integration and testing of a novel approach to simulating a wired network and the use of UDP protocol for the transmission of video data. This network is subsequently used to simulate the impact of packet loss and network delays on optimally coded video based on the framework previously proposed for the modelling and optimisation of video CODECs. The quality of received video under different levels of packet loss and network delay is simulated, concluding the impact on transmitted video based on their content and features.

A acne vulgaris é uma patologia cutânea crônica que acomete a unidade pilossebácea resultando em hiperqueratinização, aumento da produção de sebo, inflamação e proliferação bacteriana nos folículos pilosos. Diversos fármacos estão disponíveis para o tratamento da acne, entretanto a combinação de tratamentos tópicos, com diferentes mecanismos de ação, apresentam uma melhor eficácia. Desse modo, o presente trabalho visa o desenvolvimento de um sistema nanocarreador contendo adapaleno e dapsona no intuito de promover melhor seletividade, aumentando a eficiência do tratamento e minimizando efeitos adversos. Neste contexto, desenvolveu-se e caracterizou-se físico-quimicamente uma formulação de nanocápsulas de núcleo lipídico contendo adapaleno e dapsona. A suspensão contendo 0,025 % de adapaleno e 0,07 % de dapsona apresentou diâmetro médio de 194,9 ± 0,42 nm, índice de polidispersão de 0,12 ± 0,02, potencial zeta -15 ± 1,2 mV, pH 5,1 ± 0,1 e eficiência de encapsulação próximos a 100 %. A avaliação da morfologia por microscopia eletrônica de transmissão demonstrou partículas esféricas, homogêneas de dimensões nanométricas. Através da análise de retroespalhamento de luz dinâmico (Turbiscan®) não se detectou fenômenos de instabilidade na formulação. Uma nanoemulsão, similarmente preparada, mas sem o uso do polímero, foi desenvolvida para comparação a fim de estabelecer o mecanismo de encapsulação dos fármacos. As suspensões foram incorporadas em veículos semissólidos, hidrogéis de Carpobol® 940, e os seguintes estudos foram realizados: comportamento reológico, pH, perfil de liberação, potencial de irritação (HET-CAM) e permeação cutânea in vitro. A determinação quantitativa dos ativos foi realizada por cromatografia líquida de alta eficiência (CLAE), e os métodos analíticos foram desenvolvidos e validados de acordo com os códigos oficiais vigentes. As formulações apresentaram perfil não-Newtoniano pseudoplástico. No estudo de liberação in vitro, não houve diferença significativa entre as formulações contendo dapsona (p>0,05). Já para o adapaleno, não foi possível estabelecer o perfil de liberação in vitro por membrana de diálise. As formulações foram avaliadas como não irritantes pela técnica de HET-CAM. Através dos estudos de permeação cutânea foi possível quantificar os fármacos na epiderme e na derme, e quantidade não-significativa foi detectada no fluído receptor (p>0,05). Comparando as formulações de nanocápsula de núcleo lipídico e nanoemulsão, as quantidades de adapaleno e dapsona em cada camada da pele foi significativamente diferente (p<0,05). Em conclusão, os resultados obtidos até o presente momento demonstram que as nanocápsulas de núcleo lipídico são um sistema promissor para aplicação cutânea de adapaleno e dapsona. Contudo, estudos clínicos fazem-se necessários a fim de avaliar a efetividade da formulação no tratamento da acne. / Acne vulgaris is a chronic skin condition that affects the pilosebaceous unit resulting in hyperkeratinization, increased sebum production, inflammation and bacterial proliferation in hair follicles. Several drugs are available for treating acne, however the combination of topical treatments with different mechanisms of action presents greater effectiveness. Therefore, this study aims to develop a nanocarrier system containing adapalene and dapsone in order to promote better selectivity, increasing efficiency of treatment and minimizing adverse side effects. In this study, a suspension of lipid-core nanocapsules containing adapalene and dapsone was prepared by interfacial deposition of preformed polymer and characterized physicochemically. The suspension containing 0.025 % of adapalene and 0.07 % of dapsone showed an average diameter of 194.9 ± 0.42 nm, a polydispersity index of 0.12 ± 0.02, zeta potential -15 mV ± 1.2, pH 5.1 ± 0.1 and encapsulation efficiency approaching 100 %. The assessment of morphology by transmission electron microscopy showed spherical particles of homogeneous nanometric dimensions. No detectable instability phenomena in the formulation was observed by dynamic light scattering analysis (Turbiscan®). A nanoemulsion, similarly prepared but omitting the polymer, was developed to establish the mechanism of drug encapsulation. The suspensions were incorporated into semisolid vehicles, hydrogels of Carpobol® 940, and the following