Shawn P. Stapleton

An adaptive predistorter for a power amplifier based on adjacent channel emissions (mobile communications)

A slowly adapting predistorter is presented. The approach is to minimize the transmitter output power in spectral regions occupied only by intermodulation (IM) products. In this way, only a spot power measurement is required. This technique relies on the principle that the power amplifiers characteristics vary slowly with time. By monitoring the out-of-band power one can obtain an estimate for the distortion introduced by the power amplifier. Adaptation is accomplished by iterative adjustment of the predistorter parameters to minimize the IM power. For a polynomial predistorter, the authors analytically demonstrate that the IM power is a quadratic function of the coefficients. A variety of algorithms therefore apply. The authors present an analog static predistortion linearization circuit that uses the envelope of the baseband signal to generate the nonlinear functional used in predistorting the input signal. The improvement obtained with an amplitude-modulated input signal was 15 dB in the third- and 5 dB in the fifth-order intermodulation products. The IM improvement could be maintained with the use of a robust direct search algorithm. >

RF Class-D Amplification With Bandpass Sigma–Delta Modulator Drive Signals

The power efficiency of a RF Class-D amplifier with a bandpass sigma-delta (SigmaDeltaM) modulator is analyzed for a complementary voltage-switched configuration. The modulator broadens the application of the amplifier to include signals with time varying envelopes such as W-CDMA. The addition of a modulator introduces new design variables which affect amplifier power efficiency including coding efficiency and the average transition frequency of the pulse train. Design equations are derived for the optimum load impedance, output power, conduction losses, capacitive switching losses, and drain efficiency. The general design equations are consistent with both periodic and aperiodic drive signals. Analytic and simulated results are compared for an example design with pseudomorphic high-electron mobility transistor and metal-semiconductor field-effect transistor switches with a fourth-order bandpass SigmaDeltaM. The results show a drain efficiency of 52% with a 10-dB peak-to-average power ratio W-CDMA source signal at a frequency of 500 MHz

A highly efficient Doherty feedforward linear power amplifier for W-CDMA base-station applications

This paper presents a RF high-power Doherty amplifier for improving the efficiency of a 30-W feedforward linear amplifier used in wide-band code-division multiple-access (W-CDMA) base-station applications. A high-power Doherty amplifier using a single push-pull LDMOS field-effect transistor is proposed as the main amplifier of a feedforward linear amplifier. The peaking amplifiers compensation line and gate bias effects are analyzed at the 6-dB backoff point. From the experimental results of a forward-link one-carrier W-CDMA, a 2.2% power-added efficiency improvement at an adjacent channel leakage power ratio linearity of -60 dBc is achieved in comparison to a conventional feedforward class-AB amplifier.

An adaptive predistortion system

A slowly adapting predistorter is presented. The approach is to minimize the transmitter output power in spectral regions occupied only by intermodulation (IM) products. For single channel data signals, a suitable region is in the adjacent channel. This technique relies on the principle that the power amplifiers characteristics vary slowly with time. In general, the main performance drift in power amplifiers occurs because of transistor degradation, temperature changes, channel switching, and power supply variations; these characteristics do not require fast adaptation. By monitoring the out-of-band power one can obtain an estimate for the distortion introduced by the power amplifier and use this quantity to optimize the predistorter. The approach utilized to obtain a measurement of the out-of-band power is to perform a complex convolution of the RF input signal to the predistorter with the power amplifiers output signal.<<ETX>>

Simulation and analysis of an adaptive predistorter utilizing a complex spectral convolution

An adaptive predistorter for linearizing a power amplifier in a mobile transmitter is studied, and the analytical, simulation, and measured results are presented. This predistorter does not have the problems of loop delay or phase shifting in its feedback path. The feedback is used only periodically to update the predistorter parameters so that it adapts to changes in the amplifier characteristics. An adaptation method for predistorters of the polynomial type is described. Its complexity is significantly lower than that of the previously described methods and its convergence speed, though low, is more than sufficient to track amplifier drift. Analytical verification that the measurement of out-of-band power is sufficient to drive the adaptation, a complex convolution method for measuring the out-of-band power that requires no additional local oscillator or phase reference, and a demonstration of the system performance utilizing a 16-QAM signal in a 25-kHz bandwidth, centered at 850 MHz, are provided. >

