This paper presents a comprehensive study of intermodulation-distortion response asymmetries often observed in microwave nonlinear systems subject to a two-tone or multitone test. The reasons for the different amplitudes of the two adjacent tones are first investigated under small- and large-signal regimes, using a general circuit with frequency-dependent embedding impedances and resistive and reactive nonlinearities. It is shown that this intriguing phenomenon can be mainly attributed to the terminating impedances at the baseband or difference frequencies. Multitone behavior is also addressed and its main differences from the two-tone case explained. Those theoretical conclusions are then extrapolated for real circuits and validated by measured results obtained from microwave power amplifiers of two different technologies, i.e., a GaAs MESFET and an Si bipolar junction transistor.

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A comprehensive explanation of distortion sideband asymmetries

In this paper, the intermodulation distortion (IMD) behavior of LDMOS transistors is treated. First, an analysis is performed to explain measured IMD characteristics in different classes of operation. It is shown that the turn-on region plays an important role in explaining measured IMD behavior, which may also give a clue to the excellent linearity of LDMOS transistors. Thereafter, with this knowledge, a new empirical large-signal model with improved capability of predicting IMD in LDMOS amplifiers is presented. The model is verified against various measurements at low as well as high frequency in a class-AB power amplifier circuit.

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Prediction of IMD in LDMOS transistor amplifiers using a new large-signal model

The intermodulation performance of an FET in the common-source configuration is dependent on the impedance presented to its gate and drain terminals, not only at fundamental, but also at harmonic and baseband frequencies. At baseband frequencies, these terminating impedances are usually determined by the bias networks, which may have varying impedance over the frequencies involved. This can give rise to asymmetry in two-tone intermodulation levels, and changing intermodulation levels with tone spacing, as previous studies have shown. In this paper, an FET is analyzed to gain an understanding, useful to the circuit designer, of the contributing mechanisms, and to enable the prediction of bias points and the design of networks that can minimize or maximize these effects. Compact formulas are given to facilitate this. An amplifier was tested, showing good agreement between the theoretical and measured results.

Effect of baseband impedance on FET intermodulation

In this paper a new large-signal metal semiconductor field effect transistor (MESFET) model suitable for applications to nonlinear microwave CAD has been developed and the different phenomena involved in the nonlinear behavior of the transistor have been studied. The importance of this work lies in the fact that multibias starting points (hot and cold device) for pulsed measurements are used to derive a single expression for I/sub ds/ that describes the dc as well as the small and large signal behavior of the transistor, while taking into account the quiescent point dependence. The algorithms of this new model can easily be incorporated into commercially available nonlinear simulators. The operating-point dependent current I/sub ds/ is modeled by two nonlinear sources: one of them is the dc characteristic nonlinear equation, and the other represents the differences between dc and pulsed characteristics at every bias point. A complete large-signal model is presented for a 10*140 /spl mu/m GaAs-MESFET chip (F20 process) from the GEC-MARCONI Foundry and a 16*250 /spl mu/m MESFET chip (DIOM process) from the Siemens Foundry. Comparisons have been made between simulations and measurements of pulsed characteristics at different operating points. There was very good agreement between the P/sub in//P/sub out/ measurements and the MDS simulations using the complete large signal model.

Extracting a bias-dependent large signal MESFET model from pulsed I/V measurements

The authors show that the Taylor-series coefficients of a FET's gate/drain I/V characteristic, which is used to model this nonlinearity for Volterra-series analysis, can be derived from low-frequency RF measurements of harmonic output levels. The method circumvents many of the problems encountered in using DC measurements to characterize this nonlinearity. This method was used to determine the incremental gate I/V characteristic of a packaged Aventek AT10650-5 MESFET biased at a drain voltage of 3 V and drain current of 20 mA. The FET's transconductance was measured at DC, and its small-signal equivalent circuit (including the package parasitics) was determined by adjusting its circuit element values until good agreement between calculated and measured S parameters was obtained. The FET was then installed in a low-frequency test fixture. Excellent results were obtained. >

Modeling the gate I/V characteristic of a GaAs MESFET for Volterra-series analysis

