Optimization of Third-Order Intermodulation Product and Output Power from an X-Band MESFET Amplifier Using Volterra Series Analysis

A third-order analysis for accurately predicting large-signal power and intermodulation distortion performance for GaAs MESFET amplifiers is presented. The analysis is carried out for both single- and two-tone input signals using the Volterra series representation and is based only on small-signal measurements. Simple expressions for the nonlinear power gain frequency response, the output power, the gain compression factor, and the third-order intermodulation (IM/sub 3/) power are presented. The major sources of gain compression and intermodulation distortion are identified. Based on the developed nonlinear analysis in conjunction with the device nonlinear model, a systematic procedure for designing a MESFET amplifier under large-signal conditions for optimum output power and IM/sub 3/ performance is proposed. The method utilizes out of band computed matching compensation through a nonlinear model of the amplifier. The accuracy of the device large-signal and IM/sub 3/ distortion characterization and the practicability of the proposed method are illustrated through comparison between measured and predicted results.     

Third-Order Intermodulation Distortion and Gain Compression in GaAs Fet's

A simple unilateral nonlinear circuit model of a GaAs FET is used in the analysis of the third-order intermodulation distortion and gain compression characteristics of a single-stage amplifier. Expressions are obtained for these characteristics, relating them to the input power level and to the device load admittance. The expressions are illustrated with contours on the load admittance plane of constant intermodulation distortion ratio, intercept point gain compression, AM-to-PM conversion, and output power, and as output power versus input power plots for fixed terminations. Agreement with experimentally measured distortion characteristics is good.     

Prediction of Gain Expansion and Intermodulation Performance of Nonlinear Amplifiers

A mathematical model for the input‐output characteristic of an amplifier exhibiting gain expansion and weak and strong nonlinearities is presented. The model, basically a Fourier‐series function, can yield closed‐form series expressions for the amplitudes of the output components resulting from multisinusoidal input signals to the amplifier. The special case of an equal‐amplitude two‐tone input signal is considered in detail. The results show that unless the input signal can drive the amplifier into its nonlinear region, no gain expansion or minimum intermodulation performance can be achieved. For sufficiently large input amplitudes that can drive the amplifier into its nonlinear region, gain expansion and minimum intermodulation performance can be achieved. The input amplitudes at which these phenomena are observed are strongly dependent on the amplifier characteristics.     

Calculation of Intermodulation from a Single Carrier Amplitude Characteristic

The intermodulation analysis presented in this paper is based on the important observation that any output component of a memoryless nonlinear device can be expressed as an integral of the product of two functions, one being the single carrier amplitude characteristic of the nonlinear device and the other being a function of the statistical parameters of the input signal. The analysis can also be extended to nonlinear devices with AM-PM conversion by expressing the amplitude characteristic as a complex function. The signal-dependent function is given analytically for two simple but important types of signals, for two sinusoidal carriers of equal level, and for Gaussian noise, and it is demonstrated that good agreement has been obtained when the method has been applied to microwave devices like high powerX-band klystrons and traveling wave tubes. It is also shown that the method is convenient for analysis of intermodulation in cascaded nonlinear elements for which the individual single carrier amplitude characteristics are known.     

Intermodulation Distortion and Gain Compression in Varactor Frequency Converters (Short Papers)

Using the nonlinear theory of Gardiner and Ghobrial gain compression in varactor frequency converters is characterized and related to the distortion performance of the device. It is shown that gain compression results from the generation of a current component at the sideband frequency which is in antiphase with the main sideband current component. It is also shown that under certain conditions, high levels of intermodulation distortion can exist with practically zero gain compression.     

Current-Pumped Abrupt-Junction Varactor Power-Frequency Converters

This paper presents equations and design curves for a noninverting frequency converter which will enable the engineer to design efficient, high-level parametric devices using abrupt-junction varactors. In addition, the excellent intermodulation characteristic and extremely wide dynamic range (in excess of 140 dB) of the parametric frequency converters enables their use immediately as low-frequency downconverters or microwave upconverters without any deterioration of other parameters, such as noise figure, in system performance. It has been shown that these abrupt-junction diode devices possess the largest known dynamic range, in addition to being relatively spuria free with respect to intermodulation products produced by the diode nonlinearity, intermodulation distortion being generated in the device due only to gain saturation. The design curves also indicate the maximum conversion efficiency possible with a given abrupt-junction diode. An inflection point for 50 per cent converison efficiency occurs for all diodes. Any additional improvement in pump-to-sideband efficiency greater than 50 per cent gained by adjusting the diode and circuit performance, requires relatively large increases in the diode cutoff frequency and reduction in overall circuit losses. Other design curves include impedance variation with drive powers and the overall limiting output power capability for a given diode. A design example is presented to demonstrate the usefulness of the derived results and design curves. The experimental results obtained with this design have demonstrated a microwave, C band, tunable converter with almost 50 per cent conversion efficiency.     