Linearity optimization of a high power Doherty amplifier based on post-distortion compensation

The linearity of a 30-W high-power Doherty amplifier is optimized using post-distortion compensation. A balanced high-power Doherty amplifier using two push-pull laterally-diffused metal-oxide semiconductor (LDMOS) field-effect transistors (FETs) is linearized by optimum adjustment of the peaking compensation line, shunt capacitors, and gate biases. The measured results of an optimized Doherty amplifier for a four-carrier wideband code division multiple access (W-CDMA) signal, achieved -43 dBc adjacent channel leakage ratio (ACLR) at a /spl plusmn/5 MHz offset frequency. This is an ACLR improvement of 12.2dB and 6.5dB in comparison to the Doherty amplifier before optimization and a ClassAB amplifier, respectively.

Piecewise Pre-Equalized Linearization of the Wireless Transmitter With a Doherty Amplifier

This paper describes a predistorter (PD) based on piecewise pre-equalizers for use in multichannel wideband applications. The predistortion linearizer consists of piecewise pre-equalizers along with a look-up-table-based digital PD that together compensate for nonlinearities, as well as memory effects of power amplifiers (PAs). It takes advantage of multiple finite-impulse-response filters that significantly reduce the complexity when compared to memory polynomial methods. The proposed method was also compared with the conventional Hammerstein structure. A 300-W peak envelope power Doherty PA was first modeled by measured time-based samples in order to verify the adjacent channel power ratio (ACPR) performance in simulation using a multitone and single wideband code-division multiple-access (W-CDMA) carrier. Furthermore, the experimental results applying two W-CDMA carriers verify that the proposed method provided similar improvement to that of the memory polynomial approach. The experimental results verified the complexity reduction and superior ACPR performance over the conventional Hammerstein structure

A new technique for adaptation of linearizing predistorters

A technique for the adaptation of a predistorter is demonstrated. This technique does not provide real-time adaptation but only adjusts to the drifting characteristics of the power amplifier. This technique is based on the ability to sample the out-of-band power and use this scalar quantity to adjust the predistorter coefficients. The predistorter could be implemented in analog form or digitally. Analog predistorters can take the form of cubic predistorters consisting of antiparallel diodes with gain and phase adjustments, intermediate frequency analog polynomials, etc. Digital predistorters using this technique can consist of adjustments to the vector lookup table entries. It is demonstrated that this method requires only a simple adaptation strategy because of the quadratic shape of the surface contour.<<ETX>>

Equivalent circuit and capacitance of double barrier resonant tunneling diode

By measuring the scattering parameters (S parameters) with on‐wafer probing techniques, the small signal impedances of double barrier resonant tunneling diodes up to 18 GHz have been obtained. The high‐frequency impedance found can be successfully reproduced by a parallel R‐C circuit along with a series R‐L circuit. Our study indicates that the double barrier quantum well structure can be represented by the parallel R‐C circuit alone, while the inductance results from the air‐bridge lines to the probe contacts. The capacitance‐voltage (C‐V) relationship is also obtained. Well‐defined structures corresponding to the tunneling effect are found. An increase in capacitance after tunneling is found in the C‐V curve, and it is attributed to the charge accumulation at the quasibounded state within the quantum well.

Available load power in a RF class D amplifier with a sigma-delta modulator driver

A conventional RF class D amplifier is nonlinear and is limited to constant envelope signals. By augmenting a RF class D amplifier with a sigma-delta (/spl Sigma//spl Delta/) modulator, the class of signals is extended to include time varying envelope signals. The available load power delivered by a class D amplifier depends on the spectral properties of the /spl Sigma//spl Delta/ pulse train. A measure called coding efficiency is defined to quantify the spectral characteristics of the pulse train and the parameter is incorporated in class D design equations. The average pulse frequency of the /spl Sigma//spl Delta/ pulse train affects switching losses in the amplifier and the average pulse frequency for a fourth order bandpass modulator is simulated. A 1W 1 GHz voltage switched class D amplifier is designed and simulated to verify the coding efficiency and average pulse frequency parameters.