A switched-capacitor power amplifier for EER/polar transmitters

Abstract

Wireless high-speed communication standards such as WiFi, WiMax and LTE use spectrally-efficient OFDM modulation that encodes signal information in both amplitude and phase. Use of this non-constant envelope modulation requires a linear PA, operating at a less-than-peak signal level to realize higher linearity and inherently reduced efficiency. Because the PA is the dominant power consumer in most RF transceivers, operation with reduced efficiency leads to short battery lifetime and reduced mobility. Consequently, many efforts to utilize more efficient switching amplifiers with linearization circuitry have been made, notably through pulse-width modulation [1], outphasing [2] and envelope elimination and restoration (EER) [3,4]. Of the three, EER offers the best performance tradeoff between linearity, output power and efficiency; however, most previous implementations have come at the cost of large, power-hungry analog supply modulators. Additionally, conventional EER techniques are subject to nonlinearity induced by delay mismatch between the amplitude-and phasemodulated signal components. An alternative solution modulated the output power by selecting multiple PA unit cells [5], but this exhibits low efficiency at low output power levels because the power control is achieved by changing the total PA transconductance through switching of inefficient cells.This paper introduces an EER 90nm CMOS experimental prototype switchedcapacitor power amplifier (SCPA) that achieves high output power, efficiency and linear output-power control using a switched-capacitor-based switching PA without the use of a supply modulator. While amplifying 64-QAM OFDM modulation with a 20MHz signal bandwidth it achieves an average output power of 17.7dBm, an average PAE of 32.1%, and an EVM of 2.9%.Switched-capacitor circuit techniques are widely used in analog/mixed-signal design because capacitors are area-efficient native devices and CMOS transistors are excellent switches [6]. High-accuracy capacitor ratios coupled with digital signal processing techniques are easily applied to switched-capacitor circuits. These techniques can now be adopted directly at RF frequencies because of the higher operating speeds with scaled CMOS. In a switched-capacitor circuit, any voltage can be generated based on the ratio of the capacitors switched to V DD or ground (V GND ). It is important to note that there is no loss of energy ideally in the charge redistribution among capacitors ( Fig. 24.3.1). To efficiently generate a desired output voltage, capacitors are selectively connected to either V GND or switched between V GND and V DD . Hence, the ratio of capacitors switching (ΣC on ) between V GND and V DD compared to the total capacitance (ΣC on +ΣC off ) defines the output voltage. The capacitors are switched at the desired RF carrier frequency; a bandpass filter (BPF) (e.g., matching network) is created by connecting an inductive reactance in series with the capacitor array to select the RF signal to be broadcast by the SCPA. Because V DD and V GND...

A 2.4-GHz low power polar transmitter for wireless body area network applications

“…Exhaustive research has already been done in the area of power amplification [2][3][4][5]. However, most publications concerning the high efficiency amplifiers are dealing with power levels much higher than those needed in the WBAN applications, which allows more freedom in the design.…”

Analysis of a 5.5-V Class-D Stage Used in $+$30-dBm Outphasing RF PAs in 130- and 65-nm CMOS

“…With the improved speed of CMOS transistors, highly efficient switched PAs, like Class-D/E, have gained increased interest in polar modulation [1], [2] and outphasing [3]- [10]. In the outphasing amplifier, an input signal, s(t), containing both amplitude and phase modulation is divided into two constant envelope phase-modulated signals, s 1 (t) and s 2 (t), as in Fig.…”

“…The output power of Class-D RF PAs has, until recently [8]- [10], been lower than , [2], [4]- [8]. In [1] and [4] non-overlapping driver signals were used.…”

“…+30 dBm [1]- [7]. This can be explained as follows: for a given supply voltage and load resistance the P out from a Class-D PA is -3.9 dB (1) lower compared to Class-A/B PAs.…”

“…A high supply voltage (and swing) can be used in Class-D PAs by utilizing cascoding techniques to operate at two or three times the transistors' nominal supply voltage [1], [2], [4]- [8], [14]. In [1], [2], [4]- [8], the voltage stress on the devices is limited to the nominal supply voltage by using two supplies in the output stage and in the drivers, i.e. 2 x V DD and V DD as shown in Fig.…”