This relates generally to wireless systems and, particularly, to power amplifiers for wireless systems.
Power amplifiers are an important element in wireless transceivers. Because they are one of the main power consumers in most wireless radio systems, power amplifiers are important in the design consideration of a radio system. In many cases, power consumption is an important design criteria for wireless systems, including those that are battery powered.
One known technique for controlling power consumption of wireless transceivers is envelope tracking used for signals with moderate peak to average power ratio, such as Enhanced Data Rates for GSM Evolution (EDGE) applications. In these signals, the power of the signal is changing with time. The goal of envelope tracking is to change the supply voltage according to the momentary voltage swing of the signal, such that the supply voltage will be able to support the swing without compressing the signal. If the voltage supply regulator is highly efficient, the power amplifier is maintained at its peak efficiency and the overall efficiency of the wireless transceiver is improved.
However, the current implementations of envelope tracking systems have limited usefulness in wide band applications, such as WiFi, (IEEE 802.11-2007), Worldwide Interoperability for Microwave Access (WiMAX) (IEEE 802.16e-2005) and WCDMA ((Third Generation Partnership Project 3GPP), TS 25.2B Version 3.2.0 Mar. 2000), because of the need to achieve a wide bandwidth supply regulator on one hand with high efficiency and relatively low supply ripple on the other hand. The low ripple is needed to achieve a clean spectrum with low spurs at the output of the power amplifier.
Referring to
The envelope tracking output voltage is limited in range and does not follow the entire input signal to avoid chocking the power amplifier in some embodiments. Specifically, referring to
Referring to
A buck switching supply modulator conventionally includes drivers 24 and an inverter including a PMOS transistor 26 over an NMOS transistor 28 and an output inductor 30. The output inductor 30 is coupled to a capacitor 32 and to capacitor 36 coupled to the main buck switching supply modulator 22. The output from the stacked buck switching supply modulators is coupled to the power amplifier 16, as indicated in
In some embodiments, a distributed capacitance 50 may be inserted between the high and low supply voltages of the supply modulators 20 and 22. In some embodiments, the distributed capacitance 50 may be a few nanoFarads and have very low equivalent series inductance (ESL). It is important to keep the inductance as low as possible since otherwise a large supply drop and ripple at the switches' supply nodes may result that can degrade circuit performance and efficiency.
As shown in
The non-overlap circuit 38 ensures that both transistors 26 and 28 of the same modulator 20 or 22 are never on at the same time in some embodiments. Thus, the circuit 38 provides a control that ensures that once one transistor of each inverter turns completely off before the other transistor of the inverter turns on. This prevents lower and higher supply voltages from being connected together.
In some embodiments, the envelope tracking switching supply regulator topology enjoys both wide spectrum and high efficiency. Instead of using a single supply, the design includes two supplies, one higher than the other. The auxiliary AC coupled supply modulator 20 is added on top of the original main supply modulator 22. The distributed capacitance 50 is an on-chip distributed capacitance between supplies used to improve the switching regulator efficiency.
The higher supply voltage (Vcch) may be the same voltage used in conventional buck switching regulators used for envelope tracking, while the lower supply voltage (Vccl) may be a voltage, already available within the system, typically used for analog and digital circuits. In general, the lower supply voltage can be achieved using a regular low bandwidth buck converter having a bandwidth of approximately 200 kiloHertz that can be implemented today with 90 to 95 percent efficiency. For example, the lower supply voltage may be about half the higher supply voltage. Also, due to its low bandwidth, it can have very low ripple. Another option, relevant mainly for battery driven devices with low supply voltages, is to use a low bandwidth boost converter to generate the higher supply voltage.
The use of two supply voltages relaxes the voltage head room on the switching regulator, allowing the use of thin gate, fast transistors instead of thick gate transistors in some embodiments. This may improve the on resistance and may reduce the load capacitance to the drivers, thereby reducing both the dynamic switching losses and the resistive losses through the switches in some embodiments.
Furthermore, since all driving stages work between the higher and lower supply voltages, the swing at the circuit nodes is reduced from the higher supply voltage to the delta between the higher and lower supply voltages. Since the dynamic losses of switching digital circuits are proportional to the square of the swing, the dynamic losses are gradually reduced in some embodiments. In one embodiment, the lower supply voltage may be half the higher supply voltage, which results in dynamic losses being reduced by a factor of four in some embodiments. For example, the higher supply voltage may be 3.3 volts, while the lower supply voltage is 1.7 volts, as one example.
As shown in
The auxiliary supply modulator 20 may be driven with complementary signals (indicated at 42 and 46) to the main supply modulator 22. While the main supply modulator 22 inductor 34 carries the DC current sawtooth current ripple at the switching frequency (indicated at 48), the auxiliary modulator inductor carries zero DC current and sawtooth current ripple (indicated at 44) with the opposite direction of the ripple current in the main modulator inductor, as shown in the voltage and current over time curves inserted in
Referring next to
References throughout this specification to “one embodiment” or “an embodiment” mean that a particular feature, structure, or characteristic described in connection with the embodiment is included in at least one implementation encompassed within the present invention. Thus, appearances of the phrase “one embodiment” or “in an embodiment” are not necessarily referring to the same embodiment. Furthermore, the particular features, structures, or characteristics may be instituted in other suitable forms other than the particular embodiment illustrated and all such forms may be encompassed within the claims of the present application.
A similar approach can also be used for envelope elimination and restoration systems (EER) for wideband signals. A class AB power amplifier may be replaced with a switch mode class E or class D power amplifier.
While the present invention has been described with respect to a limited number of embodiments, those skilled in the art will appreciate numerous modifications and variations therefrom. It is intended that the appended claims cover all such modifications and variations as fall within the true spirit and scope of this present invention.