J. Vuolevi

Measurement technique for characterizing memory effects in RF power amplifiers

Memory effects are defined as changes in the amplitude and phase of distortion components caused by changes in modulation frequency. These are particularly important in cancelling linearizer systems, e.g., when distortion is reduced by similar distortion in the opposite phase. This paper begins by describing electrical and electrothermal causes for memory effects. A three-tone test setup is then constructed to measure the phase of third-order intermodulation distortion products. This paper also presents the measured results for a bipolar junction transistor and a MESFET amplifier.

Cancelling the memory effects in RF power amplifiers

Memory effects in RF power amplifiers render IM3 sidebands functions of tone spacing, thereby limiting the cancellation performance of predistortion techniques. Envelope filtering and envelope injection techniques can be employed to compensate for these effects and to achieve significant improvements in cancellation performance over a wide range of modulation frequencies. Predistortion with envelope injection is a promising choice of linearization in future applications.

Analysis of third-order intermodulation distortion in common-emitter BJT and HBT amplifiers

This transactions brief presents an electro-thermal Volterra model for calculating third-order intermodulation distortion (IM3) in common emitter (CE) bipolar junction transistor (BJT) RF amplifiers. The model includes nonlinearities caused by input-output cross products, which previous studies have tended to overlook, in spite of their significance for RF devices. The nonlinear I-V and Q-V sources of the model are presented also as functions of temperature to analyze how distortion is affected by dynamic temperature variations inside the device. The model is organized to facilitate the recognition of different IM3 components, especially those arising from out-of-band second-order distortion voltages. In addition, this transactions brief presents a technique for characterizing the nonlinearity coefficients of a RF power BJT and studies the behavior of intermodulation distortion as a function of bias point and of out-of-band impedance matching. Optimum bias and matching points are established for the test amplifier, and a good correlation is demonstrated between the calculated and measured data. Finally, this transactions brief shows that some serious memory effects cannot be seen when simulated using the traditional Spice BJT model, but can be detected using the polynomial Volterra model.

Extraction of a nonlinear AC FET model using small-signal S-parameters

The nonlinearities of an RF FET can be obtained by a set of small-signal circuit elements extracted over a range of terminal voltages and temperatures. This study used pulsed S-parameter measurements on a 3 /spl times/ 3 DC-bias-point grid at two different temperatures to obtain electrical and electrothermal nonlinearity coefficients up to the third order. The extracted nonlinear AC model can be used in a Volterra analysis to gain an insight into the distortion mechanisms. The measurement results were in good agreement with the calculated third-order intermodulation values.

Memory effects compensation in RF power amplifiers by using envelope injection technique

This paper addresses the compensation of memory effects in RF power amplifiers (PAs). Such effects impair the cancellation performance of most linearization methods. This study applies the envelope injection technique together with memoryless RF predistortion to a MESFET PA. The results, corroborated by simulations and measurements, show a good improvement in cancellation performance both in the modulation frequency and the amplitude domain. Also, the paper presents new simulation and measurement techniques for characterizing the optimum injection signal.

The effects of source impedance on the linearity of BTJ common-emitter amplifiers

Source impedance Zs plays an important role by minimizing the distortion of a common-emitter BJT-stage. Care has to be taken to achieve the proper impedance not only at the fundamental, but also at the frequencies of the 2nd-order signal. Volterra analysis is applied in order to gain an insight into distortion mechanisms. By providing the optimum Zs at different frequencies, significant improvement in linearity can be achieved.

Extracting a polynomial ac FET model with thermal couplings from S-parameter measurements

To simulate the linearity properties of RF FET amplifiers operating at moderate signal levels, this paper extends small-signal FET modelling and characterization to include the effects of nonlinearities. Pulsed S-parameter measurements are employed to avoid the problem of self-heating and to characterise electro-thermal distortion mechanisms. The extracted nonlinear ac model can be used in Volterra analysis to gain an insight into distortion mechanisms.

Measurement technique for increasing the linearity by optimising the source: impedance of RF power amplifiers

Out-of-band source impedances have a significant impact on the linearity of RF power stages. The source-pull measurement technique presented in this paper shows that linearity improvements up to 8 dB for a CE BJT stage can be achieved by providing optimum source impedance at the envelope frequency. If predistorters are used, the memory effects caused by the stage and predistorter can be partially cancelled by providing optimum source impedances at different envelope frequencies. As a result, the cancellation performance of the predistorter will be significantly improved.

Extraction of a polynomial LDMOS model for distortion simulations using small-signal S-parameter measurements

A method based on measured S-parameters for extracting a polynomial electro-thermal model of a 30W RF power transistor is presented. Once the package is de-embedded, the model for the intrinsic transistor can be calculated and distortion can be simulated. Simulated results match well with the measured values, revealing both thermal and electrical bandwidth dependencies of 3rd order intermodulation distortion (IM3). Component-wise Volterra analysis also shows that IM3 distortion is dominated by the nonlinearities of the input capacitance and gm in LDMOS type of transistors.

Detailed Analysis of IMD of HBT PA Based on VBIC Model

In this paper a recently proposed detailed distortion analysis technique is extended to apply in a rather complex VBIC model that contains multidimensional current and charge models. The technique is implemented on top of normal harmonic balance simulation, and it is based on fitting polynomial models for all nonlinear I-V and Q-V sources of the device, and using these to calculate the various contributions of IM3 distortion. The technique is used to analyze a 2 GHz InGa pHBT power amplifier that shows strong bandwidth dependent signal-induced bias shift