High Bandwidth Class-AB Amplifier with HighSlew Rate and Fast Current Sensing for Envelope

Tracking Applications

Punith R. Surkanti, Aditya A. PatH, Sri Harsh Pakala and Paul M. FurthVLSI Laboratory, Klipsch School of Electrical and Computer Engineering,

New Mexico State University, Las Cruces, NM 88003, USAEmail: {punith.aditya30.sriharsh and pfurth}@nmsu.edu

II. ENVELOPE TRACKING SYSTEM

Fig. 1: (a) Average power tracking and (b) envelope tracking power supply options fordriving RF power amplifier from [81

RFoUT

(b)

VSAT

RFoUT RF1N

(a)

VSAT

The envelope tracking system detects the envelope of theRF input signal and modulates the supply voltage of RFPA.The bandwidth of the envelope signal ranges from 1.4 MHzto 20 MHz. The supply modulator needs to be highly efficient

signal VAPT , that is proportional to average RF output powertransmitting over a single time frame. The APT output istypically generated by a high efficient DC-DC converter. Whilethe discrete Vee levels aid in improving the PA efficiencywhen compared to a fixed Vee, the maximum achievableefficiency is severely limited by the very slow transitions inthe supply voltage relative to the rapid variations in RFIN'Spower level.

An evolutionary method building on the concept of averagepower tracking is the envelope tracking (ET) technique [8]. Inthis scheme, shown in Fig. 1(b), the power supply of the PAis constantly adjusted based on the envelope information ofthe RF1N signal, VENV . An adaptive supply that is drivenby VENV is used to modulate Vee of the RFPA. This schemeallows a close tracking of the instantaneous power levels of theinput signal through the envelope information, thus providingsubstantial improvement in efficiency. The adaptive supplyblock is typically implemented using DC-DC converters, low­dropout voltage regulators, and/or linear amplifiers.

This work presents a class-AB linear amplifier with a novelcurrent sensing circuit that has high-accuracy and speed, whileconsuming low quiescent power. The proposed linear amplifiercan be implemented in envelope tracking applications due toits accurate current-sense block.

Recent advances in wireless communications have resultedin the introduction and implementation of portable communi­cation systems that are capable of high data rates [1]. Longterm evolution (LTE) is one such communication standardthat is widely used in cellular phones [1]-[3]. High-rate datatransmission is achieved through complex (I1Q) modulationsschemes, carrier aggregation and wide channel bandwidths [4].LTE signals specifically exhibit a high peak-to-average powerratio (PAPR), which leads to efficiency issues in power am­plifiers (PAs) [1], [2], [4]. A major technique to improve PAefficiency is to operate the PA in back-off power, that is, atlower supply voltages [5]. However a trade-off exists betweenPA efficiency and linearity. Consequently, a constant DC powersupply cannot be used to power an LTE PA. This leads toa major design challenge in portable communication systemswhere system run-time cannot be sacrificed while still enablinghigh data transmission [4], [6]. Instead, several techniquessuch as envelope elimination and restoration (EER), averagepower tracking (APT), and envelope tracking (ET) have beenproposed, aimed at improving PA efficiency while preservinglinearity [1]-[3], [7], [8].

Fig. lea) depicts a typical RFPA whose supply Vee isadjustable based on the input power level RF1N. The RFPA's adjustable supply is driven by an average power tracking

Keywords-Envelope Tracking, class-AB, mixed-signal.

Abstract-This work presents the design of a high-bandwidthand high slew rate c1ass-AB amplifier in a linear assisted hybridconverter for envelope tracking (ET) applications. ET has becomeprevalent for improving the efficiency of RF power amplifiers (PA)in portable devices when transmitting LTE signals. The c1ass-ABamplifier in the hybrid converter provides the AC power to thePA, whereas the DC power is provided by a DC-DC converter. Thec1ass-AB amplifier is designed to track the LTE signal envelope,up to 20 MHz in bandwidth. Optimization is required to improvethe efficiency of the system. A novel high-speed current-senseblock is implemented to accurately sense the output stage currentsof the c1ass-AB amplifier. The amplifier is implemented in a 0.5·{tm CMOS process, operates from a 3.6-5.0 V supply and iscapable of driving a resistive load range from 20-4 n. The class·AB amplifier achieves 80 MHz UGF at a 4 n load, consumingroughly 33 rnA quiescent current. Simulation results shows thetracking of 20 MHz LTE signals with an RMS error better than-34 dB.

