Small-signal Conditions Research Articles

A new model for the dual-gate GaAs MESFET

The development of a novel GaAs dual-gate MESFET model suitable for the design and analysis of microwave circuits is described. This quasi-two-dimensional physical model is numerically efficient due to a unique formulation of the carrier transport equations. The model includes a comprehensive description of the geometric and material parameters accounting for recess structures, nonuniform doping profiles, current injection into the buffer layer, forward-biased gate conduction, and surface depletion. The accuracy of the model under DC, small-signal, and large-signal operating conditions is assessed by comparing simulated and measured performance.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

The small-signal theory of the cyclotron maser and other gyrotron-type devices

The simplified cyclotron maser model presented by Lau (ibid., vol.ED-29, p.320-35, 1982) is analyzed under small-signal conditions with an exact analytical solution to the proper boundary condition taken across a thin, hollow rotating beam. Not only is the transverse electric field discontinuous but so are the axial and radial magnetic field components. The simultaneous presence of TE and TM modes are considered, leading to modifications in the dispersion relationships. The simultaneous inclusion of both modes and space charge leads to the main contribution to the theory: TE-TM coupling, off-resonance behavior, and subcutoff operation in the drift spaces of klystron-type devices. Also, since radial displacement of the beam is taken into account, the theory predicts a peniotron interaction in the cyclotron maser. Examples are presented of gyrotron or peniotron interactions in either the TE and TM modes as well as off-resonance and subcutoff. >

Radiative lifetime in semiconductors for infrared detection

In a previous paper, it was shown that the performance limit of purely radiative IR photon detectors is the background limit, and that the radiative lifetime of van Roosbroeck and Shockley is greatly modified by reabsorption effects. This paper extends the treatment to partially radiatively limited materials in a more general way not restricted to small signal conditions. It is shown that the widespread use of the internal radiative lifetime as a limiting factor in IR detector performance is incorrect by a factor of 25 or more, (the same factor as relates internal and external luminescence efficiencies) and the correct form of the radiative lifetime including reabsorption effects is derived. The detectivity of photoconductive detectors is calculated for a range of internal radiative efficiencies. Large improvements in performance are fundamentally possible if material quality can be improved.

Linearity and efficiency improvement by active compensation in IMPATT amplifiers

Active compensation is a successful microwave circuit technique previously developed to improve the gain-bandwidth performance of reflection amplifiers under small-signal conditions. The letter describes how this technique can be used to enhance the linearity of IMPATT amplifiers under large-signal conditions.

Dispersive transport of charge carriers in polycrystalline pentacene layers

This paper presents an experimental study of carrier drift in polycrystalline pentacene layers under small-signal conditions. The shape of the transient current, caused by pulsed bulk carried generation, and the transit time field-dependence are analysed in detail for various temperatures. The data obtained indicate a dispersive transport regime. A theory of dispersive transport with bulk generation is formulated in order to explain the experimental results. The theory is based on the concept of transport controlled by carrier trapping on localised levels distributed over a wide energy region. Particular attention is given to the exponential and Gaussian models of trap energy distribution. The Gaussian model seems more realistic for interpretation of experimental data on polycrystalline pentacene layers.

Predicted large-signal performance improvement by active compensation in microwave IMPATT amplifiers

The letter describes a technique whereby the large-signal linearity of a one-port microwave amplifier can be improved significantly. It uses a technique known as active reactance compensation previously developed to improve the gain-bandwidth behaviour of amplifiers under small-signal conditions.

Physics and classification of fast-wave devices†

Abstract This paper presents a new approach to the analysis and understanding of beam wave interaction taking place in fast-wave devices (beamlet theory). The theory takes fully relativistic effects into account, and, as it turns out, is applicable to slow-wave interactions as well. This is achieved by defining a generalized electromagnetic field whose polarization, phase velocity and origin need not be known to solve the relativistic equation of motion under small-signal conditions. As a result, two distinct but fundamental classes of interactions can be defined, referred to herein as synchronous and resonant. The characteristics of both classes of interactions are discussed in details and exemplified by the interactions taking place in gyrotrons and peniotrons. As a consequence, a systematic classification of commonly used high power microwave tubes is presented, and two new TM interactions occurring close fo waveguide cut-off are identified. The Pierce gain parameter is defined in terms of coupling and beam impedances, showing the gain of synchronous and resonant interactions to be respectively proportional to the cube and square root of the beam current.

