WO2011115533A1

WO2011115533A1 - Envelope tracking switching hybrid

Info

Publication number: WO2011115533A1
Application number: PCT/SE2010/050292
Authority: WO
Inventors: Jan Johansson; Mikael Appelberg; Anders Aronsson
Original assignee: Telefonaktiebolaget LM Ericsson AB
Current assignee: Telefonaktiebolaget LM Ericsson AB
Priority date: 2010-03-16
Filing date: 2010-03-16
Publication date: 2011-09-22
Anticipated expiration: 2012-09-16

Abstract

A hybrid envelope tracking system capable of transition between a passive, switching, and/or active mode of operation with minimized parasitic oscillations is presented. The system may include a voltage control circuit comprising first and second node interfaces configured to receive first and second voltages respectively. An input node interface may be configured to receive an input envelope signal. An enablement node interference may be configured to receive an enablement signal, the enablement signal setting a transition between a passive or switching mode of operation to an active mode of operation. The circuit may further include a modulation transistor driven by a single floating operational amplifier. The modulation transistor may be used to modulate a supplied voltage with respect to the received envelope signal during the active operating regime.

Description

ENVELOPE TRACKING SWITCHING HYBRID

TECHNICAL FIELD

Example embodiments are directed towards a hybrid envelope tracking system capable of transitioning between an active, passive, and/or switching operating regime with minimized parasitic oscillations.

BACKGROUND Radio frequency power amplifiers (RFPA) are a type of electronic amplifier used to convert low-power radio-frequency signals into larger signals of significant power, which are generally employed in communication technologies. Typically, RFPAs are utilized in a passive operating region which results in the dissipation of energy as heat or unwanted oscillations. In an attempt to eliminate the loss of energy and prevent oscillations, envelope tracking (ET) is sometimes employed. Many ET systems use a linear power amplifier and a controlled supply voltage, which closely tracks the output envelope of an input signal. ET provides a method of maintaining a minimum drain voltage marginal, whereby the RFPA transistor may steadily operate in the vicinity of its compression point, thus providing improved efficiency.

Modern ET techniques include (1 ) active solutions offering high efficiency and low bandwidth; (2) linear or passive solutions offering low efficiency and high bandwidth; and most common (3) hybrids of switched and linear solutions. Figure 1 is an example of a hybrid ET system 100 configured to supply an output to a RFPA 102. The hybrid system 100 may include a linear ET subsystem 104 and an active subsystem 106 interconnected via regulation circuitry 108. The active subsystem 106 is configured to track the envelope of an input signal V_E. For high frequency signals, the active subsystem 106 may have difficulty tracking the input signal V_E. In such instances, the linear subsystem 104 may be utilized to compensate for the active subsystem 106 by supplying a constant voltage above the level of the envelope of the input signal. The linear subsystem may also be utilized to filter noise generated by the active subsystem 106 with use of a feedback system. The operation of the linear 104 and active 106 subsystems may be controlled via the regulation circuitry 108. The regulation circuitry may include a number of devices, for example a number of operational amplifiers connected in parallel to drive a modulation transistor 1 10 of the switching subsystem 106.

SUMMARY

Hybrids of active and linear ET solutions may offer a combination of high bandwidth and high efficiency. However, modern hybrid solutions involve the interaction of hybrid parts, or the 'change-of-state' in the control signals to the hybrid parts. These interactions may introduce unwanted communication energies which cause phase or amplitude disturbances. These effects compromise the output impedance and a clean transient step response of the RFPA. Furthermore, active ET solutions typically require large amounts of circuitry to drive the modulation transistor in order to accurately track the envelope of the input signal.

Thus, example embodiments described herein are directed to a novel RFPA design which combines the desirable parameters of high efficiency and high bandwidth, without compromising the output impedance and transient step response of the RFPA. Example embodiments are also directed towards reducing the amount of circuitry required for driving the active subsystem of switching ET subsystems. According to example embodiments the hybrid envelope tracking circuit may include first and second voltage node interfaces that may be configured to receive first and second voltages, respectively. The circuit may further include an input node interface that may be configured to receive an input envelope signal. The circuit may further include an enablement node interface that may be configured to receive an enablement signal, where the voltage control circuit may be configured to transition from a passive or switching operating regime to an active operating regime when the enablement signal is supplied. A modulation transistor may also be included in the voltage control circuit. The modulation transistor may be driven by a single operational amplifier with a floating input and a floating reference point. The modulation transistor may be configured to modulate the second voltage with respect to the envelope signal during the active operating regime. The voltage control circuit may also include an output node interface that may be configured to receive the first voltage during the passive operating regime and the modulated second voltage during the active operating regime.