studies were performed: rheological behavior, pH, release profile, irritation potential (HET-CAM) and skin permeation in vitro. Drug quantification was carried out by high-performance liquid chromatography (HPLC) and analytical methods were developed and validated according to current official guidelines. The formulations exhibited non-Newtonian pseudoplastic profile. In the in vitro release study, no significant difference was found between the formulations containing dapsone (p>0.05). However, for adapalene, it was not possible to establish the in vitro release profile by dialysis membrane. The formulations were evaluated as non-irritant by HET-CAM technique. Through the skin permeation studies, it was possible to quantify the drug in epidermis and dermis, and non-significant quantity was detected in the receptor fluid (p>0.05). Comparing the lipid-core nanocapsules and nanoemulsion formulations, the amount of adapalene and dapsone in each skin layer was significantly different (p<0.05). In conclusion, the results obtained show that the lipid-core nanocapsules are a promising system for cutaneous application of adapalene and dapsone. However, clinical studies are required in order to evaluate the effectiveness of the formulation in the treatment of acne.

In this thesis, various issues regarding normally-ON silicon carbide (SiC)Junction Field-Effect Transistors (JFETs) are treated. Silicon carbide powersemiconductor devices are able to operate at higher switching frequencies,higher efficiencies, and higher temperatures compared to silicon counterparts.From a system perspective, these three advantages of silicon carbide can determinethe three possible design directions: high efficiency, high switchingfrequency, and high temperature.The structure designs of the commercially-available SiC power transistorsalong with a variety of macroscopic characteristics are presented. Apart fromthe common design and performance problems, each of these devices suffersfrom different issues and challenges which must be dealt with in order to pavethe way for mass production. Moreover, the expected characteristics of thefuture silicon carbide devices are briefly discussed. The presented investigationreveals that, from the system point-of-view, the normally-ON JFET isone of the most challenging silicon carbide devices. There are basically twoJFET designs which were proposed during the last years and they are bothconsidered.The state-of-the-art gate driver for normally-ON SiC JFETs, which wasproposed a few years ago is briefly described. Using this gate driver, theswitching performance of both Junction Field-Effect Transistor designs wasexperimentally investigated.Considering the current development state of the available normally-ONSiC JFETs, the only way to reach higher current rating is to parallel-connecteither single-chip discrete devices or to build multichip modules. Four deviceparameters as well as the stray inductances of the circuit layout might affectthe feasibility of parallel connection. The static and dynamic performance ofvarious combinations of parallel-connected normally-ON JFETs were experimentallyinvestigated using two different gate-driver configurations.A self-powered gate driver for normally-ON SiC JFETs, which is basicallya circuit solution to the “normally-ON problem” is also shown. This gatedriver is both able to turn OFF the shoot-through current during the startupprocess, while it also supplies the steady-state power to the gate-drivecircuit. From experiments, it has been shown that in a half-bridge converterconsisting of normally-ON SiC JFETs, the shoot-through current is turnedOFF within approximately 20 μs.Last but not least, the potential benefits of employing normally-ON SiCJFETs in future power electronics applications is also presented. In particular,it has been shown that using normally-ON JFETs efficiencies equal 99.8% and99.6% might be achieved for a 350 MW modular multilevel converter and a40 kVA three-phase two-level voltage source converter, respectively.Conclusions and suggestions for future work are given in the last chapterof this thesis. / I denna avhandling behandlas olika aspekter av normally–ON junction–field–effect–transistorer (JFETar) baserade på kiselkarbid (SiC). Effekthalvledarkomponenteri SiC kan arbeta vid högre switchfrekvens, högre verkningsgradoch högre temperatur än motsvarigheterna i kisel. Ur ett systemperspektivkan de tre nämnda fördelarna användas i omvandlarkonstruktionen för attuppnå antingen hög verkningsgrad, hög switchfrekvens eller hög temperaturtålighet.Såväl halvledarstrukturen som de makroskopiska egenskaperna för kommersiellttillgängliga SiC–transistorer presenteras. Bortsett från de vanligakonstruktions–och prestandaproblemen lider de olika komponenterna av ettantal tillkortakommanden som måste övervinnas för att bana väg för massproduktion.