View Full Document

Linearity of X-band Class-B Power Amplifiers



View the full content.
View Full Document
View Full Document

9 views

Unformatted text preview:

Linearity of X band Class B Power Amplifiers in a Digital Polar Transmitter Narisi Wang Nestor D Lopez Vahid Yousefzadeh John Hoversten Dragan Maksimovic and Zoya Popovic Department of Electrical and Computer Engineering University of Colorado Boulder Colorado 80309 USA Abstract This paper discusses the linearity of a class E Xband PA in a digital polar transmitter The PA is nonlinear since it is compressed by 2 2 dB when achieving a 59 PAE at 10 GHz Load pull is performed under a two4one test and the resulting optimal efficiency impedance is found to be over 30 different from the single4one load pull In addition standard two4one load pull measurements indicate that the impedance for max P0ut coincide with worst nonlinearity IMD Surprisingly in the two4one polar load pull impedance for max P0ut differs significantly from the impedance corresponding to worst IMD Therefore under polar modulation a different matching circuit should be designed for optimal linearity and efficiency The PA is part of a digital polar transmitter which enables linearization including phase predistortion with IMD levels over 11 dB lower than in the standard two4one test Load pull performed on the polar class E PA shows that the level of intermodulation products varies for different signals input into the PA Index Terms Envelope elimination and restoration polar modulation envelope amplifier class E amplifier I INTRODUCTION M ODERN communication systems are forced to continually improve their spectral efficiency in an effort to achieve higher bitrates in limited bandwidth This improvement is typically manifested as a complex scheme in which both amplitude and phase of the RF carrier are modulated at a high rate and over a wide dynamic range Such modulated signals demand very linear power amplifiers which are operated typically in class A or backed off class AB modes These linear operating modes limit the efficiency of the PA to around 30 A number of researchers have investigated supply modulating techniques that can simultaneously provide high efficiency and good linearity and most notable examples are Envelope Elimination and Restoration EER polar modulation and dynamic biasing 1 4 These types of transmitters are more complex and the total transmitter efficiency depends on both the PA and the supply modulating circuit efficiency In this paper we examine the linearity of a ultra nonlinear high efficiency switched mode PA in a digital polar transmitter The RF carrier is at 10GHz which is a higher frequency than most current commercial communication systems but is a common frequency in other applications such as radar At carrier frequencies above 2 GHz the circuit parasitics and device nonlinearities are more pronounced and difficult to model The class E X band PA used in this study has been demonstrated in a two stage efficient PA 5 and its linearity 1 4244 0688 9 07 20 00 C 2007 IEEE Liner assisted sAitchirg iplifier Fig 1 Block diagram of a polar system with FPGA digital control The RF PA is a class E 1O GHz MESFET amplifier The envelope signal is split into a low frequency component which controls a DC DC converter and a high frequency component which provides additional envelope AC variations Phase variation is achieve with a digitally controlled phase shifter Fig 2 Pout solid line and PAE dashed line load pull contours for MESFET AFM04P2 Alpha industries The class E impedance 27 2 j31 4Q is indicated with x and EER operation were investigated in 6 The class E mode of operation lends itself naturally to EER and polar transmitter architectures It can be shown e g in 5 that the output voltage into a fixed resistive RL load is proportional to the supply voltage VDD Vout 26 fs Co RERL VDD 1 where RE is the real part of the optimal impedance presented to the device for class E switched mode operation fs is the operating frequency which is also the switching frequency and C0ut is the output capacitance of the active device In addition 1083 20 10 7 E O 1 8dBc 19 5dB 0 0 2O2020 0 1 30 1 E 0 0 1 o 40 o 5 Input Power dBm 50 1 0 0 0 5 Offset Frequency MHz a b c Fig 3 a Single tone power sweep for class E PA at 10 GHz b two one output spectrum for 200 kHz offset between tones carrier frequency is 10 GHz and c MESFET Pout and worst IMD3 load pull contours Class E impedance is indicated with an x for this impedance PAE is 45 with 17 5 dBm of Pout Optimum PAE of 48 is obtained for 22 j38Q o with 17 5 dBm of Pout Worst IMD3 and maximum Pout contours coincide the optimal efficiency and optimal load impedance are ideally not affected by the bias variation since the transistor current and voltage amplitudes only change with bias but not their time domain waveform shapes The power can theoretically vary from zero to the maximal available power but in practice the lowest power is limited by feedthrough and the maximal power is constrained by the power handling of the device As a result of the PA property described by Eq 1 the output voltage of a class E PA can be varied proportionally to variations in the supply voltage which is required for a polar transmitter in which the envelope of the modulated RF carrier amplitude modulates the output voltage while the phase of the modulated signal is directly input into the PA through the drive RF input This is similar to the analog EER technique 4 but can be implemented digitally as shown with the schematic in Fig 1 The digitally controlled drain bias provides amplitude modulation of the output voltage through a ultra efficient slow DC DC converter in combination with a fast less efficient linear amplifier which provides the AC portion of the signal envelope The signal is generated digitally and converted from IQ to polar form The digital control of the bias allows allocation of the amplitude modulation between the slow and fast circuits in order to optimize efficiency for different modulated signals possibly adaptively II CLASS E PA PERFORMANCE WITH A SINGLE AND Two TONE INPUT Practical class E PAs have achieved 96 drain efficiency in the low MHz range 9 around 90 at VHF 10 and as high as 70 at X band 5 In 6 class E PAs are examined and it is shown they can be linearized to some degree using polar modulation The class E PA used throughout this work was designed using design equations as in 8 and load pull characterization of a GaAs MESFET at 10GHz shown in Fig 2 The theoretical class E impedance calculated from the output capacitance of this device is 24 7 j28 4Q The maximum PAE


Access the best Study Guides, Lecture Notes and Practice Exams

Loading Unlocking...
Login

Join to view Linearity of X-band Class-B Power Amplifiers and access 3M+ class-specific study document.

or
We will never post anything without your permission.
Don't have an account?
Sign Up

Join to view Linearity of X-band Class-B Power Amplifiers and access 3M+ class-specific study document.

or

By creating an account you agree to our Privacy Policy and Terms Of Use

Already a member?