Analog Frontend Circuits for Avalanche Photodiodes

The aims of this work is to design low noise electronics for optical sensing and X‐ray spectroscopy using Sheffield‐grown Avalanche photodiodes(APD). A transimpedance amplifier(TIA) for a 2.0 μm LIDAR system is designed and tested as part of a project funded by ESA. Numerical analysis is provided for the TIA in addition to SPICE and experimental analysis. Characterisation of the TIA shows that a noise equivalent power of less than 100 fW/√Hz can be achieved with an optimised InAs APD. Preliminary results of a TIA‐InAs module at 2.0 μm is presented. A low noise charge sensitive preamplifier(CSP) with a novel local feedback is designed and characterised. The CSP shows a better noise performance than commercially available CSP such as the CoolFet 250. The CSP is also characterised for APD dark current of up 4 μA and the CSP is found to behave well for such relatively high dark current. Discrepancies between the SPICE model and measured characteristic of the CSP’s input JFET is presented and discussed. The first ever Aluminium Indium Phosphide (AlInP) APD X‐ray spectroscopy measurement is presented in this work. AlInP is the widest band material that can be grown latticematched on a GaAs substrate. Due to its wide bandgap, AlInP can offer reverse dark current of less than 2 pA at gain of 100 for a 200um device, making it desirable for room temperature operation. An energy resolution of 647 eV is obtained for AlInP APD coupled to the CSP and exposed to 55Fe X‐rays. Using the CSP presented in this work, previously reported GaAs/AlGaAs APD is characterised and compared with results obtained using a commercial CSP. A 21% improvement in X‐ray energy resolution is reported, despite degradation in the APD

Techniques for signal to noise ratio adaptation in infared optical wireless for optimisation of receiver performance

The challenge of creating a new environment of links for wireless infrared and optical local area networks (LANs) is driving new innovations in the design of optical transceivers. This thesis is concerned with a systematic approach to the design of receivers for indoor optical wireless communication. In particular, it is concerned with how to offer bandwidth adjustment capability in a receiver according to the dynamic service quality of the incoming signals. Another part of the discussion of the thesis is how one can properly choose the front-end preamplifier and biasing circuitry for the photodetector. Also, comparison is made between different types of amplifier, and the methods of bandwidth enhancement. The designs of six different techniques of integrating transimpedance amplifiers, with photodetectors to adapt an adjustable bandwidth control receiver are discussed. The proposed topologies provide an adjustable range of bandwidths for different frequency ranges, typically between 52Hz to 115MHz. The composite technique designs were used to incorporate into a system with an automatic gain control to study its effect, on an optical wireless receiver which had bandwidth adjustment and automatic gain adjustment. Theoretical analysis of noise performance for all the designed circuits is also presented. The theory and design of obstacles of indoor optical wireless receiver delivery, in addition to techniques for mitigating these effects, are discussed. This shows that infrared is a viable alternative to ratio for certain applications

A Bulk Driven Transimpedance CMOS Amplifier for SiPM Based Detection

The contribution of this work lies in the development of a bulk driven operationaltransconducctance amplifier which can be integrated with other analog circuits andphotodetectors in the same chip for compactness, miniaturization and reducing thepower. Silicon photomultipliers, also known as SiPMs, when coupled with scintillator materials are used in many imaging applications including nuclear detection. This thesis discuss the design of a bulk-driven transimpedance amplifier suitable for detectors where the front end is a SiPM. The amplifier was design and fabricated in a standard standard CMOS process and is suitable for integration with CMOS based SiPMs and commercially available SiPMs. Specifically, the amplifier was verified in simulations and experiment using circuit models for the SiPM. The bulk-driven amplifier’s performance, was compared to a commerciallyavailable amplifier with approximately the same open loop gain (70dB). Bothamplifiers were verified with two different light sources, a scintillator and a SiPM.The energy resolution using the bulk driven amplifier was 8.6% and was 14.2% forthe commercial amplifier indicating the suitability of the amplifier design for portable systems

Distributed Circuit Analysis and Design for Ultra-wideband Communication and sub-mm Wave Applications