Även framtida SiC–komponenter diskuteras.Ur ett systemperspektiv är normally-ON JFETen en av de mest utmanandeSiC-komponenterna. De två varianter av denna komponent som varittillgängliga de senaste åren har båda avhandlats.State–of–the–art–drivdonet för normally-ON JFETar som presenteradesför några år sedan beskrivs i korthet. Med detta drivdon undersöks switchegenskapernaför båda JFET-typerna experimentellt.Vid beaktande av det aktuella utvecklingsstadiet av de tillgängliga normally–ON JFETarna i SiC, är det möjligt att uppnå höga märkströmmar endastom ett antal single–chip–komponenter parallellkopplas eller om multichipmodulerbyggs. Fyra komponentparametrar samt strö-induktanser för kretsenkan förutses påverka parallellkopplingen. De statiska och dynamiska egenskapernaför olika kombinationer av parallellkopplade normally-ON JFETarundersöks experimentellt med två olika gate–drivdonskonfigurationer.Ett självdrivande gate-drivdon för normally-ON JFETar presenteras också.Drivdonet är en kretslösning till “normally–ON–problemet”. Detta gatedrivdonkan både stänga av kortslutningsströmmen vid uppstart och tillhandahållaströmförsörjning vid normal drift. Med hjälp av en halvbrygga medkiselkarbidbaserade normally–ON JFETar har det visats att kortslutningsströmmenkan stängas av inom cirka 20 μs.Sist, men inte minst, presenteras de potentiella fördelarna med användningenav SiC-baserade normally-ON JFETar i framtida effektelektroniskatillämpningar. Speciellt visas att verkningsgrader av 99.8% respektive 99.5%kan uppnås i fallet av en 350 MW modular multilevel converter och i en40 kVA tvånivåväxelriktare. Sista kaplitet beskriver slutsatser och föreslagetframtida arbete. / <p>QC 20130527</p>

Chosen as one of six Generation‒IV nuclear-reactor concepts, SuperCritical Water-cooled Reactors (SCWRs) are expected to have high thermal efficiencies within the range of 45 ‒ 50% owing to reactor‘s high outlet temperatures. A generic pressure-channel (or pressure-tube)SCWR operates at a pressure of 25 MPa with inlet- and outlet-coolant temperatures of 350°C and 625°C. Consequently, the sheath and fuel centerline temperatures are higher in SCWRs than those of the current nuclear reactors. Previous studies have shown that the sheath and fuel centerline temperatures could exceed the design and industry accepted limits of 850°C and 1850°C, respectively. These studies correspond to UO2 enclosed in a 43-element fuel bundle at an average thermal power per channel of 8.5 MWth. Additionally, these high operating conditions in the range of 350 - 625°C lead to high heat losses from the coolant to the moderator, which in turn reduces the overall thermal efficiency of the Nuclear Power Plant (NPP). Therefore, there is a need for alternative fuels or fuel bundles for future use in SCWRs. Hence, it is also necessary to determine the amount of heat losses from a number of fuel-channel designs for SCWRs. The objectives of this study are to investigate the possibility of using alternative fuels and to determine the heat losses from a fuel-channel design at SCWR conditions. The investigated fuels are categorized as low thermal-conductivity (e.g., UO2, MOX, and ThO2), high thermal-conductivity (e.g., UC, UC2, UN), and enhanced thermal-conductivity (e.g., UO2‒SiC, UO2‒C, and UO2‒BeO) fuels. Additionally, the examined fuel channel is the High Efficiency Channel (HEC), which has been designed by the Atomic Energy of Canada Limited (AECL) for the proposed CANDU SCWR. In order to achieve the objectives of this study, a steady-state one-dimensional heat-transfer analysis was conducted. The MATLAB© and NIST REFPROP© software were used for programming and retrieving thermophysical properties of a light-water coolant, respectively. The fuel centerline temperature was calculated for the fuel channels with the maximum thermal power, i.e., +15% above average channel power. Results of this analysis showed that the fuel centerline temperatures of low thermal-conductivity fuels exceed the industry limit; therefore, either a fuel with a higher thermal conductivity should be used or the fuel bundle geometry must be modified. Among the high thermal-conductivity fuels, UC has been shown to be a candidate for future use in SCWRs. However, the chemical compatibility of UC with water at high operating temperatures of SCWRs remains ambiguous. Therefore, further studies are required before selecting UC. In regards to enhanced thermal-conductivity fuels, UO2‒BeO is the most suitable candidate; however, its mechanical and neutronic properties must be thoroughly studied before any decision is made with regards to the selection of a fuel. In regards to the heat losses from the examined fuel channel, the heat loss was between 70 kW and 110 kW per fuel channel based on an average thermal power per channel of 8.5 MWth and a moderator pressure of 0.1 MPa at 80°C. A sensitivity analysis of the fuel channel shows that the heat loss can be reduced by increasing the operating pressure of the moderator, which in turn allows for increasing the operating temperature of the moderator. Higher operating temperatures of the moderator result in smaller temperature differences between the coolant and the moderator, which leads to lower heat losses. Therefore, either the thickness of the insulator or the pressure of the moderator should be increased in order to reduce the heat losses from the fuel channel. / UOIT

