Efficient symbolic computation of approximated small-signal characteristics of analog integrated circuits

A symbolic analysis tool is presented that generates simplified symbolic expressions for the small-signal characteristics of large analog integrated circuits. The expressions are approximated while they are computed, so that only those terms are generated which remain in the final expression. This principle causes drastic savings in CPU time and memory, compared with previous symbolic analysis tools. In this way, the maximum size of circuits that can be analyzed, is largely increased. By taking into account a range for the value of a circuit parameter rather than one single number, the generated expressions are also more generally valid. Mismatch handling is explicitly taken into account in the algorithm. The capabilities of the new tool are illustrated with several experimental result

Algorithm for efficient symbolic analysis of large analogue circuits

An algorithm is presented that generates simplified symbolic expressions for the small-signal characteristics of large analogue circuits. The expressions are approximated while they are computed, so that only the most significant terms are generated which remain in the final expression. This principle leads to dramatic savings in CPU time and memory compared to existing techniques, significantly increasing the maximum size of circuits that can be analysed. By taking into account a range for the value of a circuit parameter rather than one single number the generated symbolic expressions are also generally valid

Symbolic analysis of large analog integrated circuits by approximation during expression generation

A novel algorithm is presented that generates approximate symbolic expressions for small-signal characteristics of large analog integrated circuits. The method is based upon the approximation of an expression while it is being computed. The CPU time and memory requirements are reduced drastically with regard to previous approaches, as only those terms are calculated which will remain in the final expression. As a consequence, the maximum circuit size amenable to symbolic analysis has largely increased. The simplification procedure explicitly takes into account variation ranges of the symbolic parameters to avoid inaccuracies of conventional approaches which use a single value. The new approach is also able to take into account mismatches between the symbolic parameters

A Family of matroid intersection algorithms for the computation of approximated symbolic network functions

In recent years, the technique of simplification during generation has turned out to be very promising for the efficient computation of approximate symbolic network functions for large transistor circuits. In this paper it is shown how symbolic network functions can be simplified during their generation with any well-known symbolic network analysis method. The underlying algorithm for the different techniques is always a matroid intersection algorithm. It is shown that the most efficient technique is the two-graph method. An implementation of the simplification during generation technique with the two-graph method illustrates its benefits for the symbolic analysis of large analog circuits

Gradual Internal Reforming of Ethanol in Solid Oxide Fuel cells

AbstractElectrolyte (yttria-stabilised zirconia, YSZ) supported solid oxide fuel cells (SOFCs) were fabricated using spin coating of standard LSM cathode and Ni-YSZ cermet anode. A ceria-based catalytic layer was deposited onto the anode with a special current collector design. Such a single cell configuration allows operation by gradual internal reforming of direct carbon-containing fuels. First, the fabricated single cells were operated with hydrogen to determine the optimised conditions of fuel concentration and flow rate regarding faradaïc efficiency. Then, the fuel was switched to dry ethanol and the cells were operated for several hours (100h) with good stability. Post-operation electron microcopy analyses revealed no carbon formation in the anode layer. The results indicate that the gradual internal reforming mechanism is effective, opening up the way to multi-fuel SOFCs, provided that a suitable catalyst layer and cell design are available

Correlated hopping of electrons: Effect on the Brinkman-Rice transition and the stability of metallic ferromagnetism

We study the Hubbard model with bond-charge interaction (`correlated hopping') in terms of the Gutzwiller wave function. We show how to express the Gutzwiller expectation value of the bond-charge interaction in terms of the correlated momentum-space occupation. This relation is valid in all spatial dimensions. We find that in infinite dimensions, where the Gutzwiller approximation becomes exact, the bond-charge interaction lowers the critical Hubbard interaction for the Brinkman-Rice metal-insulator transition. The bond-charge interaction also favors ferromagnetic transitions, especially if the density of states is not symmetric and has a large spectral weight below the Fermi energy.Comment: 5 pages, 3 figures; minor changes, published versio

Quantum phase transitions in the Bose-Fermi Kondo model

We study quantum phase transitions in the Bose-Fermi Kondo problem, where a local spin is coupled to independent bosonic and fermionic degrees of freedom. Applying a second order expansion in the anomalous dimension of the Bose field we analyze the various non-trivial fixed points of this model. We show that anisotropy in the couplings is relevant at the SU(2) invariant non Fermi liquid fixed points studied earlier and thus the quantum phase transition is usually governed by XY or Ising-type fixed points. We furthermore derive an exact result that relates the anomalous exponent of the Bose field to that of the susceptibility at any finite coupling fixed point. Implications on the dynamical mean field approach to locally quantum critical phase transitions are also discussed.Comment: 13 pages, 9 figures, some references added/correcte

Two-site dynamical mean-field theory

It is shown that a minimum realization of the dynamical mean-field theory (DMFT) can be achieved by mapping a correlated lattice model onto an impurity model in which the impurity is coupled to an uncorrelated bath that consists of a single site only. The two-site impurity model can be solved exactly. The mapping is approximate. The self-consistency conditions are constructed in a way that the resulting ``two-site DMFT'' reduces to the previously discussed linearized DMFT for the Mott transition. It is demonstrated that a reasonable description of the mean-field physics is possible with a minimum computational effort. This qualifies the simple two-site DMFT for a systematic study of more complex lattice models which cannot be treated by the full DMFT in a feasible way. To show the strengths and limitations of the new approach, the single-band Hubbard model is investigated in detail. The predictions of the two-site DMFT are compared with results of the full DMFT. Internal consistency checks are performed which concern the Luttinger sum rule, other Fermi-liquid relations and thermodynamic consistency.Comment: LaTeX, 14 pages, 8 eps figures included, Phys. Rev. B (in press