High Bandwidth Class-AB Amplifier 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 amplifiers (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) modulations 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 transistor-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 cascode differential amplifier 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 transistors 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 .