In this work, spin-on processes were used to improve front- and rear-side technologies of solar cells to increase efficiencies to >20 %. A limited source diffusion process was developed using phosphoric acid dopant solutions developed in-house. An optimal emitter was obtained to be used in conjunction with screen-printed contacts. This emitter was used to improve the efficiency of conventional full aluminum back surface field solar cells to 19.6 %. A streamlined process was then developed to fabricate high-efficiency dielectric rear passivated cells in a single high temperature step. This process combined the diffusion process described earlier with a spin-on dielectric for rear passivation to achieve solar cell efficiencies of ~20%. Several laser candidates were investigated to improve process reproducibility and throughput. Ultra-violet laser with nanosecond pulse width was identified as the optimal choice. Cell efficiencies of ~20% were reproduced using UV laser for ablation of rear dielectric. This cell design and process were transferred to low-cost low-lifetime commercial grade substrates after identifying the optimal substrate characteristics using modeling.

The continuous increase in wireless usage forces an immense pressure on wireless communication in terms of increased demand and spectrum scarcity. Service providers for communication services had no choice but to allocate new parts of the spectrum and present new communication standards that are more spectrally efficient. Communication is not only limited to mobile phones but recently attention has been given to intelligent transportation systems (ITS) where cars will be given a significant place in the communication network. Vehicular Ad-Hoc Network (VANET) is already assigned a slice of the spectrum at 5.9GHz using the IEEE802.11p standard also known as Dedicated Short-Range Communication (DSRC); however, this assignment will have limited range and functionality at first, and users are expected to depend on existing wireless mobile channels for some services such as video streaming and car entertainment. Therefore, it is essential to integrate existing wireless mobile communication standards into the skeleton of ITS at launch and most probably permanently. An investigation was carried out regarding the existing communication standards including wireless local area networks (WLAN) and it was found that frequency bands from 400MHz up to 6GHz are being used in various regions around the world. It is also noted that current state of the art transceivers are composed of several transmitter front-ends targeting certain bands and standards. However, the more standards to be supported the more components to be added and the higher the cost not to mention the limited space in mobile devices. Multimode Multiband (MMMB) transmitters are therefore proposed as a potential solution to the existing redundancy in the number of front-end paths in modern transmitters. Broadband amplifiers are an essential part of any MMMB transmitter and they are also among the most challenging especially for high power requirements. This work explains why single ended topologies with efficiencies higher than 50% have a fundamental bandwidth limit such that the highest frequency of operation must be lower than twice the lowest frequency of operation. Hence, Push-Pull amplifier topology is being proposed as it was found that it has inherent broadband capabilities exceeding those of other topologies with comparable efficiency. The major advantage of Push-Pull power amplifiers is its capability of isolating the even harmonics present in the even mode operation of a Push-Pull amplifier from the less critical odd mode harmonics and the fundamental frequency. This separation between even and odd signals comes from the inclusion of a Balun at the output of push-pull amplifiers. Such separation makes it possible to operate amplifiers beyond the existing limit of single ended power amplifiers. To prove the concept, several Baluns were designed and tested and a comparison was made between different topologies in terms of balance, bandwidth and odd and even mode performances; moreover, to illustrate the concept a Push-Pull power amplifier design was implemented using the multilayer Low Temperature Co-fired Ceramics (LTCC) technology with a bandwidth ratio of more than 100%.

