High Bandwidth Class-AB with High Slew Rate and Fast Current Sensing for Envelope Tracking Applications

Punith R. Surkanti, Aditya A. PatH, Sri Harsh Pakala and Paul M. Furth VLSI Laboratory, Klipsch School of Electrical and Computer Engineering, New Mexico State University, Las Cruces, NM 88003, USA Email: {punith.aditya30.sriharsh and pfurth}@nmsu.edu

Abstract-This work presents the design of a high-bandwidth VSAT VSAT and high slew rate c1ass-AB amplifier in a linear assisted hybrid converter for envelope tracking (ET) applications. ET has become prevalent for improving the efficiency of RF power (PA) in portable devices when transmitting LTE signals. The c1ass-AB amplifier in the hybrid converter provides the AC power to the PA, whereas the DC power is provided by a DC-DC converter. The RFoUT RF 1N RFoUT c1ass-AB amplifier is designed to track the LTE signal envelope, up to 20 MHz in bandwidth. Optimization is required to improve the efficiency of the system. A novel high-speed current-sense (a) (b) block is implemented to accurately sense the output stage currents of the c1ass-AB amplifier. The amplifier is implemented in a 0.5· Fig. 1: (a) Average power tracking and (b) envelope tracking power supply options for {tm CMOS process, operates from a 3.6-5.0 V supply and is driving RF power amplifier from [81 capable of driving a resistive load range from 20-4 n. The class· AB amplifier achieves 80 MHz UGF at a 4 n load, consuming roughly 33 rnA quiescent current. Simulation results shows the signal VAPT, that is proportional to average RF output power tracking of 20 MHz LTE signals with an RMS error better than transmitting over a single time frame. The APT output is -34 dB. typically generated by a high efficient DC-DC converter. While Keywords-Envelope Tracking, class-AB, mixed-signal. the discrete Vee levels aid in improving the PA efficiency when compared to a fixed Vee, the maximum achievable efficiency is severely limited by the very slow transitions in I. INTRODUCTION the supply voltage relative to the rapid variations in RFIN'S Recent advances in wireless communications have resulted power level. in the introduction and implementation of portable communi­ An evolutionary method building on the concept of average cation systems that are capable of high data rates [1]. Long power tracking is the envelope tracking (ET) technique [8]. In term evolution (LTE) is one such communication standard this scheme, shown in Fig. 1(b), the power supply of the PA that is widely used in cellular phones [1]-[3]. High-rate data is constantly adjusted based on the envelope information of transmission is achieved through complex (I1Q) the RF1N signal, VENV . An adaptive supply that is driven schemes, carrier aggregation and wide channel bandwidths [4]. by VENV is used to modulate Vee of the RFPA. This scheme LTE signals specifically exhibit a high peak-to-average power allows a close tracking of the instantaneous power levels of the ratio (PAPR), which leads to efficiency issues in power am­ input signal through the envelope information, thus providing plifiers (PAs) [1], [2], [4]. A major technique to improve PA substantial improvement in efficiency. The adaptive supply efficiency is to operate the PA in back-off power, that is, at block is typically implemented using DC-DC converters, low­ lower supply voltages [5]. However a trade-off exists between dropout voltage regulators, and/or linear amplifiers. PA efficiency and linearity. Consequently, a constant DC power supply cannot be used to power an LTE PA. This leads to This work presents a class-AB linear amplifier with a novel a major design challenge in portable communication systems current sensing circuit that has high-accuracy and speed, while where system run-time cannot be sacrificed while still enabling consuming low quiescent power. The proposed linear amplifier high data transmission [4], [6]. Instead, several techniques can be implemented in envelope tracking applications due to such as envelope elimination and restoration (EER), average its accurate current-sense block. power tracking (APT), and envelope tracking (ET) have been proposed, aimed at improving PA efficiency while preserving II. ENVELOPE TRACKING SYSTEM linearity [1]-[3], [7], [8]. The envelope tracking system detects the envelope of the Fig. lea) depicts a typical RFPA whose supply Vee is RF input signal and modulates the supply voltage of RFPA. adjustable based on the input power level RF1N. The RF The bandwidth of the envelope signal ranges from 1.4 MHz PA's adjustable supply is driven by an average power tracking to 20 MHz. The supply modulator needs to be highly efficient

978-1-5090-6389-5/17/$31.00 ©2017 IEEE 1220 VBAT average c1ass-AB output current to zero. This results in the buck converter attempting to provide the maximum possible load current. Therefore, a fast and accurate current sensing circuit is necessary for the control of the buck converter and also to reduce the peak AC and average currents from the c1ass-AB amplifier.