Intermodulation distortion of cascaded transistors

Intermodulation distortion performance of cascaded transistors is analyzed using a nonlinear frequency-dependent model for the transistor and with Volterra series as an analysis tool. The objective of this study is to determine the optimum cascade that has high gain and good modulation performance especially for higher frequencies. A particular application is in long-haul, analog, solid-state coaxial systems where intermodulation distortion is a critical problem. Both experimental as well as computer results are used in this analysis. It is shown, for example, that certain low distortion configurations can exhibit high distortion when cascaded. A few optimum cascade configurations are discussed in detail.     

Reduced intermodulation distortion in 1300 nm gain-clamped MQWlaser amplifiers

A 1300 nm gain-clamped DFB multiple quantum well laser amplifier with negligible pass band ripple, 20 dB fiber to fiber gain, and 10 dB reduction in gain saturation is demonstrated. The remaining gain saturation is attributed to longitudinal hole burning. After some modifications the reduction in gain saturation is improved to more than 30 dB for an input signal having the same polarization state as the lasing mode. From these experiments and a theoretical analysis it is concluded that there is a potential for realizing highly linear 1300 nm CATV semiconductor laser amplifiers using gain-clamping with less intermodulation distortion than today's directly modulated linear semiconductor lasers     

State-space synthesis of oscillators based on the MOSFET square law

A state-space synthesis of oscillators based on the large-signal square-law characteristic of a MOSFET in saturation is presented. The approach is an extension of the recently proposed MOSFET square-law filter synthesis methodology. Simulation results demonstrate that oscillation at megahertz frequencies is attainable. The oscillation frequency is also electronically tunable. The total harmonic distortion (THD) is simulated as 0.1-3% for output voltages between 80 mV and 1.1 V peak-to-peak, respectively. This oscillation amplitude can be controlled by an additional CMOS limiter     

AlGaAs/GaAs HBT linearity characteristics

Communication systems require linear power amplifiers with high efficiency and very low intermodulation distortion. AlGaAs/GaAs heterojunction bipolar transistors (HBT's) were found to have very low intermodulation distortion in power operation. Two-tone tests were carried out on both common-emitter (CE) and common-base (CB) power HBT's. At 7 GHz, the CE HBT showed -20 dBc IM₃ (third-order intermodulation ratio) and 12% power added efficiency (PAE) per tone at the 1 dB gain compression point; IM₃ dropped to -30 dBc at 1.5 dB output power backoff. The CE HBT has lower intermodulation distortion than CB HBT. Load pull data were collected to aid the understanding of the intermodulation. Parameters of the Gummel-Poon model (as used in SPICE) were derived for HBT's based on dc data and small-signal S parameters at various bias points. The accuracy and validity of the model were confirmed by comparison to experimental two-tone results. SPICE predicts that the emitter and base resistances linearize the HBT and reduce the third-order intermodulation distortion. The excellent third-order intermodulation performance of the CE HBT makes it a very attractive choice for linear power amplifiers     

A method to predict harmonic distortion in small-geometry MOS analog integrated circuits

A method to predict the small-signal linear gain and level of harmonic distortion in analog MOS circuits is presented. This method, based on a generalized nonlinear transfer function approach, lends itself to implementation in the AC small-signal analysis routine of the circuit simulation program SPICE. A low-frequency nonlinear distortion model based on the CSIM simulator MOSFET model is applied to three simple MOSFET circuits. Results presented emphasize the need to consider small-signal quantities in the development of MOSFET models and in the determination of device parameters. The method can be easily extended to include capacitive effects and a prediction of intermodulation distortion.     

Third-order intermodulation distortion in microwave amplifier

Third-order intermodulation distortions of a microwave GaAs f.e.t. amplifier are analysed using a simple nonlinear model for the device. If the amplifier has strong feedback at low frequencies, the second-order interactions may dominate and degrade the third-order distortions. Solutions are presented to eliminate this distortion degradation. The analysis is very well substantiated by measurements.     

采用90°分支电桥的C波段预失真线性化器

为了改善功率放大器的三阶交调失真,提出了一种基于90°分支线电桥的C波段预失真线性化器,使用肖特基二极管产生非线性信号。通过改变线性化器的偏置电压及电容,可调整线性化器的增益扩张和相位延迟特性,与功放级联后对功放的三阶交调失真有改善作用。将该线性化器应用到工作频率为7 GHz,饱和功率为20 dBm的放大器上,在输出功率回退5 dBm处对放大器的三阶交调有10 dBc的改善。     

双抽运光参量放大的增益特性

在相同条件下(如抽运光功率、信号光功率等),与单抽运相比,双抽运光参量放大(OPA)的增益大,增益带宽宽.由于信号光的增益特性与参量增益系数直接相关,因此,在建立高非线性光纤(HNLF)光参量放大的理论模型的基础上,对相位匹配条件、参量增益以及信号光饱和增益的特性进行了分析讨论,采用有限差分法,对增益饱和的特性进行了模拟.结果表明,参量增益和光纤参数、输入抽运光以及信号光的波长、功率有关,而信号光的饱和增益和输入抽运功率有关.     