According to example embodiments, the first voltage may be provided by a constant DC power voltage source with high efficiency. The second voltage may be provided by a variable voltage source with high efficiency. Alternatively, the second voltage may be provided by a constant DC power voltage with high efficiency. Both the first and second voltages may be regulated to adjust a voltage level, thereby increasing efficiency.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing will be apparent from the following more particular description of example embodiments, as illustrated in the accompanying drawings in which like reference characters refer to the same parts throughout the different views. The drawings are not necessarily to scale, emphasis instead being placed upon illustrating

embodiments.

FIG. 1 is a circuit diagram hybrid envelope tracking system;

FIG. 2 is a schematic of a hybrid envelope tracking system according to example embodiments;

FIG. 3 is a circuit diagram of a voltage control circuit included in the system of FIG.

2; and

FIG. 4 is a flow chart depicting an example of operational steps which may be provided by the system of FIG. 2.

DETAILED DESCRIPTION

Figure 2 is a hybrid envelope tracking (ET) system 200 capable of providing oscillation free transitions between a switching, active, and/or passive mode of operation, according to example embodiments. The ET system 200 may include a voltage control circuit 204 configured to supply an output voltage 228 to a RFPA 202 via an output node interface 230. The voltage control circuit 204 may be configured to receive an envelope signal 212 and an enablement signal 216 from the input stage 210 via input stage node interfaces 214 and 218, respectively. The enablement signal 216 may provide an indication of when a transition to an active operational mode is required. In example embodiments, the enablement signal 216 may be a digital signal. In other example embodiments, the envelope signal 212 may provide an indication of a transition, therefore eliminating the need of a separate enablement signal.

A passive operational mode may be utilized when the incoming envelope signal 212 includes a low bandwidth and low amplitude. During a passive operational mode, the voltage control circuit 204 may be configured to receive a first voltage 224 from a constant DC voltage source 208 via a first voltage node interface 226. The constant DC voltage source 208 may further include an adaptive regulator 209. The adaptive regulator 209 may be configured to receive a level signal 213 from the input power stage 210. The level signal 213 may provide the voltage source 208 an indication of a amplitude level of the envelope signal 212. In response to the level signal 213, the regulator 209 may gradually adjust an amplitude level of the supplied first voltage 224.

If a large adjustment of amplitude is required of the first voltage source 208, a switching mode of operation may be employed. A transition between a switching a passive operational mode may be indicated, for example, by the envelope signal 212, the level signal 213, the enablement signal 216, or the regulator 209. During a switching operational mode, the voltage control circuit 204 may be configured to receive a second voltage 220 from a switching power supply 206 via a second voltage node interface 222. The switching power supply 206 may be a variable power source or may be a constant power source. The voltage 220 supplied by the switching power supply 206 may be a constant voltage capable of being adjusted to various preconfigured amplitude levels. The amplitude levels of the second voltage 220 may be of a greater value than the amplitude levels of the first voltage 224. The switching power supply 206 may also include a regulator 21 1. Similarly to the regulator 209 included in constant power supply, the regulator 21 1 of the switching power supply 206 may also gradually adjust an amplitude level of the second voltage 220 according to a level signal 213. In example embodiments, the regulators 21 1 and 209 may include one or several switches that may connect different voltage outputs from one transformer. Adjustments of the levels of the first and second voltages 224 and 220, respectively, may improve efficiency with different peak to average signals during operational mode transitions.

If the incoming envelope signal 212 has a high bandwidth, an active operational mode may be employed. In an active operational mode, the second voltage 220 may be modulated to closely track the envelope of the envelope signal 212. The modulation may be provided with circuitry included in the voltage control circuit 204. An indication that an operational transition to an active mode is required may be provided, for example, by the envelope signal 212, the level signal 213, the enablement signal 216, or regulators 209 and 21 1 .