III. CLASS-AB AMPLIFIER WITH CURRENT SENSOR V ENV + Class AS >-_---+-~>--V:..:c::::.c----+-- The -level schematic of the high bandwidth and r------I I I high slew rate c1ass-AB amplifier with a fast current sensing I I I I circuit is shown in Fig. 3. c ICPA RpAI I I I I I 1 1 A. Class-AB Amplifier Because the c1ass-AB amplifier must be able to drive a resistive load, a two-stage architecture with a c1ass-AB output Fig. 2: Block-level architecture of linear assisted hybrid converter for envelope tracking system from [6] stage is adapted. Since the amplifier is driving a low resistive load, the gain of the second-stage is low. The higher the open­ loop gain of the amplifier, the lower the error between the input and fast enough to track such high bandwidth LTE envelopes. and output signals in a closed-loop configuration. To achieve Fig. 2 shows the block-level architecture of a hybrid converter higher overall amplifier gain, the first stage needs high gain. in which a c1ass-AB amplifier assists a parallel DC-DC buck Therefore a high-gain folded in converter. Together they drive the RFPA that is modelled as series with a pUSh-pull output stage forms a two-stage c1ass­ a resistor RpA in parallel with the capacitor CPA. The c1ass­ AB amplifier with good gain [6]. Conventional folded cascode AB amplifier tracks the AC portion of the envelope signal amplifiers have high gain with wide output voltage swing such to modulate the output voltage. In order to track the fast­ that the slew rate is limited by the bias current. A push-pull transient segments of the envelope signal, the amplifier should output stage is carefully biased to operate the output have high bandwidth, high slew rate and good gain. The c1ass­ in saturation and achieve c1ass-AB operation. The push-pull AB amplifier consumes high quiescent current to achieve high c1ass-AB output stage is conventionally biased using a floating bandwidth and slew rate. In order to achieve high system current source as a separate branch that consumes extra power. efficiency, the buck converter is operated with low switching To avoid power loss, a modified version of the folded cascode frequency and provides the average power to the load. In this amplifier with inherent floating current source to bias the push­ implementation, the c1ass-AB amplifier is configured as a non­ pull output-stage is introduced in [9]. inverting amplifier to scale the envelope signal according to This c1ass-AB amplifier is adapted for an envelope tracking . RFBi system, as shown in Fig. 3. The folded cascode amplifier with = = (1) Gam 1 + -R 1.25 V/V inherent floating current source is formed by transistors M ­ FB2 i M 12. In order to achieve rail-to-rail output swing, the input The buck converter and the c1ass-AB amplifier can be swing of the amplifier is 1.25 times lower than the output. considered as two voltage sources combine together to power As such, the PMOS input differential pair M i -M2 is able to the RFPA. Consequently, the amount of load current deliv­ operate over the full range of the input envelope signal. To ered from each source depends on their respective output achieve higher bandwidth all the transistors are designed with impedance. The buck converter's output resistance is lower minimum length and biased with high current. The output than that of a c1ass-AB amplifier due to the presence of an side of the folded cascode amplifier contains two floating inductor at the output node. This results in the majority of current sources formed by transistors M5 /M7 and M 6 /Ms the low-frequency load current being provided by the buck in both branches that are used to bias the pUSh-pull output­ converter. As mentioned earlier, the buck converter operates stage transistors M13-M14, such that c1ass-AB operation is with low switching frequency for high efficiency, whereas the achieved. Unlike the conventional folded-cascode amplifier, to c1ass-AB amplifier has high bandwidth and high slew rate but achieve high slew rate at the output, both PMOS sourcing and consumes more quiescent power. NMOS sinking current sources are dynamically controlled with their respective diode-connected transistors M 3 and M n . In Therefore, even a small amount of DC current sourced addition, the floating current sources set the bias currents in from the c1ass-AB amplifier highly degrades system efficiency. the two output branches of the folded cascode stage that are To increase the efficiency, the DC load current sourced from biased with voltages V and V . These bias voltages are the c1ass-AB amplifier should be minimized to approximately FN FP generated from a biasing stage formed by transistors M15-MiS zero while increasing the load current sourced by the buck whose expression are given by converter to a maximum. To achieve zero DC current from the c1ass-AB amplifier, the output-stage current of the class-AB VFP = VBAT - 2Vsc, (2) amplifier is sensed (hB) and fed back to the buck converter VFN = 2Vcs (3) control-loop. As the sourcing or sinking current of the c1ass­ AB increases the feedback current sets the buck converter where Vsc and Vcs are the gate-to-source voltages of the to source or sink higher current and eventually reduces the PMOS and NMOS diode-connected transistors in the bias