Space charge perturbed transient transport in a dispersive medium a-As 2Se 3

Transport of an injected sheet of flash photoexcited holes is compared under small and large signal conditions in the same a-As 2Se 3 specimen films. Under small signal conditions the injected charge packet transits the specimen bulk in a uniform field. Under lage signal conditions nearly a CV of charge is injected and transport therefore proceeds under space charge perturbed conditions. Many experimental comparisons between transport under small and large signal conditions have been made in non-dispersive media. Features unique to transient transport under space charge perturbed conditions in nondispersive systems are theoretically predicted and typically observed. In a-As 2Se 3, a prototypical dispersive medium, no analogous distinction is observed between transient transport behavior under small and large signal conditions. Key characteristics of dispersive transport under small signal (pulse injection) conditions for example, the computed transit times, universality of the transient current pulse shape with respect to field and the correlation between current pulse shape and field dependence of the transit time are unchanged in the large photoexcitation limit. Temperature dependence of the dispersion parameter α is observed. It is also unaffected by photoexcitation intensity.

Experiments on the single woodwind tone hole

Impedance parameters associated with the lumped T circuit representing the tone hole in the woodwind musical instrument transmission-line model are measured for two types of tone holes. One type has a diameter large relative to the height of the tone-hole chimney, as on the saxophone. The other type has a chimney height nearly equal to the hole diameter, as on the clarinet. The imaginary parts of the series and shunt impedances, measured under small signal conditions, agree with predicted values, and the closed-hole series impedance data agree with earlier data of Nederveen and van Wulfften Palthe [Acustica 13, 65–70 (1963)]. A semi-empirical expression is deduced, by use of measurements of Bouasse [Tuyaux et Resonators (Librairie Delagiare, Paris, 1929), p. 239], Benade and Murday [J. Acoust. Soc. Am. 41, 1609 (1967)], and the theory of Keefe [J. Acoust. Soc. Am. 72, 676 (1982)], for the open-hole effective length te of a tone hole with a pad suspended above it. This expression correctly predicts the values of te for the two types of tone holes measured as well as for the flute tone-hole data obtained by Coltman [J. Acoust. Soc. Am. 65, 499–506 (1979)]. The open-hole shunt resistance, which includes radiation loss from the open-hole, viscous shearing loss in the bulk of the air, and wall loss at the sharp corners of the hole and within the hole chimney, is measured. The ratio of the radiation resistance to the total shunt resistance for a single hold varies systematically with frequency. This ratio is low at low frequencies due to large losses at the corners and edges, is a maximum near 750 Hz for sax-type holes and decreases again at higher frequencies due to increased shearing losses.

Dispersion characteristics for a gyro-TWA operating under small signal conditions

Frequency characteristics representing the small signal interaction process which can occur between a ‘ fast ’ electromagnetic wave and a relativistic spiralling beam are presented in Pierce form, with gain and phase parameters x and y plotted as functions of the normalized frequency. When depieted in this way the precise nature of the set of coupled waves which is required to satisfy the beam-present boundary conditions is more clearly expressed, and the significance of neglecting the backward wave in constructing the cubic form of the dispersion relation is more readily demonstrated.

Sensory transduction in an insect mechanoreceptor: extended bandwidth measurements and sensitivity to simulus strenght

The dynamic properties of sensory transduction in an insect mechanoreceptor, the femoral tactile spine of the cockroach, Periplaneta americana, have been studied by measurement of the frequency response function between randomly varying movement of the tactile spine and afferent action potentials from the sensory neuron which innervates it. The frequency response function of the mechanoreceptor has been characterized over a frequency range which is more than ten times larger than has previously been used for this preparation. Also the effects of varying the amplitude of the stimulating signal have been studied by the use of a range of input signal strengths from about 0.5 to 10 μm R.M.S. displacement. The measured frequency response functions can all be well fitted by a theoretical relationship which is a fractional exponent of complex frequency, provided that the time delay caused by conduction of the action potentials from the sensory dendrite to the recording electrodes is taken into account. Under small signal conditions the exponent of complex frequency is close to 0.5 but with larger displacements its value decreases to about half this value. The overall sensitivity of the receptor, as measured by the gain of the frequency response function at a natural frequency of 1 radian/s, is not significantly altered by changes in the input movement amplitude, so that the receptor behaves linearly in this respect. However, the mean rate of action potential occurrence is not linearly related to input movement amplitude. These results are discussed in terms of current theories of sensory transduction and the possible role of tubular bodies in the dynamic behaviour of insect cuticular mechanoreceptors.