This paper discusses, in a mathematical form and supported by a complete special experimental characterization, the gate-to-source nonlinear capacitor contribution on small-signal intermodulation distortion (IMD) as well as other nonlinear related phenomena such as the onset of phase distortion and gain compression in GaAs FET devices. A simplified one-sided version of our previously proposed test setup and its corresponding characterization formulation are shown to conform a direct technique to extract the second- and third-order coefficients for the Cgs(Vgs) Taylor-series expansion. The extracted terms let us evaluate some of the most widely employed equations for this reactive nonlinearity according to their capability of reproducing its small-signal nonlinear distortion contribution. They are also shown to be responsible for some previously detected differences on IMD behavior at high frequencies and for significant variations on the load selection criteria for high carrier-to-intermodulation ratio and high output-power tradeoffs.

Characterizing the gate-to-source nonlinear capacitor role on GaAs FET IMD performance

This paper describes a systematic methodology for complete stability analysis of nonlinear microwave multifunction circuits. The proposed strategy has two different stages: the stability analysis of a nominal steady-state solution and the use of continuation techniques to efficiently determine the unstable operation ranges. The stability analysis is demanding due to the multiple loops contained in the large multifunction circuit. The first step is to check the possible fulfillment of the oscillation startup conditions at different circuit nodes followed by pole-zero identification. Given the complexity of the circuit topology, a systematic technique is necessary for the selection of the observation nodes. This has been applied at both the lumped-element schematic and the layout levels. These stability analyses have been carried out at small-signal (linear) and large-signal (nonlinear) since the multifunction circuit includes a nonlinear mixer. In the case of instability, the origin of the oscillation and its characteristics are analyzed versus the critical circuit parameters through the application of continuation techniques to the steady-state oscillatory solution. Moreover, sensitivity yield analysis and variations of environmental conditions combined with the stability techniques have also been taken into account and integrated into the design cycle. The proposed systematic approach has been successfully applied to determine and correct an oscillation of a multifunction monolithic-microwave integrated-circuit converter. It has also been proven in other multifunction circuits in the same way.

Complete Stability Analysis of Multifunction MMIC Circuits

In this work, a continuous nonlinear model for the I/sub ds/ current source in recessed gate pseudomorphic HEMT heterostructures is proposed. A careful characterization of the DC, pulsed, and small-signal nonlinear distortion behaviour of this predominant nonlinearity has been employed in order to extract the model parameters. Being able to reproduce the current-voltage (I-V) behaviour as well as the higher order derivatives of the transconductance and output conductance, the equation is valid for an accurate control of the critical nonlinear distortion phenomena in HEMT applications. Comparisons between measured and simulated results prove its validity under both static and dynamic conditions for either large- or small-signal operation.

Accurately modeling the drain to source current in recessed gate P-HEMT devices

A new set of pseudoempirical equations is presented in order to simulate the optical and bias dependencies of GaAs MESFET junction capacitances, which is valid for the whole I-V plane. The variations induced in the small-signal equivalent circuit by the optical illumination are extracted from on-wafer scattering parameter measurements. New linear and quasi-logarithmic variations versus the incident optical power are shown for gate-drain and gate-source (Cgd and Cgs) capacitances. Furthermore, experimental results are in very good agreement with the simulated values for a wide range of optical power and bias conditions. Large signal MESFET models show a better fit with measured S-parameters than those previously published, leading to a greater degree of confidence in the design of photonic monolithic microwave integrated circuits.

An accurate photonic capacitance model for GaAs MESFETs

This paper discusses the gate to source nonlinear capacitor contribution on small signal intermodulation distortion (IMD) performance of FET devices. The second and third order coefficients for the Cgs(Vgs) Taylor-series expansion, experimentally extracted with a simplified one-sided version of our previously proposed test set-up, are shown to be responsible for some detected differences on IMD behaviour at high frequencies.

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J.L. Garcia

Papers

Characterizing the gate-to-source nonlinear capacitor role on GaAs FET IMD performance

Complete Stability Analysis of Multifunction MMIC Circuits

Accurately modeling the drain to source current in recessed gate P-HEMT devices

An accurate photonic capacitance model for GaAs MESFETs

Characterizing the gate to source nonlinear capacitor role on FET IMD performance