Design Considerations for CMOS Current Mode Operational Amplifiers and Current Conveyors

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    Abstract

    This dissertation is about CMOS current conveyors and current mode operational amplifiers (opamps). They are generic devices for continuous time signal processing in circuits and systems where signals are represented by currents.Substantial advancements are reported in the dissertation, both related to circuit implementations and system configurations and to an analysis of the fundamental limitations of the current mode technique.In the field of system configurations and circuit implementations different configurations of high gain current opamps are introduced and some of the first implementations of current mode opamps in CMOS technology are described. Also, current conveyor configurations with multiple outputs and flexible feedback connections from outputs to inputs are introduced. The dissertation includes several examples of circuit configurations ranging from simple class A and class AB conveyor implementations to implementations based on purely digital circuit structures and on more complex analog subsystems such as a voltage mode opamp with feedback to provide a voltage follower action. An important by-product of the investigation of current mode structures is the definition of a non slew-rate limited voltage mode opamp.In the field of defining the fundamental limitations of the current mode technique a substantial contribution is given in the analysis of the frequency limitations of operational amplifiers with feedback. It is concluded that - when taking technology constraints into account - the type of opamp providing the highest bandwidth potential is the 'native' opamp type of the feedback system. For instance, if the feedback amplifier system is a transimpedance system (current input, voltage output) the highest bandwidth can be achieved by selecting a transimpedance opamp element. Another important contribution is given in the analysis of noise properties and dynamic range limitations of current mode systems where it is concluded that the essential parameters in determining the dynamic range are the supply voltage and the transistor geometries. Unfortunately, the dynamic current range increases linearly with the supply voltage which implies that no significant advantages with respect to low supply voltage operation are obtained compared to voltage mode techniques. A substantial contribution is also given in the development of an analytical model for the distortion in CMOS current mirrors. Only the distortion caused by device mismatch is considered since this provides the fundamental limit to the performance. Distortion due to electrical mismatch can be cancelled by proper circuit techniques. The analytical model is developed to yield a calculation of the worst case distortion expected from devices with a statistical mismatch and it is concluded that harmonic distortion levels somewhat below 1% are attainable.Altogether, in the dissertation the current mode technique is demonstrated as an interesting and relevant technique for processing of analog signals. A good reason for this is that the controlled output signal of the fundamental active devices (MOS transistors and bipolar transistors) is a current. This implies that at the circuit level current will almost by definition be a signal representing quantity and a good knowledge of current mode signal processing at the circuit level is important for the optimization of circuit functions. At the system level, current mode signal processing is relevant if the input signals or output signals of the system are currents (e.g. signals from transducers or signals to actuators). In general, significant advantages cannot be expected from a systematic conversion of the signal processing to the current domain since the fundamental performance limitations in current mode signal processing are no less restrictive than the limitations in voltage mode signal processing.
    Original languageEnglish
    Place of PublicationLyngby
    PublisherDepartment of Information Technology, Technical University of Denmark
    Publication statusPublished - 1999

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