I. INTRODUCTION

978-1-5090-6389-5/17/$31.00 ©20 17 IEEE 1220

VBAT average c1ass-AB output current to zero. This results in thebuck converter attempting to provide the maximum possibleload current. Therefore, a fast and accurate current sensingcircuit is necessary for the control of the buck converter andalso to reduce the peak AC and average currents from thec1ass-AB amplifier.

Fig. 2: Block-level architecture of linear assisted hybrid converter for envelope trackingsystem from [6]

and fast enough to track such high bandwidth LTE envelopes.Fig. 2 shows the block-level architecture of a hybrid converterin which a c1ass-AB amplifier assists a parallel DC-DC buckconverter. Together they drive the RFPA that is modelled asa resistor RpA in parallel with the capacitor CPA. The c1ass­AB amplifier tracks the AC portion of the envelope signalto modulate the output voltage. In order to track the fast­transient segments of the envelope signal, the amplifier shouldhave high bandwidth, high slew rate and good gain. The c1ass­AB amplifier consumes high quiescent current to achieve highbandwidth and slew rate. In order to achieve high systemefficiency, the buck converter is operated with low switchingfrequency and provides the average power to the load. In thisimplementation, the c1ass-AB amplifier is configured as a non­inverting amplifier to scale the envelope signal according to

. RFBiGam = 1 +-R = 1.25 V/V (1)FB2

The buck converter and the c1ass-AB amplifier can beconsidered as two voltage sources combine together to powerthe RFPA. Consequently, the amount of load current deliv­ered from each source depends on their respective outputimpedance. The buck converter's output resistance is lowerthan that of a c1ass-AB amplifier due to the presence of aninductor at the output node. This results in the majority ofthe low-frequency load current being provided by the buckconverter. As mentioned earlier, the buck converter operateswith low switching frequency for high efficiency, whereas thec1ass-AB amplifier has high bandwidth and high slew rate butconsumes more quiescent power.

Therefore, even a small amount of DC current sourcedfrom the c1ass-AB amplifier highly degrades system efficiency.To increase the efficiency, the DC load current sourced fromthe c1ass-AB amplifier should be minimized to approximatelyzero while increasing the load current sourced by the buckconverter to a maximum. To achieve zero DC current from thec1ass-AB amplifier, the output-stage current of the class-ABamplifier is sensed (hB) and fed back to the buck convertercontrol-loop. As the sourcing or sinking current of the c1ass­AB increases the feedback current sets the buck converterto source or sink higher current and eventually reduces the

(2)(3)

VFP = VBAT - 2Vsc ,VFN = 2Vcs

A. Class-AB Amplifier

Because the c1ass-AB amplifier must be able to drive aresistive load, a two-stage architecture with a c1ass-AB outputstage is adapted. Since the amplifier is driving a low resistiveload, the gain of the second-stage is low. The higher the open­loop gain of the amplifier, the lower the error between the inputand output signals in a closed-loop configuration. To achievehigher overall amplifier gain, the first stage needs high gain.Therefore a high-gain folded cascode differential amplifier inseries with a pUSh-pull output stage forms a two-stage c1ass­AB amplifier with good gain [6]. Conventional folded cascodeamplifiers have high gain with wide output voltage swing suchthat the slew rate is limited by the bias current. A push-pulloutput stage is carefully biased to operate the output transistorsin saturation and achieve c1ass-AB operation. The push-pullc1ass-AB output stage is conventionally biased using a floatingcurrent source as a separate branch that consumes extra power.To avoid power loss, a modified version of the folded cascodeamplifier with inherent floating current source to bias the push­pull output-stage is introduced in [9].

This c1ass-AB amplifier is adapted for an envelope trackingsystem, as shown in Fig. 3. The folded cascode amplifier withinherent floating current source is formed by transistors M i ­

M 12 . In order to achieve rail-to-rail output swing, the inputswing of the amplifier is 1.25 times lower than the output.As such, the PMOS input differential pair M i -M2 is able tooperate over the full range of the input envelope signal. Toachieve higher bandwidth all the transistors are designed withminimum length and biased with high current. The outputside of the folded cascode amplifier contains two floatingcurrent sources formed by transistors M5 /M7 and M6 /Msin both branches that are used to bias the pUSh-pull output­stage transistors M13-M14 , such that c1ass-AB operation isachieved. Unlike the conventional folded-cascode amplifier, toachieve high slew rate at the output, both PMOS sourcing andNMOS sinking current sources are dynamically controlled withtheir respective diode-connected transistors M 3 and M n . Inaddition, the floating current sources set the bias currents inthe two output branches of the folded cascode stage that arebiased with voltages VFN and VFP . These bias voltages aregenerated from a biasing stage formed by transistors M15-MiSwhose expression are given by