1221 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 of down to VOL by diode-connected transistors M l9 and PMOS the folded-cascode stage are also biased with higher current in M 20 , respectively. These level-shifted voltages VOH and VOL order to achieve high slew rates at nodes Vcp and VCN . are shifted back by M 25 and M 22 transistors to the drains of the NMOS and PMOS sense FETs, respectively, as shown The push-pull output stage is designed such that it provides in Fig. 3. This helps in achieving accurate current sensing. high sinking and sourcing transient currents to the RFPA to The current sense circuit presented here offers an inherent modulate the supply voltage. Thus the size of those transistors 180 °phase shift between the class-AB output current and the are huge. The input stage is biased with high bias current to respective sense currents through current mirrors. Since the drive the high gate capacitance of the output transistors. The current sensing circuit is formed by current mirrors with high push-pull output is also biased with high current using the quiescent currents, the sensing is fast. floating current source in order to achieve fast slew rate at the output. IV. SIMULATION RESULTS For the purpose of stabilizing the two-stage class-AB am­ The proposed linear amplifier with current sensing circuit plifier, symmetric Miller compensation with nulling resistors is designed in the 0.5 /lm CMOS process. The circuit operates formed by COl-RcI and CC2-Rc2 are introduced across the with a supply voltage of 5 V and quiescent current of 33 rnA. gate and drain of both output-stage transistors, as shown in The amplifier is stable for a wide range of resistive loads from Fig. 3. This compensation helps in stabilizing the class-AB 4 to 20 n. AC and transient simulations are presented below amplifier for a wide range of loads from 20 to 4 n. which characterize the amplifier's performance. In order to accurately track a 20 MHz envelope signal with A. AC Analysis minimal , the linear amplifier should exhibit a high unity gain frequency (UGF). In this implementation a UGF An AC simulation to analyze the small-signal stability of ?': 4x the frequency of VENV is achieved. High bandwidth is the linear amplifier was performed. The simulated magnitude essential, resulting in a high quiescent current (lQ) utilization. and phase plots of the amplifier in open-loop configuration driving 4 and 20 n resistive loads are shown in Fig. 4. A DC B. Proposed Current Sense Circuit gain of 39 dB with a unity-gain frequency of approximately 80 MHz and phase margin of 60 ° is achieved while driving In an ET system, feedback from class-AB amplifier to the 4 n load. Similarly, driving a resistive load of 20 n, the buck converter is necessary to minimize the average output linear amplifier's open loop DC gain and unity-gain frequency current from the class-AB amplifier. A feedback signal hB increase to 52 dB and 134 MHz, respectively. generated from the class-AB amplifier is used to modulate the power provided by the buck converter. As the buck converter's B. Transient Analysis tracking ability is limited by its low bandwidth, it is essential The designed linear class-AB amplifier's transient perfor­ to accurately sense the current from the class-AB amplifier. mance is evaluated through transient simulations. The linear The current in the pUSh-pull output stage of the class-AB amplifier is configured with a gain of 1.25 VN in order to amplifier is sensed through the current sense circuit formed scale the envelope information signal VENV as per the input by transistors M I9-M28 , as shown in Fig. 3. Sense FETs M21 common-mode requirements.Referring to Fig. 5, the first graph and M 26 in parallel with the PMOS and NMOS transistors in contains two waveforms, the input envelope signal and the the output stage sense the current with a scaling factor K. In output envelope signal. The input signal is DC shifted by order to sense the current accurately the VDS of sense FETs 0.5 V for clarity. Icc and hB are also shown to display the must be same as the main FETs. To achieve this the output of designed class-AB amplifier's output current and the inverted class-AB amplifier Vcc is level shifted up to VOH and shifted scaled output of the current sense circuit.

