Power amplifier with output power control

This disclosure relates systems and methods for a power amplifier with output power control. The power amplifier can include multiple stages of amplification. An RF signal is fed to the power amplifier with output control that amplifies the power of the RF signal to meet operational requirements. A first stage of the power amplifier controls the output power via voltage regulation. An isolating device is introduced in the transmission path of the RF signal between the first stage and the following stages of the power amplifier. The isolating device ensures that the load impedance of the first stage remains fixed at a constant value.

BACKGROUND

Generally, in mobile communication devices based on systems such as GSM (Global System for Mobile Communication), EDGE (Enhanced Data Rates for GSM Evolution), DECT (Digital Enhanced (formerly European) Cordless Telecommunications), etc., the output power of transmitters is adjusted to reduce power consumption and minimize interference with other signals. The adjustments in the output power are usually performance in accordance with pre-defined standards that govern the implementation of such systems. For example, GSM specifications list nominal output power levels, and accepted tolerances of GSM mobile transmitters under various operating conditions. In existing systems, the output power is adjusted by regulating either the operating voltage or the quiescent current in a power amplifier.

Control of output power by regulating the operating voltage depends on the load impedance of the power amplifier. The load impedance should be maintained at a constant value for effective power control; however, in general, the load impedance may be susceptible to variations. For example, if a cell phone is placed on a metallic object, the antenna of the cell phone may pick up some noise, causing variations in load impedance of the power amplifier inside the cell phone. Power control rising voltage regulation is typically most effective when the power amplifier operates in the saturation region. In some systems, such as GSM and DECT systems, the power amplifier operates in the saturation region, while in other systems, such as EDGE, 3G (WCDMA), Wireless LAN, the power amplifier operates in the linear region. Therefore, power control by voltage regulation cannot be used effectively in all systems.

Output power control by regulating quiescent current involves adjusting the quiescent current to change the output power. This approach works well for small outputs since the current consumption is low; however, there remain issues related to transistor parameters, temperature, and dependence on input power.

DETAILED DESCRIPTION

This disclosure is directed to techniques for power control in power amplifiers. More particularly, the techniques involve implementation of an output power amplifier with output power control. The disclosed power amplifier with power control can be implemented in a variety of communication devices or systems. For example, a power amplifier with output power control can be implemented in mobile phones, base stations, etc. The following systems and methods are described with reference to a mobile communication system; however, it will be appreciated that the disclosed power amplifier with power control can be implemented generally in any electronic communication system.

Mobile communication systems include components such as mobile communication devices, base stations, etc., that can receive input signals, modulate the input signals into RF signals, amplify the RF signals as per requirement, and transmit the amplified signal. The components include a power amplifier to amplify the RF signals and provide power efficiency.

The disclosed technique for implementing a power amplifier with power control involves a combination of voltage regulation and quiescent current regulation. The disclosed power amplifier includes multiple amplification stages. A first stage involves power control via voltage regulation while the later stages may involve power control via current regulation. In one implementation, voltage regulation is performed when a small part of the output is to be converted, such as in the first stage that generally accounts for a small share of the overall efficiency of the power amplifier. Following stages, which account for the maximum share of the overall efficiency, are controlled via quiescent current. In another implementation, one or more of the following stages may also be controlled by voltage regulation.

Exemplary System

FIG. 1illustrates an exemplary system100including a power amplifier with power control. The order in which the blocks of the system are described is not intended to be construed as a limitation, and any number of the described system blocks can be combined in any order to implement the system, or an alternate system. Additionally, individual blocks may be deleted from the system without departing from the spirit and scope of the subject matter described herein. Furthermore, the system can be implemented in any suitable hardware, software, firmware, or a combination thereof, without departing from the scope of the invention.

The system100represents an RF (radio frequency) transmission section of a mobile communication device, such as a cell phone. The system100receives an input signal or incoming data-stream102. The input signal102can correspond to voice data, text data, or audio-video data. The input signal102is passed to an RF-modulator104.

The pre-amplifier and buffer106amplifies the modulated signal105, making the RF input signal suitable for further processing. The pre-amplifier and buffer106can provide a (programmable) voltage gain to the input signal105; however, the pre-amplifier and buffer106may not provide significant current gain. The pre-amplifier and buffer106can additionally provide electrical impedance transformation to the RF input signal (modulated signal)105and can be realized to have a programmable or variable gain (PGA) for systems requiring a high dynamic range for the output power levels such as 3G (UMTS). The amplified input signal is then sent to the power amplifier110.

The RF modulator104converts the amplified signal into a radio frequency-modulated signal, or RF signal105, where modulation conditions the amplified signal to be capable of being transmitted through free space. The RF signal108is then sent to a power amplifier with power control110or power amplifier110.

The power amplifier110amplifies and increases the power efficiency of the RF signal108. To provide variable power output depending upon the operating conditions, the power amplifier110can implement the disclosed techniques for power control.

In one implementation, the power amplifier110includes multiple stages of amplification. Each stage may be implemented with electronic components, such as transistors. The power control is achieved by voltage regulation in the first stage and by current regulation in the following stages. The power amplifier110includes an isolating device, for example as further described below, between the first stage and the second stage. The isolating device serves to maintain constant load impedance at the first stage by preventing return signals in the transmission path. For instance a 90°-Hybrid would be a device to maintain constant impedance. Therefore, by using the isolating device, the output of the first stage can be maintained a pre-defined value. After amplification by the power amplifier110, the amplified RF signal output112is transmitted via an antenna114.

