DESIGN AND IMPLEMENTATION OF ANALOG CIRCUITS USING CURRENT MODE BUILDING BLOCKS

Abstract

Rapid advancements in semiconductor technology has made integration of millions of transistors on a single die. The analog – digital boundaries are diminishing and integrated solutions for complete system are in vogue where both analog and digital subsystems are placed on single die. Though in last two decades’ electronic circuit design has a paradigm shift from analog to digital domain yet analog circuit design is going to remain in mainstay. For signal acquisition and processing, amplifiers, filters, sample and hold circuits, signal comparators, analog to digital converter (ADC) and digital to analog converter (DAC) are required. Further, new applications continue emerging and require high performance analog interface circuits. There are challenges in designing analog circuits due to continual scaling of device dimensions and also the power supply voltages. The lowering in power supply lead to reduction in input common mode range, linearity and output voltage swing. It is well known that signal processing may be done by manipulating node voltages or branch currents. The analog circuits are termed as voltage mode (VM) or current mode (CM) if the information medium is represented by the nodal voltages or branch currents. The performance parameters of VM circuits such as dynamic range, slew rate and common mode range are severely affected by scaling. The CM circuits show advantageous features such as improved slew rate due to smaller time constant, wider bandwidth, as transistors in current amplifiers may be used till unity gain bandwidth. Further as current depends nonlinearly on voltage, a smaller voltage swing allows larger current change, thus improving dynamic range. Further, these circuits may be compact as addition of current can done by simply joining branches. To exploit the advantages of CM signal processing, a wide variety of analog building blocks have been developed. Operational floating current conveyor (OFCC) is one

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among these building block which combines the features of current conveyor and the current feedback operational amplifier along with additional current outputs that adds flexibility in the circuit design. It can readily be used for processing and providing both current and voltage at appropriate impedance levels. Numerous OFCC based signal processing applications such as basic amplifiers, filters, oscillators, instrumentation amplifiers (IAs), variable/programmable gain amplifiers (VGA/PGA), read out circuits, wheat-stone bridge and logarithmic amplifier, as available in open literature. The main focus of the work presented in this thesis is to develop circuits which are more versatile, modular, integrable and can provide better quality response. A first order Transadmittance mode (TAM) filters is presented that processes signal from voltage sensor and provide current output for further processing. It provides three responses i.e. low pass, high pass and all pass at high impedance. A feature that is gainfully used for developing an oscillator without using any extra active blocks. Second order CM SIMO filter is put forward that can provide low pass, high pass, band pass and notch responses simultaneously. The filter parameters can be adjusted independently. As an application a CM shadow bandpass filter is developed for controlling filter parameters through an amplifier gain. Electronic tunability of filter parameters is achieved via MOS based resistors. Instrumentation amplifier (IA) is widely used in applications pertaining to medical instrumentation, sensor read out integrated circuits, data acquisition systems, industrial process control, automotive transducers, bio-potential acquisition systems and linear position sensing. Two generalized structure of IA is put forward and its usefulness is shown through OFCC. These structures are termed as: Structure-I and Structure-II. Each structure is used to generate IA topologies operating in VM, CM, TIM and TAM, thus eight IAs are generated in total. A

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modification to Structure I and Structure II are suggested to obtain IA topologies with reduced resistor count. Considering the modifications, six more topologies are put forward. The input and output impedance of all proposed topologies are proper therefore no additional active block is needed for interfacing. Amplifiers, in particular PGA, find applications in instrumentation, photodiode circuits, ultrasound preamplifiers, sonar, wide dynamic range sensors, driving ADCs and automatic gain control (AGC) loops. A new OFCC based PGA is the key contribution towards such amplifiers which offers programmable gain feature. The rectifiers are widely used in applications pertaining to telecommunication, instrumentation and measurement. OFCC based CM half wave rectifiers (HWRs) and full wave rectifier (FWR) are put forward. These rectifiers do not use diode and are thus suitable for low voltage rectification. It is pertinent to mention here that all proposed rectifiers use single active block and are resistor-less. The behavior of all the proposed circuits under the influence of nonidealities of OFCC is also examined. The functionality of proposed circuits is verified through SPICE simulations and/or through experimental observations.