CMOS voltage/current controlled oscillator designs and applications




Yadav, Rohit

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In advanced short-channel CMOS processes, analog circuit-design poses significant challenges imposed by the low power-supply voltage, and the reduced intrinsic-gain of short channel-effects of CMOS-transistors. However, continued scaling of CMOS devices makes them faster and thus enables the design of high-speed circuits such as oscillators at lower power-levels. Voltage and current-controlled oscillators are essential building blocks for designing Delta-Sigma analog-to-digital converters (ADC) that do not require a sample and hold circuits up front. This research focuses on designing scaling-friendly, linear, low-power CMOS relaxation-oscillators for Delta-Sigma analog to digital conversion.

Ring oscillators using an odd number of digital-inverters provide a simple reliable-solution, and thus are popular. So the first approach is to increase the speed (time-resolution) by simply lowering the inverter-gain using the translinear and temperature-invariant voltage-mirror circuits instead of open-loop inverters. Here two scaled-inverters are used to obtain gain exploiting the function and its functional inverse idea to obtain the required loop-gain. Here we are simply trading gain to increase speed at the expense of power. An alternative approach is to use the gyrator circuit to emulate an inductor. Two transconductors with opposite polarity current-output are needed to form a gyrator. Inverting the output-polarity is achieved using a translinear, unity-gain current-mirror achieving stability of the gyrator circuit that uses negative feedback. Using these, an LC -coupled ring-oscillator is emulated. These provide a more robust solution.

Process, supply-voltage and temperature (PVT) variations are traditionally minimized by the well-established technique of negative-feedback in circuits using Bandgap reference voltage and passive elements like MIM capacitors. Thus the design of an operational current to frequency converter (OCFC) is proposed. This is inspired by the success of the notion of an operational amplifier (OPAMP) that has a very large gain. It is always configured to trade gain for obtaining linearity using negative- feedback making the feedback network set the precision of the transfer function. The OCFC uses the same idea using a switched-capacitor linear frequency-to-current detector in the feedback path to make the closed-loop tuning-curve linear, having a low temperature-coefficient.

The design of a simple linear oscillator based on a traditional multivibrator-circuit is explored for two specific applications. There is a need for a medium-resolution low-speed 10-bit ADC for IOT applications. The other is for a 6-bit converter for high-speed data-communication systems. The idea is to obtain the performance requirements with no need for power-hungry digital-calibration. Manufacturability, and cost are the prime focus here.

Finally, an open loop Delta-Sigma modulator designed using pseudo-differential ring oscillators is explored for low bandwidth and medium resolution analog-to-digital conversion applications. It employs a nonlinearity cancellation technique in the voltage-to-current (V/I) converter to improve resolution. Most of the circuit can be synthesized using digital toolchain except for the front-end V/I converter to reduce turn-around time. All the circuits presented in this thesis are verified at multiple process nodes ranging from 180nm to 22nm. The simulation-results presented here are mostly using the models for the 180nm TSMC process.


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