This thesis explores research into new distributed circuit design techniques and topologies, developed to extend the bandwidth of amplifiers operating in the mm and sub-mm wave regimes, and in optical and visible light communication systems. Theoretical, mathematical modelling and simulation-based studies are presented, with detailed designs of new circuits based on distributed amplifier (DA) principles, and constructed using a double heterojunction bipolar transistor (DHBT) indium phosphide (InP) process with fT =fmax of 350/600 GHz. A single stage DA (SSDA) with bandwidth of 345 GHz and 8 dB gain, based on novel techniques developed in this work, shows 140% bandwidth improvement over the conventional DA design. Furthermore, the matrix-single stage DA (M-SSDA) is proposed for higher gain than both the conventional DA and matrix amplifier. A two-tier M-SSDA with 14 dB gain at 300 GHz bandwidth, and a three-tier M-SSDA with a gain of 20 dB at 324 GHz bandwidth, based on a cascode gain cell and optimized for bandwidth and gain flatness, are presented based on full foundry simulation tests. Analytical and simulation-based studies of the noise performance peculiarities of the SSDA and its multiplicative derivatives are also presented. The newly proposed circuits are fabricated as monolithic microwave integrated circuits (MMICs), with measurements showing 7.1 dB gain and 200 GHz bandwidth for the SSDA and 12 dB gain at 170 GHz bandwidth for the three-tier M-SSDA. Details of layout, fabrication and testing; and discussion of performance limiting factors and layout optimization considerations are presented. Drawing on the concept of artificial transmission line synthesis in distributed amplification, a new technique to achieve up to three-fold improvement in the modulation bandwidth of light emitting diodes (LEDs) for visible light communication (VLC) is introduced. The thesis also describes the design and application of analogue pre-emphasis to improve signal-to-noise ratio in bandwidth limited optical transceivers

Techniques for the design of a low noise, high dynamic range, high gain, wideband amplifier for analogue OEIC applications

Various techniques for the Design of a Low Noise, High Dynamic Range, High Gain, Wideband Amplifier for Analogue OEIC Applications, such as radar receiver arrays for electronic warfare applications were developed and investigated in this work. Firstly, the available transistor technologies, semiconductor technologies and photodetector technologies and their pros and cons in light of the target application type are investigated in order to decide on the technologies best suited for this research, and justification of the chosen options are presented. Secondly, three different known amplifier topologies are discussed and their linearity, gain, bandwidth, SFDR, and other performances are compared via simulations calibrated against measured results. The results from the comparisons are analysed and the amplifier topology most suitable for this work is chosen based on these results. Thirdly, three different circuit alteration techniques for improving the linearity and SFDR of the previously chosen amplifier topology are developed, analysed and verified through simulations. It is shown that these techniques can be combined to gain further improvement in overall performance. And finally, the influence of various geometrical and doping alterations of the transistor on desired figures of merit, i.e. gain, bandwidth, linearity, etc. are investigated in detail using two-dimensional physical device simulations calibrated against measured results of an InP/InGaAs single heterojunction bipolar transistor. The device simulations were carried out using Technology-Computer-Aided-Design (TCAD) within the SILVACO software package. The results are then used to suggest techniques to improve performance at the transistor level

Modelling, Analysis and Design of Optimised Electronic Circuits for Visible Light Communication Systems

This thesis explores new circuit design techniques and topologies to extend the bandwidth of visible light communication (VLC) transmitters and receivers, by ameliorating the bandwidth-limiting effects of commonly used optoelectronic devices. The thesis contains detailed literature review of transmitter and receiver designs, which inspired two directions of work. The first proposes new designs of optically lossless light emitting diode (LED) bandwidth extension technique that utilises a negative capacitance circuit to offset the diode’s bandwidth-limiting capacitance. The negative capacitance circuit was studied and verified through newly developed mathematical analysis, modelling and experimental demonstration. The bandwidth advantage of the proposed technique was demonstrated through measurements in conjunction with several colour LEDs, demonstrating up to 500% bandwidth extension with no loss of optical power. The second direction of work enhances the bandwidth of VLC receivers through new designs of ultra-low input impedance transimpedance amplifiers (TIAs), designed to be insensitive to the high photodiode capacitances (Cpd) of large area detectors. Moreover, the thesis proposes a new circuit, which modifies the traditional regulated cascode (RGC) circuit to enhance its bandwidth and gain. The modified RGC amplifier efficiently treats significant RGC inherent bandwidth limitations and is shown, through mathematical analysis, modelling and experimental measurements to extend the bandwidth further by up to 200%. The bandwidth advantage of such receivers was demonstrated in measurements, using several large area photodiodes of area up to 600 mm^2, resulting in a substantial bandwidth improvement of up to 1000%, relative to a standard 50 Ω termination. An inherent limitation of large area photodiodes, associated with internal resistive elements, was identified and ameliorated, through the design of negative resistance circuits. Altogether, this research resulted in a set of design methods and practical circuits, which will hopefully contribute to wider adoption of VLC systems and may be applied in areas beyond VLC

