A brief survey of filter technologies is given and the requirement for a high frequency continuous time monolithic filtering capability is identified. The ideal transconductor is defined and is compared to the other most common forms of integrated amplifier. A distinction is drawn between 'open loop' and 'closed loop' transconductor integrators. Although closed loop integrators have several advantages, filters using open loop integrators are emphasised since they have greater potential for high frequency operation. A detailed review is given of transistor level transconductor circuits in CMOS, JFET, GaAs MESFET and bipolar technologies, proposed by other researchers. Also reviewed are the techniques that have been used for the design of transconductor filters including automatic frequency and phase tuning circuits. It is demonstrated that standard methods for active-RC and switched capacitor ladder filter design are not satisfactory when applied to transconductor ladder design, particularly for bandpass responses. An original CMOS transconductance cell is described which is well suited to applications requiring high frequency of operation and high linearity within a low power supply. The advantages of designing transconductors with folded cascade ouput stages are demonstated. Also, two enhancements to the standard folded cascade structure are proposed. The first is the addition of low impedance inputs (in addition to the normal inputs). These allow the use of unidirectional capacitive branches in filters based upon open loop integrators, and thereby increase greatly the number of ladder filter structures that can be designed. The second enhancement provides control of the phase response of the transconductor by means of a variable d.c. voltage. This may be used to compensate actively for the effects of parasitic poles. A set of algebraic methods for the design of transconductor ladder filters is presented. These represent a structured method which may be used as the basis for computer aided design tools. More importantly they provide an abstract representation of the ladder which can be used to find superior active filter circuits that are not intuitively obvious. In particular, new circuits for bandpass ladders are derived which could not be obtained using conventional methods. Applying the developments described above, a set of transconductor ladder filters and a frequency control loop have been designed and fabricated on a 1 micron CMOS process. These include 1 MHz lowpass filters, along with 400 KHz and 1 MHz bandpass filters. Detailed experimental results are given for these circuits.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:651798 |
| Date | January 1992 |
| Creators | Greer, N. P. J. |
| Publisher | University of Edinburgh |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
| Source | http://hdl.handle.net/1842/14951 |
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