Phototransduction in the fly compound eye exhibits temporal resonances and a pure time delay.

Photoreceptor cells in both vertebrates and invertebrates respond to a flash of light with a slow graded change in membrane potential, which is generally depolarising in invertebrates and hyperpolarising in vertebrates. Although some of the early photochemical and biochemical stages of the transduction process have been elucidated in both cases, these reactions are fast compared with the time course of the electrical response, which is typically hundreds of milliseconds long. To explain this slow response in the eye of Limulus, Fuortes and Hodgkin proposed a mechanism in which the light signal passes through a cascade of simple low pass filters. The model was later defined more specifically in terms of a chain of chemical reactions linked together through products and reactants, and has been used with small modifications and different numbers of stages to account for the behaviour of various vertebrate and invertebrate photoreceptors including the fly compound eye. I have now obtained evidence that phototransduction in the fly in small signal conditions involves underdamped resonance behaviour and a significant pure time delay, neither of which can be accounted for by the conventional cascade model.

The transient response of quasi-two-dimensional semiconductors under hot electron conditions

In calculating the transport properties of semiconductors in the presence of high electric fields, one must consider the effects of both the momentum, energy, and population-transfer relaxation times of the individual valleys, or quantum levels in a quasi-two-dimensional system. In recent calculations of hot electron microwave conductivities of quasi-two-dimensional silicon 〈100〉 samples it was found that the microwave conductivity at a fixed bias electric field showed non-monotonic behavior when plotted as a function of frequency, and has a conductivity peak in the high frequency region. To further clarify this conductivity peak and its implications for velocity overshoot, we have carried out transient response solutions to the electron transport for both large-signal and small-signal conditions. In this paper, the results of these calculations for electrons in a 〈100〉 inversion layer are reported. The response of the electron gas to a large dc field step and to a small step applied upon a large dc bias field are calculated. Much of the velocity overshoot and ac conductivity peaking are related to the differential repopulation among the inequivalent valley sub-bands.

The transmission of information by first and second order neurons in the fly visual system

1. Random (white noise) fluctuations of both point and wide field light sources were presented to eyes of the fly,Calliphora stygia. At the same time, intracellular recordings were made from photoreceptor cells and from large monopolar cells (LMCs) in the lamina ganglionaris. 2. Fourier analysis was used to determine the linear frequency response functions and coherence functions between the input light fluctuations and the resultant fluctuations in membrane potentials of the two types of cells. 3. The frequency response function of the receptor-to-LMC pathway was estimated by dividing the overall light-to-LMC frequency response function by the light-to-receptor frequency response function. For small signal conditions this function is primarily a phase shift of 180° (inversion) and some increase in sensitivity with frequency. 4. Comparison of results obtained with point and wide field stimuli did not reveal any significant differences between the frequency response functions of receptors or LMCs to either stimulus. No evidence of lateral interaction between receptors or LMCs was found. 5. For a linear system the coherence function provides a measure of the signal-to-noise ratio as a function of frequency. Comparison of the coherence functions obtained from receptors and LMCs showed that there is no significant improvement or deterioration in the signal-to-noise ratio as the signal passes from receptors to LMCs.

Electrode kinetics, equivalent circuits, and system characterization: Small-signal conditions,

Electrode kinetics, equivalent circuits, and system characterization: Small-signal conditions,

Electrode kinetics, equivalent circuits, and system characterization: Small-signal conditions

