A.K. Betts

Design issues for a switched-capacitor filter using GaAs technology

Various aspects of the design of a high-speed GaAs switched-capacitor filter are discussed. The circuit is based on a formerly unexploited finite-region insensitive integrator-pair, taking advantage of a recently recognized property of its inverting integrator. Device modeling and the influence of transistor characteristics on subcircuit design are discussed. Details of the filter-optimization procedures are given. The circuit has been fabricated and is now being mounted for testing.<<ETX>>

Evaluation and synthesis of ultra narrow band switched-capacitor-filters employing multirate techniques

The problem of designing ultra-narrowband switched-capacitor filters (relative bandwidth <0.1%) using current design techniques is considered. These are evaluated using a statistical simulation approach to quantify the effects of mismatch in the multipath structures used. The limitations of available techniques are highlighted, and a structure that is significantly less sensitive to mismatch is proposed. As a corollary, a pseudo-N-path building block that has especially low sensitivity to finite amplifier gain-bandwidth product is introduced.<<ETX>>

Design and evaluation of a high-precision, fully tunable OTA-C bandpass filter implemented in GaAs MESFET technology

A second-order bandpass filter employing the operational transconductance amplifier-capacitor (OTA-C) method and featuring independent tuning of center frequency and Q is described. The filter, which is realized in 0.5-/spl mu/m GaAs MESFET technology, is intended for use in high-precision, continuous-time (CT) IF bandpass filtering applications requiring both accurate amplitude and group delay responses. The filter center frequency is tunable in the range 12-50 MHz, Q is tunable in the range 4-60, and a transfer function accuracy of the order of 1% is achieved throughout the tuning range. Active area is 1 mm/sup 2/ and static power consumption is 230 mW. >

An improved tuning technique for switched capacitor filters

A new technique is described for tuning SC circuits that provides wide-ranging, continuous tuning of any circuit parameter that is determined by a capacitor ratio. The technique employs voltage scaling circuits to modify effective capacitor ratios. Although based on the master-slave control principle, these strategies are considerably more economical than other widely used master-slave tuning methods as they do not require duplication of a substantial part of the main filter.<<ETX>>

Implementation issues for ultra narrow band switched-capacitor filters

The authors describe a project to investigate the properties of the same sample correction finite gain insensitive pseudo-N-path (SSC FGI-PNP) filters. They explore the N*M-path filter architecture and its building blocks. The final circuit was laid out using MIETEC 2.4- mu m double-poly and double-metal CMOS technology. The simulated circuit behavior is described and its limitations are identified. Simulation results suggest that the performance of the SSC FGI-PNP filters is considerably enhanced when compared to the conventional PNP filters at an extra silicon cost of approximately 50%. The effect of gain on the center frequency and Q of the PNP circuit, which are the crucial parameters for N*M-path filters has also been investigated.<<ETX>>

A continuously variable tuning technique for switched capacitor filters

A new tuning technique for SC filters is described which allows separate, continuous tuning of centre frequency and Q. Feedforward control is used with this arrangement and so the complexities and hence cost of feedback control are largely avoided. Measured results from a discrete component model are provided.

The variable gain bootstrap active load: a new approach to amplifier voltage gain enhancement and control

This work describes a novel technique for obtaining controllable, high-voltage gain in amplifier circuits where current mirrors and cascading techniques cannot be used. An example of this is in high-frequency design where suitable complementary devices are generally not available. Although the method was originally intended for use in the design of operational transconductance amplifiers (OTAs) implemented in GaAs MESFET technology for use in high-precision filtering applications, it can be applied to other types of gain stage and implemented in other technologies. For example, using the new technique, controllable gains in excess of 40 dB can be obtained using standard GaAs MESFET technology without reliance on device width ratioing or on early saturation. The application of the OTA design in an integrated continuous-time bandpass filter chip is presented with encouraging preliminary results. >

A high precision 2nd order bandpass continuous time filter realised with novel OTA-C integrators in GaAs MESFET technology

A preliminary investigation into the effects of excess phase on the accuracy of the amplitude and group delay responses of a second-order operational-transconductance-amplifier-C (OTA-C) state-variable bandpass filter, used as one section of a high-order, high-precision filter, is reported. A comparison is made of using ideal OTA-Cs and introducing excess phase by means of first-order RC phase shift networks. A recently reported integrator using GaAs MESFET technology suitable for a second-order bandpass filter is also compared.<<ETX>>

Novel tunable GaAs MESFET OTA-C integrators suitable for high precision filtering applications

The design of novel, differential input operational-transconductance-amplifier (OTA-C) integrators for realization in 0.5- mu m, 20-GHz GaAs MESFET technology is described. It is shown that due to certain unique features of the single-ended OTA, extension to a fully balanced differential form requires considerably less than doubling of the number of components. The circuits, which feature separate control of gain and cutoff frequency, are particularly suited to high-precision, very-high-Q filtering applications. The fully balanced integrator is very linear, and searching tests conducted over a wide range of transconductance and operating conditions reveal that the total harmonic distortion levels are low.<<ETX>>

Implementation of a very narrow band finite gain insensitive pseudo-N-path filter

This paper describes a project to implement very narrow band filters using switched capacitor techniques. Past attempts to obtain such filters are reviewed and their limitations highlighted. The implementation of a highly reconfigurable 3rd order pseudo-N-path filter (N/spl les/7), which is based on new circuits that have been developed by the authors, are then described. This filter has been fabricated using MIETEC 2.4 /spl mu/m CMOS process and measured results are presented and any deviations from die simulation results are explained.<<ETX>>