III. CLASS-AB AMPLIFIER WITH CURRENT SENSOR

The transistor-level schematic of the high bandwidth andhigh slew rate c1ass-AB amplifier with a fast current sensingcircuit is shown in Fig. 3.

where Vsc and Vcs are the gate-to-source voltages of thePMOS and NMOS diode-connected transistors in the bias

r-----------I

I II II I

Ic ICPA RpAI

I II I1 1

+Class AS >-_---+-~>--V:..:c::::.c----+--

V ENV

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Fig. 3: Transistor-level schematic of two-stage c1ass-AB amplifier with current sensing circuit and bias circuit.

stage. In the balanced condition, the two output branches ofthe folded-cascode stage are also biased with higher current inorder to achieve high slew rates at nodes Vcp and VCN .

The push-pull output stage is designed such that it provideshigh sinking and sourcing transient currents to the RFPA tomodulate the supply voltage. Thus the size of those transistorsare huge. The input stage is biased with high bias current todrive the high gate capacitance of the output transistors. Thepush-pull output is also biased with high current using thefloating current source in order to achieve fast slew rate at theoutput.

For the purpose of stabilizing the two-stage class-AB am­plifier, symmetric Miller compensation with nulling resistorsformed by COl-RcI and CC2-Rc2 are introduced across thegate and drain of both output-stage transistors, as shown inFig. 3. This compensation helps in stabilizing the class-ABamplifier for a wide range of loads from 20 to 4 n.

In order to accurately track a 20 MHz envelope signal withminimal distortion, the linear amplifier should exhibit a highunity gain frequency (UGF). In this implementation a UGF?': 4x the frequency of VENV is achieved. High bandwidth isessential, resulting in a high quiescent current (lQ) utilization.

B. Proposed Current Sense Circuit

In an ET system, feedback from class-AB amplifier to thebuck converter is necessary to minimize the average outputcurrent from the class-AB amplifier. A feedback signal hBgenerated from the class-AB amplifier is used to modulate thepower provided by the buck converter. As the buck converter'stracking ability is limited by its low bandwidth, it is essentialto accurately sense the current from the class-AB amplifier.

The current in the pUSh-pull output stage of the class-ABamplifier is sensed through the current sense circuit formedby transistors M I9-M28 , as shown in Fig. 3. Sense FETs M21and M 26 in parallel with the PMOS and NMOS transistors inthe output stage sense the current with a scaling factor K. Inorder to sense the current accurately the VDS of sense FETsmust be same as the main FETs. To achieve this the output ofclass-AB amplifier Vcc is level shifted up to VOH and shifted

down to VOL by diode-connected transistors M l9 and PMOSM 20 , respectively. These level-shifted voltages VO H and VOL

are shifted back by M 25 and M 22 transistors to the drainsof the NMOS and PMOS sense FETs, respectively, as shownin Fig. 3. This helps in achieving accurate current sensing.The current sense circuit presented here offers an inherent180 °phase shift between the class-AB output current and therespective sense currents through current mirrors. Since thecurrent sensing circuit is formed by current mirrors with highquiescent currents, the sensing is fast.

IV. SIMULATION RESULTS

The proposed linear amplifier with current sensing circuitis designed in the 0.5 /lm CMOS process. The circuit operateswith a supply voltage of 5 V and quiescent current of 33 rnA.The amplifier is stable for a wide range of resistive loads from4 to 20 n. AC and transient simulations are presented belowwhich characterize the amplifier's performance.

A. AC Analysis

An AC simulation to analyze the small-signal stability ofthe linear amplifier was performed. The simulated magnitudeand phase plots of the amplifier in open-loop configurationdriving 4 and 20 n resistive loads are shown in Fig. 4. A DCgain of 39 dB with a unity-gain frequency of approximately80 MHz and phase margin of 60 ° is achieved while driving4 n load. Similarly, driving a resistive load of 20 n, thelinear amplifier's open loop DC gain and unity-gain frequencyincrease to 52 dB and 134 MHz, respectively.

B. Transient Analysis

The designed linear class-AB amplifier's transient perfor­mance is evaluated through transient simulations. The linearamplifier is configured with a gain of 1.25 VN in order toscale the envelope information signal VENV as per the inputcommon-mode requirements.Referring to Fig. 5, the first graphcontains two waveforms, the input envelope signal and theoutput envelope signal. The input signal is DC shifted by0.5 V for clarity. Icc and hB are also shown to display thedesigned class-AB amplifier's output current and the invertedscaled output of the current sense circuit.