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_ .5<) 80 -4o load L -20Qload : 60 ~ ·100 30 3S 40 45 50 55 60 65 70 7S 80 .. Quiescent Current (rnA) ·150 Fig. 6: Simulated unity-gain frequency (UGF) over the current consumption (lQ) of the 10' 10' 10:1 10" 105 10' '0' '0' 10' class-AB amplifier wilh proposed current sensing circuit. Frequency (Hz)

Fig. 4: Small-signal simulated result of class-AB amplifier at 4 nand 20 n load with REFERENCES VBAT=5y' [II Y. Li, J. Lopez, C. Schecht, R. Wu, and D. Y. C. Lie, "Design of high efficiency monolithic power amplifier with envelope-tracking and transistor resizing for broadband wireless applications," IEEE Journal of Solid-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, "Envelope tracking power amplifier robust to battery depletion," in 2010 IEEE MIT­ S 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-tracking operation of handset power amplifier," in 2014 9th European Microwave Integrated Circuit Conference. Oct 2014. pp. 408-411. [4] R. Shrestha, R. A. R. V. der Zee, A. 1. M. de Graauw, and B. Nauta, "A wideband supply modulator for 20mhz rf bandwidth polar pas in 65nm ~~E~.~:~~ cmos," in 20081EEE Symposium on VLS1 Circuits, June 2008. pp. 92-93. [5] 1. Choi, D. Kang. D. Kim. and B. Kim, "Optimized envelope tracking

.5<)0 operation of doherty power amplifier for high efficiency over an ex­ o 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 tended dynamic range," IEEE Transactions on Microwave Theory and Tlm.(~s) Techniques, vol. 57, no. 6. pp. 1508-1515, June 2009. [6] W. Y. Chu, B. Bakkaloglu. and S. Kiaei, "A 10 mhz bandwidth, 2 mv Fig. 5: Simulated transient response of class-AB amplifier with a 10MHz LTE envelope ripple pa regulator for cdma transmitters," IEEE Journal of Solid-State signal. Observe the class-AB oUlput current Jcc and sense current J P R. Circuits, vol. 43, no. 12, pp. 2809-2819, Dec 2008. [7] 1. S. Walling, S. S. Taylor, and D. J. Allstot, "A class-g supply modulator C. Relationship of Quiescent Current and UGF and class-e pa in 130 run cmos," IEEE Journal of Solid-State Circuits. vol. 44, no. 9, pp. 2339-2347, Sept 2009. The designed linear class-AB amplifier is stable for a wide [8] 1. Choi, D. Kim. D. Kang, and B. Kim. "A new power management ic bandwidth. This bandwidth can be increased by increasing architecture for envelope tracking power amplifier," IEEE Transactions the bias current through the circuit. As shown in Fig. 6 as on Microwave Theory and Techniques, vol. 59, no. 7, pp. 1796-1802, July 2011. I Q increases UGF increases. For a 4n load the amplifier's UGF increases from approximately 80 MHz to 180 MHz. [9] R. Hogervorst, J. P. Tero, R. G. H. Eschauzier, and J. H. Huijsing. "A This ability to increase the linear amplifier's UGF through compact power-efficient 3 v cmos rail-to-rail input/output operational amplifier for vlsi cell libraries," in Solid-State Circuits Conference. 1994. the bias current is advantageous in ET applications. For Digest of Technical Papers. 41st ISSCC., 1994 IEEE International, Feb example, considering a 5 MHz LTE envelope signal a UGF 1994, pp. 244-245. of approximately 50 MHz is required to track the signal. In such a scenario the linear amplifier's power consumption can be reduced by decreasing the bias current such that a minimum required UGF is achieved.

V. DISCUSSION AND CONCLUSION A high slew rate, high bandwidth linear class-AB amplifier with a novel current sense circuit is presented in detail. The proposed current sense circuit provides output current infor­ mation which is critical when utilized in an envelope tracking power supply modulator. Through the proposed current sense circuit a scaled and inverted version of the amplifier's output current is utilized to control the current sourced by the buck converter. As showcased through simulation results the pro­ posed current sense circuit for a class-AB linear amplifier aids in implementing a fast envelope tracking supply modulator.

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