This thesis work describes the implementation perspective of an integrated high efficiency DC-DC converter implemented in 65 nm CMOS. The implemented system employs the Buck converter topology to down-convert the input battery voltages. This converter offers its use as a power management unit in portable battery operated devices. This thesis work includes the description of a basic Buck converter along with the various key equations involved which describe the Buck operation as well as are used to deduce the requirements for the various internal building blocks of the system. A detailed description of the operation as well as the design of each of the building blocks is included. The implemented system can convert the input battery voltage in the range of 2.3 V to 3.6 V into an output supply voltage of 1.6 V. The system uses dual-mode feedback control to maintain the output voltage at 1.6 V. For the low load currents the PFM feedback control is used and for the higher load currents the PWM feedback control is used. This converter can supply load currents from 0 to 300 mA with efficiency above 85%. The static line regulation of the system is &lt; 0.1% and the load regulation of the system is &lt; 0.3%. A digital soft-start circuit is implemented in this system. The system also includes the capability to trim the output voltage in ~14 mV steps depending on the 4-bit input digital code.

In wireless communication systems, multiple standards have been implemented to meet the past and present demands of different applications. This proliferation of wireless standards, operating over multiple frequency bands, has increased the demand for radio frequency (RF) components, and consequently power amplifiers (PA) to operate over multiple frequency bands. In this research work, a systematic approach for the synthesis of a novel dual-band matching network is proposed and applied for effective design of PA capable of maintaining high power efficiency at two arbitrary widely spaced frequencies. The proposed dual-band matching network incorporates two different stages. The first one aims at transforming the targeted two complex impedances, at the two operating frequencies, to a real one. The second stage is a dual-band filter that ensures the matching of the former real impedance to the termination impedance to 50 Ohm. Furthermore, an additional transmission line is incorporated between the two previously mentioned stages to adjust the impedances at the second and third harmonics without altering the impedances seen at the fundamental frequencies. Although simple, the harmonic termination control is very effective in enhancing the efficiency of RF transistors, especially when exploiting the Class J design space. The proposed dual-band matching network synthesis methodology was applied to design a dual-band power amplifier using a packaged 45 W gallium nitride (GaN) transistor. The power amplifier prototype maintained a peak power efficiency of about 68% at the two operating frequencies, namely 800 MHz and 1.9 GHz. In addition, a Volterra based digital predistortion technique has been successfully applied to linearize the PA response around the two operating frequencies. In fact, when driven with multi-carrier wideband code division multiple access (WCDMA) and long term evolution (LTE) signals, the linearized amplifier maintained an adjacent channel power ratio (ACPR) of about 50 dBc and 46 dBc, respectively.

High linearity transmitters and receivers should be used to efficiently utilize the available channel bandwidth. Power consumption is also a critical factor that determines the battery life of portable devices and wireless sensors. Three base-band and RF building blocks are designed with the focus of high linearity and low power consumption. An architectural attenuation-predistortion linearization scheme for a wide range of operational transconductance amplifiers (OTAs) is proposed and demonstrated with a transconductance-capacitor (Gm-C) filter. The linearization technique utilizes two matched OTAs to cancel output harmonics, creating a robust architecture. Compensation for process variations and frequency-dependent distortion based on Volterra series analysis is achieved by employing a delay equalization scheme with on-chip programmable resistors. The distortion-cancellation technique enables an IM3 improvement of up to 22dB compared to a commensurate OTA without linearization. A proof-of-concept lowpass filter with the linearized OTAs has a measured IM3 < -70dB and 54.5dB dynamic range over its 195MHz bandwidth. Design methodology for high efficiency class D power amplifier is presented. The high efficiency is achieved by using higher current harmonic to achieve zero voltage switching (ZVS) in class D power amplifier. The matching network is used as a part of the output filter to remove the high order harmonics. Optimum values for passive circuit elements and transistor sizes have been derived in order to achieve the highest possible efficiency. The proposed power amplifier achieves efficiency close to 60 percent at 400 MHz for -2dBm of output power. High efficiency class A power amplifier using dynamic biasing technique is presented. The power consumption of the power amplifier changes dynamically according to the output signal level. Effect of dynamic bias on class A power amplifier linearity is analyzed and the results were verified using simulations. The linearity of the dynamically biased amplifier is improved by adjusting the preamplifier gain to guarantee constant overall gain for different input signal levels.