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7045 50 55 60 65Quiescent Current (rnA)

220

200

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Fig. 6: Simulated unity-gain frequency (UGF) over the current consumption (lQ) of theclass-AB amplifier wilh proposed current sensing circuit.10''0''0'10:1 10" 105 10'

Frequency (Hz)

10'10'

·150

:~ ·100..

40r-.-----------__.::--...

iii"0-; 20

~

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Fig. 4: Small-signal simulated result of class-AB amplifier at 4 nand 20 n load withVBAT=5y'

~~E~.~:~~.5<)0

o 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8Tlm.(~s)

Fig. 5: Simulated transient response of class-AB amplifier with a 10MHz LTE envelopesignal. Observe the class-AB oUlput current Jcc and sense current J P R.

C. Relationship of Quiescent Current and UGF

The designed linear class-AB amplifier is stable for a widebandwidth. This bandwidth can be increased by increasingthe bias current through the circuit. As shown in Fig. 6 asI Q increases UGF increases. For a 4n load the amplifier'sUGF increases from approximately 80 MHz to 180 MHz.This ability to increase the linear amplifier's UGF throughthe bias current is advantageous in ET applications. Forexample, considering a 5 MHz LTE envelope signal a UGFof approximately 50 MHz is required to track the signal. Insuch a scenario the linear amplifier's power consumption canbe reduced by decreasing the bias current such that a minimumrequired UGF is achieved.

REFERENCES

[II Y. Li, J. Lopez, C. Schecht, R. Wu, and D. Y. C. Lie, "Design ofhigh efficiency monolithic power amplifier with envelope-tracking andtransistor resizing for broadband wireless applications," IEEE Journal ofSolid-State Circuits, vol. 47, no. 9, pp. 2007-2018, Sept 2012.

[21 1. Choi, D. Kim, D. Kang, 1. Park, B. Jin, and B. Kim, "Envelopetracking power amplifier robust to battery depletion," in 2010 IEEE MIT­S International Microwave Symposium. May 2010, pp. I-I.

[3] 1. Kim, D. Kim, Y. Cho, D. Kang, S. Jin, B. Park, K. Moon, H. Jin.S. Koo, and B. Kim. "Wideband envelope amplifier for envelope-trackingoperation of handset power amplifier," in 2014 9th European MicrowaveIntegrated Circuit Conference. Oct 2014. pp. 408-411.

[4] R. Shrestha, R. A. R. V. der Zee, A. 1. M. de Graauw, and B. Nauta, "Awideband supply modulator for 20mhz rf bandwidth polar pas in 65nmcmos," in 20081EEE Symposium on VLS1 Circuits, June 2008. pp. 92-93.

[5] 1. Choi, D. Kang. D. Kim. and B. Kim, "Optimized envelope trackingoperation of doherty power amplifier for high efficiency over an ex­tended dynamic range," IEEE Transactions on Microwave Theory andTechniques, vol. 57, no. 6. pp. 1508-1515, June 2009.

[6] W. Y. Chu, B. Bakkaloglu. and S. Kiaei, "A 10 mhz bandwidth, 2 mvripple pa regulator for cdma transmitters," IEEE Journal of Solid-StateCircuits, vol. 43, no. 12, pp. 2809-2819, Dec 2008.

[7] 1. S. Walling, S. S. Taylor, and D. J. Allstot, "A class-g supply modulatorand class-e pa in 130 run cmos," IEEE Journal of Solid-State Circuits.vol. 44, no. 9, pp. 2339-2347, Sept 2009.

[8] 1. Choi, D. Kim. D. Kang, and B. Kim. "A new power management icarchitecture for envelope tracking power amplifier," IEEE Transactionson Microwave Theory and Techniques, vol. 59, no. 7, pp. 1796-1802,July 2011.

[9] R. Hogervorst, J. P. Tero, R. G. H. Eschauzier, and J. H. Huijsing. "Acompact power-efficient 3 v cmos rail-to-rail input/output operationalamplifier for vlsi cell libraries," in Solid-State Circuits Conference. 1994.Digest of Technical Papers. 41st ISSCC., 1994 IEEE International, Feb1994, pp. 244-245.

V. DISCUSSION AND CONCLUSION

A high slew rate, high bandwidth linear class-AB amplifierwith a novel current sense circuit is presented in detail. Theproposed current sense circuit provides output current infor­mation which is critical when utilized in an envelope trackingpower supply modulator. Through the proposed current sensecircuit a scaled and inverted version of the amplifier's outputcurrent is utilized to control the current sourced by the buckconverter. As showcased through simulation results the pro­posed current sense circuit for a class-AB linear amplifier aidsin implementing a fast envelope tracking supply modulator.

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