Figure 3 illustrates an example of the circuitry which may be included in the voltage control circuit 204 of the ET system 200. Figure 3 includes numeric value examples for a number of circuit elements, it should be appreciated that this is merely an example and other values may be used. As shown in Figure 3, the voltage control circuit 204 may include a modulation transistor 234 driven by an operational amplifier 232. In example embodiments the modulation transistor 234 may be a field effect transistor (FET) or any other known transistor in the art. The operational amplifier may be driven by a floating input 236 and utilize a floating reference. A diode 238 associated with the first voltage source 208 may also be included in the voltage control circuit 204. An interconnection between the modulation transistor 234 and the diode 238 is represented as a summation point (SP). The summation point SP is interconnected to the operational amplifier 232 via a feedback connection. The feedback connection allows the operational amplifier 232 to monitor and adjust the current flowing from the summation point SP to the output node 230, thereby providing a floating reference for the modulation transistor 234. The current adjustment helps reduce oscillations during operational transitions.

In an example embodiment, the ET system 200 may be configured to maintain a low voltage at the output interface node 230 for a majority of its operation in order to reduce power dissipation. Specifically, the ET system 200 may mainly operate in the passive mode. In the passive mode the voltage 228 supplied to the PA 202 may be provided by the constant DC voltage source 208 and the modulation transistor 234 may be kept in a disabled state. Once the amplitude of the envelope signal 212 reaches a threshold value, an operational transition to a switched mode may occur. During the switched operational mode, the modulation transistor 234 may stay in a disabled state while the second voltage 220 is supplied to the PA 202. In the switched operational mode, the diode 238 may become disabled as voltage is no longer being supplied by power source 208.

Once the input stage 210 transmits the enablement signal 216, the modulation transistor 234 may become enabled while the diode associated with power supply 208 remains disabled. Thereafter, the voltage control circuit 204 may operate in an active mode and vary the output voltage 228 according to the signal amplitude of the envelope signal 212. During the active mode of operation the second voltage 220 may be modulated according to the envelope of the envelope signal 212.

Therefore, during operation, the voltage control circuit 204 may continuously adjust the voltage 228 being supplied to the RFPA 202 by transitioning to and from a passive, switching, and/or active mode of operation. The voltage 228 supplied by the voltage control circuit 204 may be provided with the switching power supply 206 (during a switching and/or active operation mode) defining the highest possible output voltage level transmitted to the PA 202, or the constant DC voltage source 208 (during a passive operation mode) defining the lowest possible output voltage level. The maximum output swing of the voltage 228 may be limited by the difference between the voltage outputs of from the two voltage sources 206 and 208. Since the output voltage 228 is defined by the currents flowing in relation to the DC 208 and switching 206 power supplies, the transitions from these two power sources must be smooth and include minimal oscillations. The voltage control circuit 204 may measure the individual current in the diode 238 and the output PA node 230 at the summation point (SP) and minimize oscillations due to energy transfers between the diode 238 and the modulation transistor 234, regardless of any diode parasitics that may be present. It should be appreciated that a second modulation transistor may be placed in parallel with the diode 238. The use of a second modulation transistor may increase efficiency during a time between transitions.

Figure 4 illustrates a sequence of example operational steps which may be taken by the ET system 200 during a transition. The voltage control circuit 204 may be configured to receive an envelope signal 212 via the input node interface 214 (Fig. 4, 402). The voltage control circuit 204 may initially be functioning in a passive operative mode. Thus, the modulation transistor 234 may be pre-biased to Class A, or a disabled status, and the constant power supply 208 will provide the output voltage. A passive mode of operation may be best suited for an incoming envelope signal having low signal amplitude and low bandwidth. Therefore, there is no need for peak power feeding and the voltage 224 supplied by the constant power source 208 may be slowly adjusted as needed via the regulator 209. Based on the received envelope signal 212, level signals 213 may continuously, or at predetermined times, supply information to power sources 206 and 208 for regulation of voltage levels via the regulators 209 and 21 1 (Fig. 4, 403). Upon any necessary adjustments, the first voltage 224 may be sent to the output node 230 as the output voltage 228 to the PA 202 (Fig. 4, 404).