The small-signal steady-state response around the point of zero charge of an electrode/material system is examined for an unsupported electrolyte (material) with two species of charge carrier of arbitrary mobilities and valence numbers and with arbitrary intrinsic/extrinsic conduction character, taking full account of bulk, electrode reaction, sequential adsorption, and diffusion processes. The exact solution of the transport equations of the problem for generalized Chang-Jaffé single-point boundary conditions is compared with the responses of a variety of plausible equivalent circuits, using a complex least squares fitting technique. A hierarchical circuit is found which closely reproduces the exact results when charge of one sign is completely blocked without adsorption, except for some of the cases in which diffusion and reaction effects interfere with each other. The circuit is composed of frequency-independent lumped capacitances and resistances separately identified with bulk, reaction, and adsorption/reaction processes and a single, finite-length, Warburg-like impedance for diffusion effects. Relations between the circuit elements and microscopic electrode/material parameters are found and apply irrespective of the time-frequency overlap between bulk, reaction, and adsorption processes. It is also found that the reaction and adsorption resistances and the adsorption capacitance are all strongly interrelated. The circuit may be used with simultaneous non-linear least squares fitting of the real and imaginary parts of experimental impedance data to obtain estimates of the values of circuit elements and thus of the values of the microscopic parameters characterizing the electrode/material system. The relationship of small-signal response for overpotential-dependent electrode kinetics to that obtained for Chang-Jaffé boundary conditions is then considered. The reaction resistance and adsorption capacitance are found to be formally identical for Butler-Volmer (or Butler-Volmer-like) and Chang-Jaffé conditions. In the d.c. limit these quantities are unchanged, for the boundary conditions just mentioned, when a supporting electrolyte is added. A transformation of variables method is described which permits one to determine the smallsignal impedance of an electrode/material system with general overpotential-dependent firstorder electrode reaction kinetics from a compact-layer model for the small-signal overpotential and the small-signal response obtained for Chang-Jaffé boundary conditions. Exact impedance results are given for the case of a single species of mobile charge carrier. Analysis of this case indicates that the overpotential dependence of the boundary conditions has negligible effect on the small-signal response unless the cell is of microscopic thickness or the Debye length is comparable to the compact layer thickness and the electrode reaction is slow.

Electron trajectories in the PPM-focused coupled-cavity TWT

Trajectories of electrons in a coupled-cavity TWT with ppm focusing are plotted in both <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">r-z</tex> and <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x-y</tex> views. Plots are given for magnetic fields from 1.0 to 1.9 times the Brillouin field, and for small signal and saturated RF conditions. It is shown that a suitably programmed field can control the beam entry problem and minimize the RF debunching, while significantly reducing the beam exit angle. The computer program is briefly described.

Rotational nonequilibrium and line-selected operation in CW DF chemical lasers

A numerical analysis of rotational nonequilibrium effects in CW DF chemical lasers is presented. The rotational master equations incorporate the effects of simultaneous mixing, chemical pumping, rotational cross relaxation, and radiative emission. Rotational cross relaxation is described by exponential gap models in which ΔJ is unrestricted. Results of this analysis indicate that rotational equilibrium can be a poor assumption for high J levels even under small-signal conditions. The model predictions are in good qualitative agreement with observed multiline resonator spectra. Performance loss due to rotational nonequilibrium is predicted to be modest (∼15 percent) for multiline operation, with larger losses predicted for line-selected modes. Criteria for efficient line-selected operation are presented.

On the large-signal behavior of transferred-electron amplifiers with a cathode notch

Transferred-electron amplifiers (TEA) are finding applications where medium powers are required as driving stages. Most work has been done up to now under small-signal conditions. The present paper describes theoretical results of a large-signal analysis. From this analysis, it appears that different large-signal behaviors of TEA devices can be associated with their corresponding dc field distribution. We compare, by computer analysis, the evolution of the large signal impedance with the RF driving signal for different doping profiles (different stationary fields) and give design figures to increase the device added power.

IMPATT Pump Sideband Noise and its Effect on Parametric Amplifier Noise Temperature

This paper describes an investigation of the amplitude modulation (AM) noise sidebands of silicon IMPATT microwave oscillators and the effect these noise sidebands have on the excess noise temperature of parametric amplifiers. It is shown that the noise temperature may be affected under both large- and small-signal conditions to an extent which depends on the level of pump sideband noise. Simple relationships between easily measurable amplifier properties and pump noise power are given which enable the performance of any combination of pump and amplifier under the two signal conditions to be predicted. A method which simultaneously eliminates the small-signal effect and reduces the large-signal effect to an acceptable level is proposed and demonstrated to be a practical solution.