Circuit Model for Characterizing the Nearly Linear Behavior of Avalanche Diodes in Amplifier Circuits

A nonlinear circuit model for avalanche diodes is proposed. The model was derived by assuming that the bias dependence of the elements in a known small-signal equivalent-circuit model for existing diodes arises in a manner consistent with the theory of an idealized "Read-type" device. The model contains a nonlinear RL branch, a controlled source, and a linear depletion capacitance. The model is used in the nearly linear sense to predict intermodulation distortion (IMD) and gain compression in avalanche diode amplifiers. Computed results for amplifiers with existing diodes are shown to be in good agreement with experiment.     

Nonlinear Circuit Design Using the Modified Harmonic Balance Algorithm

A modification to a harmonic balance algorithm allows the nonlinear analysis of circuits driven by two nonharmonically related input frequencies. The algorithm was implemented on an IBM AT Personal Computer. Three examples are presented to illustrate the analysis. The first is a novel wide-band FET frequency doubler that achieves an average conversion loss of 3.5 dB over the 8-16-GHz output band. The second example illustrates a technique used in the design of a C-band power amplifier in which third-order intermodulation distortion was reduced by 8 dB with two tones of 34 dBm each at the output. The final example illustrates the gain suppression of a smaller tone in the presence of a larger one of slightly different frequency in a limiting amplifier. Simulations agree with measurements in which 2.5-dB gain suppression was observed in a 2-GHz FET feedback amplifier driven into saturation.     

硅微波功率静电感应晶体管(SIT)非线性失真特性分析

本文讨论了介质盖栅SIT的基本特性。在SIT小信号S参数和静态特性的基础上,得到了SIT非线性等效电路模型。采用Volterra级数法,分析了包含三种主要非线性源的SIT微波功放的互调失真和增益压缩特性。指出引起SIT互调失真和增益压缩的主要原因是跨导G_m的非线性,其次是栅源电容C_(gs)的非线性变化和漏电导G_d的非线性变化的作用。还指出了SIT非线性失真与输入功率电平和偏置条件的关系。分析结果得到了实验的证实。     

微波GaAs FET功率放大器的一种建模与互调计算方法

本文提出了一种利用测量ＡＭ－ＡＭ、ＡＭ－ＰＭ特性对微波ＧａＡｓＦＥＴ大信号建模的方法．此外，利用测量或理论求得的ＡＭ－ＡＭ、ＡＭ－ＰＭ非线性特性，可进一步分析放大器的增益、互调性能，避免多音（射频）激励下进行测量或理论分析的麻烦．结果表明，理论与实验甚为一致．     

Signal distortion and noise in AM-SCM transmission systemsemploying the feedforward linearized MQW-EA external modulator

In subcarrier multiplexing (SCM) optical video distribution systems, the nonlinear signal distortion generated by the combined action of laser chirp and fiber dispersion limits the transmission distance. This paper for the first time shows that low-chirp MQW (multiquantum well)-EA (electroabsorption) external modulators are applicable to AM-SCM transmission systems. The feedforward linearization technique is used to compensate the signal distortion due to the nonlinearity in the modulator's L-V (light power versus voltage) characteristic. The effectiveness of this type of modulator for suppressing dispersion-induced distortion is clarified experimentally. A feedforward linearized transmitter with composite second order (CSO) intermodulation distortion <-62 dBc, composite triple beat (CTB) intermodulation distortion <-59 dBc and a carrier-to-noise ratio (CNR) >50.5 dB is successfully constructed. It is shown that the transmitter can achieve a CSO <-59 dBc, a CTB <-57 dBc and a CNR >49.5 dB for a 32 channel AM signal and 10 km transmission. Furthermore, estimations of the signal distortion cancellation and the noise characteristic achieved with feedforward circuit modeling are shown. The results obtained here emphasize that MQW-EA external modulators are applicable to AM-SCM transmission systems     

Nonlinear space charge wave theory of distortion in a Klystron

We present a new view of nonlinear distortion in a klystron based on an analytically solvable nonlinear Eulerian model. The nonlinear contributions to the analytic solutions for the beam modulations are "nonlinear space charge waves" in the sense that they are produced by the nonlinear mixing of the linear space charge waves. For a single-frequency input, amplitude and phase distortion are shown to be results of "self-intermodulation" at the drive frequency, or mixing of harmonic distortions with the fundamental. The self-intermodulation contributions add out of phase with the linear space charge waves to produce gain compression and phase distortion. By comparing the results to a conventional large-signal Lagrangian model we find this physical picture is accurate for drive levels up to 1.2 dB of gain compression. For a two-frequency input we predict the third-order intermodulation distortion generation and suppression with the nonlinear space charge wave theory.