Once it is determined that the constant voltage supply 208 is not sufficient for providing the amplitude demands of the incoming envelope signal 212, a transition to the switching operational mode may be made. In the transition between the switching and passive operational modes, the diode becomes disabled and the modulation transistor remains disabled. The output voltage is supplied by the second voltage supply 206 (Fig. 4, 404).

When a transition into an active operating region is desired, the enablement signal

216 may be supplied to the voltage control circuit 204 via the input interface 218. In supplying the enablement signal, the input stage 210 may be configured to change the ground reference of the input envelope signal 212 to the virtual ground level at the summation point SP. Changing the ground reference of the input envelope signal eliminates the need for additional circuit elements, such as inductors or capacitors, in order to further reduce oscillations during operational transitions. Furthermore, altering the ground reference of the input envelope signal lessens the amount of power consumption needed by lowering the amplitude of the input signal. Lowering the amplitude of the input envelope signal decreases the frequency at which the voltage control circuit 204 may need to operate.

Upon any necessary adjustments, the enablement signal 216 may thereafter be supplied to the floating input 236 of the operational amplifier 232 used to drive the modulation transistor 234 (Fig. 4, 408). The presence of the enablement signal 216 may induce a change in the modulation transistor 234 resulting in a Class D setting, or an enabled status. It should be appreciated that the enablement of the modulation transistor 234 may occur prior to the actual modulation in order provide a smoother transition with reduced oscillations.

Upon enablement, a small constant current may be supplied to the modulation transistor 234 via the operational amplifier 232. The operational amplifier 232 may ensure that the current is held constant by taking in voltage over the series resisters within the floating input and balancing the current against the applied enablement signal. This allows the low signal amplitude (or a passive mode of operation) to continue to vary unhindered by the operational transition taking place. A constant smaller current is sent to the RFPA via the modulation transistor 234, while larger and varying currents may come via the diode 238.

Once the peak or high amplitude signal is present, the peak amplitude may be mirrored through the enablement signal interface 218, as well as an indication of the amount of current needed for the modulation transistor 234 to provide the required amount of modulation. The operational amplifier 232 may be configured to provide the modulation transistor 234 a small amount of current over what is needed.

The time for the actual active mode transition may be indicated in a demand signal embedded in the enablement signal. The operational amplifier 232 may compare the demand value with the feedback value (via the summation point) from the series resistors in the floating input. The operational amplifier 232 in time will realize that the current being supplied by the constant source 208, or switching source 206, is not enough. In turn, the operational amplifier 232 will increase the voltage supplied to the gate 'G' of the modulation transistor 234, thereby establishing the active mode. Once the active mode has been established, the voltage at the summation point (SP) will be defined higher than what the RFPA requires. The operational amplifier 232, which is emitter coupled, may use all of its bandwidth to control the modulation transistor 234. Therefore, it is only the modulation transistor 234 that is limiting.

Thereafter, the voltage control circuit 204 may vary the output voltage 228 by modulating the second voltage 220 according to the envelope of the envelope input signal 212 via the modulation transistor 234 (Fig. 4, 410). The second voltage 220 may be adjusted via regulator 21 1 as indicated by level signal 213. It should be appreciated that the second voltage 220 need not be regulated since the modulating transistor 234 handles the current regulation for the voltage control circuit 204.

The modulated voltage may be provided as the output voltage 228 to the RFPA (Fig. 4, 412). Once the enablement signal 216 has been released, the voltage control circuit may transition back to a passive or switched mode of operation (Fig. 4, 414). It should be appreciated that operational transitions may occur to and from any of the three operational modes.

In example embodiments, an external feedback system may be included in the ET system 200 in order to improve over-all linearity of the system. It should also be appreciated that the components of the ET system 200 may be cascaded to from a voltage ladder of low voltage blocks delivering highly efficient switching. Each voltage block may comprise a very low active voltage swing in both Classes A and D.

The foregoing description of embodiments of the present invention, have been presented for purposes of illustration and description. The foregoing description is not intended to be exhaustive or to limit embodiments of the present invention to the precise form disclosed, and modifications and variations are possible in light of the above teachings or may be acquired from practice of various embodiments of the present invention. The embodiments discussed herein were chosen and described in order to explain the principles and the nature of various embodiments of the present invention and its practical application to enable one skilled in the art to utilize the present invention in various embodiments and with various modifications as are suited to the particular use contemplated. The features of the embodiments described herein may be combined in all possible combinations of methods, apparatus, modules, systems, and computer program products.

