Broad-band microwave amplifier design considerations
Broad-band microwave integrated circuit (MIC) amplifier design is a complex, multi-disciplinary process. This work focuses on three important aspects: the behaviour of microstrip transmission lines, discontinuities, and related structures; the accurate measurement of components and devices mounted in microstrip circuits; and the circuit design methodology. Techniques for microstrip quasi-static analysis are reviewed in order to identify methods suitable for extension to deal with the effects of substrate anisotropy. An integral equation method is described and the anisotropic Green's function derived using an extension to the method of partial images. Proposed transform methods are assessed and the preferred option implemented by adaption of a microstrip analysis computer program. A method, by which accurate measurements of microstrip properties may be made, is developed. Involving measurements of the resonant behaviour of half-wavelength short circuit resonators with two arbitrary coupling conditions, this technique allows the unloaded properties to be deduced. Results for microstrip on a sapphire substrate concur with the analysis. A pragmatic but effective approach to the calculation of the capacity component of microstrip discontinuities, and some other three dimensional MIC structures, is described and developed to allow existing data for isotropic substrates to be applied to the anisotropic situation. The computer corrected network analyser (CCNA) is a widely used microwave measurement tool. Weaknesses in popular correction strategies are identified and remedies developed. In particular, revised calibration equations that better accommodate test port mismatch variation with s-parameter selection, and a model for quadrature error are presented. A 2-port calibration scheme suitable for use with MIC transmission lines, using only simple standards, is described. The standards are partially self-calibrating;the values of propagation constant, loss, and end effect are deduced in the calibration process. An effective jig for use with microstrip is described and the results of measurements on microwave transistors presented. Conventionally microwave amplifiers are designed using reactive components both to achieve good port matches and compensate the frequency dependent gain of the active devices. The problems associated with this approach are enumerated and the alternatives reviewed. A methodology which combines the benefits of frequency dependant dissipative networks with the elegance of reactive network synthesis is described. The device gain slope is compensated by simple lumped or distributed circuits incorporating a resistive element to produce a composite `device' with a specififed (flat) maximum available gain frequency response. Reactive matching networks are then used to interface these gain blocks. By this structured approach the amplifier gain breakdown can be defined at the outset and preserved through the design process. Other advantages stemming from the use of dissipative compensation include improved tolerance to device parameter and component value scatter, reduced group delay variations and enhanced reverse isolation. The method is demonstrated by the design and characterisation of 4 to 9 GHz amplifier having a representative specification. The close conformance of the performance of the untrimmed amplifier to that predicted by computer simulation testifies to the inherent accuracy of the design method, the microstrip (and related structures) analysis techniques and the CCNA MIC calibration scheme.