Daniel Sira

Output Power Control in Class-E Power Amplifiers

A technique is presented to facilitate power control of cascode class-E power amplifiers (PAs). It is shown that by controlling the signal applied to the gate of the cascode transistor, the transmit power is changed. The main advantage of the proposed technique is a high 36 dB output power control range (PCR) compared to 20 dB for the traditional approach. This fulfills the requirements of the GSM standard on the PCR at all power levels and all frequency bands (for GMSK modulation). The concept of the cascode power control of class-E RF PA operating at 2.2 GHz with 18 dBm output power was implemented in a 0.18 ¿m CMOS technology, and the performance has been verified by measurements. The PA has been tested by a single tone, and by a GMSK modulated input signal.

A cascode modulated class-E power amplifier for wireless communications

A cascode modulated CMOS class-E power amplifier (PA) is presented in this paper. It is shown that by applying a modulated signal to the gate of the cascode transistor the output power is modulated. The main advantage of the proposed technique is a high 35dB output power dynamic range. The peak power added efficiency (PAE) is 35%. The concept of the cascode power control of class-E RF PA operating at 2.2GHz with 18dBm output power was implemented in a 0.18@mm CMOS technology and the performance has been verified by measurements. The prototype CMOS PA is tested by single tone excitation and by enhanced data rates for GSM evolution (EDGE) modulated signal. Digital predistortion is used to linearize the transfer characteristic. The EDGE spectrum mask is met and the rms error vector magnitude (EVM) is less than 4^o in the entire output power range.

Modeling and predistortion of envelope tracking power amplifiers using a memory binomial model

Nowadays envelope tracking (ET) is considered one of the most appealing techniques for the efficiency enhancement of RF power amplifiers (PAs), but it also introduces a number of additional challenges for the system simulation and implementation. In this context, this paper aims to provide a new behavioral model capable of an improved performance when used for the modeling and predistortion of RF PAs deployed in ET transceivers. The proposed solution consists in a 2D behavioral model having as a dual-input the PA complex baseband envelope and the modulated supply waveform, peculiar of the ET case. The model definition is based on binomial series, hence the name of memory binomial model (MBM). The MBM is here applied to measured data-sets acquired from an ET measurement set-up. When used as a PA model the MBM showed an NMSE (Normalized Mean Squared Error) as low as -40dB and an ACEPR (Adjacent Channel Error Power Ratio) below -51 dB. The simulated predistortion results showed that the MBM can improve the compensation of distortion in the adjacent channel of 5.8 dB and 5.7 dB compared to a memory polynomial predistorter (MPPD). The predistortion performance in the time domain showed an NMSE improvement of 2.5 dB against the MPPD.

Memory models for behavioral modeling and digital predistortion of envelope tracking power amplifiers

Abstract New advanced Envelope Tracking (ET) techniques can provide RF (Radio Frequency) transmitters with high-efficiency Power Amplifiers (PAs). On the other hand, system complexity substantially increases, requiring more advanced PA models for the representation and compensation of ET PA distortion effects. In this context, this paper proposes some solutions for behavioral modeling and digital predistortion of ET PAs. The adopted modeling strategy consists in including the modulated supply voltage as an additional independent model variable to define more accurate behavioral models capable of an increased accuracy when applied to model and compensate ET PAs. The new model variable is included in a polynomial model with memory whose nonlinear structure is derived from a binomial power series, whence the name of Memory Binomial Model (MBM). Another modeling approach is subsequently proposed, where the Cann model for static PA AM/AM nonlinearities is extended to model both AM/AM and AM/PM dynamic distortion occurring in ET PAs. The Extended Cann model includes an MBM structure for modeling dynamic AM/PM distortion effects. Both modeling approaches are tested on measured data-sets acquired using an ET measurement set-up including a commercial PA from RFMD and an envelope modulator designed using a commercial IC from Texas Instruments. The measured results showed that the proposed models could obtain a better modeling and predistortion performance when applied to ET PAs, with respect to the Memory Polynomial Model, here considered as a reference to represent the state-of-the-art of PA modeling and digital predistortion.

On wafer X-parameter based modeling of a switching cascode power amplifier

X-parameters have been introduced as the natural extension of S-parameters capable of characterizing a nonlinear device excited by a large-signal input. This paper describes validation of the X-parameter model of a switching cascode power amplifier (PA), which has strong nonlinearity. The X-parameter model of the PA was measured and extracted by an Agilent N5245A PNA-X. Measurements were done on wafer and deem-bedded to the input and output pads of the device. An Enhanced Data rates for GSM Evolution (EDGE) signal was applied to the model for simulations. The simulated relative levels of output spectrum and RMS value of error vector magnitude (EVM) were compared with the measured data in order to validate the X-parameter model. A good match was achieved between the simulation and measurement. The maximum difference between the simulated and measured relative levels of output spectrum is 4 dB. The maximum error between the simulated and measured EVM is less than 3 %-point.

Process, Voltage and Temperature Compensation Technique for Cascode Modulated PAs

This paper presents a process, voltage and temperature (PVT) compensation method for a cascode modulated polar power amplifier (PA). It is shown that it is possible to create a baseband replica circuit of the PA that has the same AM-AM nonlinearity as the PA itself. The replica circuit, that represents a transistor level model (empirical model) of the cascode modulated PA, is utilized in a PA analog predistorter. The analog predistorter linearizes and compensates for PVT variation of the cascode modulated PA. The empirical model is placed in the negative feedback of an operational transconductance amplifier. The predistorted varying envelope signal is applied to the cascode gate of the PA. It is shown that the proposed PVT compensation technique significantly reduces the PVT spread of the PA linearity indicators and improves the PA linearity. Simulations were performed in a 0.13 μm CMOS process.

Wideband limit study of a GaN power amplifier using two-tone measurements

This paper studies the wideband limit (WBL) of a GaN RF power amplifier (PA). The WBL study is achieved by a PA characterization using two-tone measurements. The characterization method allows to identify the dependency of PA memory effects on the two-tone frequency spacing. PA memory effects (MEs) are measured using the opening in the AM/AM and AM/PM curves and they were found to be located in a limited range of tone spacings. The outcome of this characterization procedure is the identification of the PA wideband limit defined as the upper limit of the MEs frequency range. The most interesting phenomenon related to the WBL is that for all tone spacings beyond the WBL the AM/AM curves of the PA collapse to a quasi-static case, which is different from the static case. The presence of the wideband limit is not so evident from AM/PM curves, where the phase loop is not collapsing beyond WBL. The wideband limit is an important aspect of the DUT behaviour and can be used to improve the accuracy of the DUT behavioral model identifying its memory range.

Modeling of cascode modulated power amplifiers

Two models of the cascode modulated polar power amplifier (PA) are presented. The cascode modulated PA, that operates as a switch mode amplifier with class-E like output network, has a highly nonlinear transfer characteristic. The proposed empirical model is based on modeling of the peak drain current through the cascode connected transistors. A simplified analytical model, that uses mathematical expressions to describe the nonlinear transfer characteristic of the cascode modulated PA, is proposed. The behavior of the proposed baseband models is compared with the RF domain simulations in a 0.13µm CMOS process.