Furthermore, the various embodiments of the present invention described herein is described in the general context of method steps or processes, which may be

implemented in one embodiment by a computer program product, embodied in a computer-readable medium, including computer-executable instructions, such as program code, executed by computers in networked environments. A computer-readable medium may include removable and non-removable storage devices including, but not limited to, Read Only Memory (ROM), Random Access Memory (RAM), compact discs (CDs), digital versatile discs (DVD), etc. Generally, program modules may include routines, programs, objects, components, data structures, etc. that perform particular tasks or implement particular abstract data types. Computer-executable instructions, associated data structures, and program modules represent examples of program code for executing steps of the methods disclosed herein. The particular sequence of such executable instructions or associated data structures represents examples of corresponding acts for implementing the functions described in such steps or processes.

Claims

1 . A hybrid envelope tracking circuit (200) comprising:

first (226) and second (222) voltage node interfaces configured to receive first (224) and second (220) voltages respectively;

an input node interface (214) configured to receive an input envelope signal

(212),

an enablement node interface (218) configured to receive an enablement signal (216), the voltage control circuit (200) configured to transition from a passive or switching operating regime to an active operating regime when the enablement signal (216) is supplied;

a modulator unit (204) having a modulation transistor (234) driven by a floating operational amplifier (232), the modulator unit configured to modulate the second voltage (220) with respect to the envelope signal (212) during the active operating regime; and

an output node interface (230) configured to receive the first voltage (224) during the passive operating regime, the second voltage (220) during a switching operating regime, or the modulated second voltage (228) during the active operating regime.

2. The circuit of Claim 1 wherein the first voltage (224) is a constant DC power voltage source with high efficiency.

3. The circuit of Claim 1 wherein the first voltage (224) or second voltage (220) is regulated to adjust a voltage level.

4. The circuit of Claim 1 wherein the second voltage (220) is a variable voltage with high efficiency.

5. The circuit of Claim 1 wherein the second voltage (220) is a constant DC power voltage high efficiency.

6. A method for envelope tracking comprising:

receiving an input envelope signal (212); supplying a first voltage (224) to an output node (230) during a passive operating regime, or supplying a second voltage (220) to the output node (230) during a switching operating regime;

transitioning to an active operating regime when an enablement signal (216) is supplied;

thereafter, driving a voltage control circuit (204) with a floating input (236); modulating the second voltage (220) with respect to the input envelope signal (212);

supplying the modulated second voltage to the output node (230) during the active operating regime; and

transitioning back to the passive or switching operating regime when the enablement signal (216) is no longer supplied.

The method of Claim 6 further comprising:

regulating the first voltage (224) or the second voltage (220) to adjust a voltage level.

The method of Claim 6 wherein the transitioning to the active operating regime further comprises:

adjusting a ground reference of the input envelope signal (212) to a virtual ground level at a summation point (SP) defined in the voltage control circuit (204).

A hybrid envelope tracking system comprising (200):

first (208) and second (206) power sources proving first (224) and second (220) voltages, respectively;

an input stage (210) configured to transmit an input envelope signal (212) and an enablement signal (216), the enablement signal (216) configured to prompt a transition of the envelope tracking system to an active operating regime when supplied;

a voltage control circuit (204) having a modulation transistor (234) driven by a floating operational amplifier (232), the voltage control circuit (204) configured to modulate the second voltage (220) with respect to the envelope signal (216) during the active operating regime; and an output node (230) configured to receive the first voltage (224) during a passive operating regime, the second voltage (220) during a switching operating regime, or the modulated second voltage during the active operating regime.

10. The system of Claim 9 wherein the first power source (208) is a constant DC power voltage source with high efficiency.

1 1 . The system of Claim 10 further including an adaptive regulator (209, 211)

configured to adjust a voltage level of the first (224) or second (220) voltage.

12. The system of Claim 9 wherein the second power source (206) is a variable or multiple voltage converter with high efficiency.

13. The system of Claim 9 wherein the second power source (206) is a constant DC power voltage source with high efficiency.

14. The system of Claim 9 wherein the input stage (210) is further configured to adjust a ground reference of the input envelope signal (212) to a virtual ground level at a summation point (SP) defined in the voltage control circuit (204).