THE USE OF TUNED FRONT END OPTICAL RECEIVER AND PULSE POSITION MODULATION

The aim of this work is to investigate the use of tuned front-ends with OOK and PPM schemes, in addition to establish a theory for baseband tuned front end receivers. In this thesis, a background of baseband receivers, tuned receivers, and modulation schemes used in baseband optical communication is presented. Also, the noise theory of baseband receivers is reviewed which establishes a grounding for developing the theory relating to optical baseband tuned receivers. This work presents novel analytical expressions for tuned transimpedance, tuned components, noise integrals and equivalent input and output noise densities of two tuned front-end receivers employing bi-polar junction transistors and field effect transistors as the input. It also presents novel expressions for optimising the collector current for tuned receivers. The noise modelling developed in this work overcomes some limitations of the conventional noise modelling and allows tuned receivers to be optimised and analysed. This work also provides an in-depth investigation of optical baseband tuned receivers for on-off keying (OOK), Pulse position modulation (PPM), and Di-code pulse position modulation (Di-code PPM). This investigation aims to give quantitative predictions of the receiver performance for various types of receivers with different photodetectors (PIN photodetector and avalanche photodetector), different input transistors (bi-polar junction transistor BJT and field effect transistor FET), different pre-detection filters (1st order low pass filter and 3rd order Butterworth filter), different detection methods, and different tuned configurations (inductive shunt feedback front end tuned A and serial tuned front end tuned B). This investigation considers various optical links such as line of sight (LOS) optical link, non-line of sight (NLOS) link and optical fibre link. All simulations, modelling, and calculations (including: channel modelling, receiver modelling, noise modelling, pulse shape and inter-symbol interference simulations, and error probability and receiver calculations) are performed by using a computer program (PTC Mathcad prime 4, version: M010/2017) which is used to evaluate and analyse the performance of these optical links. As an outcome of this investigation, noise power in tuned receivers is significantly reduced for all examined configurations and under different conditions compared to non-tuned receivers. The overall receiver performance is improved by over 3dB in some cases. This investigation provides an overview and demonstration of cases where tuned receiver can be optimised for baseband transmission, offering a much better performance compared to non-tuned receivers. The performance improvement that tuned receivers offers can benefit a wide range of optical applications. This investigation also addresses some recommendations and suggestions for further work in some emerging applications such as underwater optical wireless communication (UOWC), visible light communication (VLC), and implantable medical devices (IMD). Keyword: Optical communications, Baseband receivers, Noise modelling, tuned front end, pulse position modulation (PPM)

Thermal noise limit for ultra-high vacuum noncontact atomic force microscopy

Lübbe J, Temmen M, Rode S, Rahe P, Kühnle A, Reichling M. Thermal noise limit for ultra-high vacuum noncontact atomic force microscopy. Beilstein Journal of Nanotechnology. 2013;4:32-44.The noise of the frequency-shift signal Delta f in noncontact atomic force microscopy (NC-AFM) consists of cantilever thermal noise, tip-surface-interaction noise and instrumental noise from the detection and signal processing systems. We investigate how the displacement-noise spectral density d(z) at the input of the frequency demodulator propagates to the frequency-shift-noise spectral density d(Delta f) at the demodulator output in dependence of cantilever properties and settings of the signal processing electronics in the limit of a negligible tip-surface interaction and a measurement under ultrahigh-vacuum conditions. For a quantification of the noise figures, we calibrate the cantilever displacement signal and determine the transfer function of the signal-processing electronics. From the transfer function and the measured dz, we predict d(Delta f) for specific filter settings, a given level of detection-system noise spectral density d(ds)(z) and the cantilever-thermal-noise spectral density d(th)(z). We find an excellent agreement between the calculated and measured values for d(Delta f). Furthermore, we demonstrate that thermal noise in d(Delta f), defining the ultimate limit in NC-AFM signal detection, can be kept low by a proper choice of the cantilever whereby its Q-factor should be given most attention. A system with a low-noise signal detection and a suitable cantilever, operated with appropriate filter and feedback-loop settings allows room temperature NC-AFM measurements at a low thermal-noise limit with a significant bandwidth