FIG. 2illustrates a power amplifier110implemented in three stages according to one embodiment. The RF signal108is sent to stage1202. The stage1202includes electronic components such as transistors, etc., for amplifying the RF signal108. The stage1202, which controls power via voltage regulation, may account for a small percentage, such as approximately 5%, of the overall efficiency of the power amplifier110. The output power of the stage1202depends on the operating voltage of the stage1202. A voltage regulator204regulates the operating voltage in accordance with a control voltage206. The control voltage206can be supplied from any standard power source, e.g. a baseband processor output in mobile phone applications. The control voltage206can be set at a constant value, such as per an operating requirement. The voltage regulator204compares the operating voltage with the control voltage206and changes the operating voltage accordingly.

The output power of the stage1202can also vary with a change in load impedance of the electronic components in the stage1202. The load impedance may vary if any noise or interference is received in the transmission path, such as from the antenna114. To avoid noise or interference, an isolating device208(as discussed above) is introduced in the transmission path from the stage1202to the next stage. The isolating device208couples the power from the stage1202to the next stage, but isolates the stage1202from the signals coming from other stages of the power amplifier110and other components of system100, such as the antenna114. Therefore, the isolating device208can prevent variation in the load impedance due to retroactivity. The isolating device208can be implemented with the use of passive electronic components, such as transmission lines (90° branch line coupler, rat-race coupler), inductors (L-C bridge), transformers, circulators and isolating devices using anisotropic ferrites or other insolating elements. In an alternate embodiment, forced adjustments through resistors can also be made to keep the load impedance at a constant value. However resistors usually imply signal losses.

The RF signal108is sent to stage2210of the power amplifier110. At stage2210, current regulation may be used for power control. Stage2210, which can include electronic components such as transistors, can receive a variable bias current via a variable current source212-1. The control voltage206dictates the output of the variable current source212-1. Therefore, depending upon the control voltage206, the variable current source212-1regulates the bias current for the stage2210. The RF signal108received from the stage1202may be amplified further and sent to stage3214of the power amplifier110.

At stage3214, current regulation may again be used for power control. Stage3214, which can include electronic components such as transistors, receives a variable bias current via a variable current source212-2. In one implementation, the variable current source212-2can be same as the variable current source212-1. The control voltage206dictates the output of the variable current source212-2. Depending upon the control voltage206, the variable current source212-2regulates the bias current for stage3214. The RF signal108can be amplified furthermore at stage3214to meet the operating requirements of the system100. The amplified RF signal112can then be transmitted via the antenna114. The power level of the amplified RF signal112can be measured via a detector216. If the power level does not meet an operating requirement of system100, further adjustments can be made in the power amplifier110.

The current sources212-1and212-2provide a minimum bias current, where the Stage2210and Stage3214are basically controlled by an RF input signal. The effect is that a certain RF level reaching an Ube voltage will be rectified by an Ube diode (not shown) of the RF transistor or stages210and214, hence resulting in a so-called “self-bias”. To avoid that this effect abruptly starts, and that device and temperature are dependent, the amplifier stages210and214gets a bias current to “smoothen” the change-over from Class-A to Class-B operation.

The result is that the RF output signal is almost fully controlled by the input signal which is very well defined by the isolating device output and the voltage regulated first stage. If a linear mode is required (e.g., EDGE), the bias can be increased to keep a power amplifier in Class-A mode.

Another advantage of regulating mainly the first stage with a follower circuit is that regulation works very fast (i.e., limited only by the transistor device speed and the control signal itself). There exist no “real” voltage regulator in terms of a feedback loop and an operational amplifier. The control current sets the voltage without looking for a feedback signal. This works best, if a bipolar transistor is used, giving a well defined relation between base and emitter. For example, In Silicon this is a 0.7V diode voltage plus the temperature dependence (˜2 mV/K). In Germanium, this would be about 0.4V and in III/V materials (GaAs) typically voltages in the region of 1.3V. A set signal introduces less noise. Basically due to the much simpler circuitry, much less noisy elements such as current references, noisy transistors are found.

The regulation speed is independent of the control signal. In difference, a feedback using system usually uses a low-pass filter (RC Filter), which gets very slow when the current is reduced. The reason for this is physically that for a fixed capacitance, a reduced charge current leads to an increased charge time. Another advantage of the follower circuit is that there is no feedback loop, hence no instability due to positive feedback can occur. Considering power-time templates as defined for GSM System, the power regulation (time-template) may not be critical, even for low current modes.

FIG. 3illustrates an exemplary power amplifier300with power control including a variable gain amplifier. The power amplifier300may be another embodiment of the power amplifier110. The power amplifier300can be used in systems where the power amplifier110is applicable. In the following description of the power amplifier300, the components common to both the power amplifier110and the power amplifier300are referred to by the same names and reference numbers.

The power amplifier300receives the RF signal108for amplification. In the stage1202, which includes electronic components such as transistors, the output power can be controlled by voltage regulation, or by using a variable gain amplifier (VGA)/programmable gain amplifier (PGA) based on the mode of operation of the stage1202.

For example, in systems where the transistors in the stage1202, operate in the saturation region, such as in the case of GSM systems, the output power can be controlled by voltage regulation. In such a case, the operating voltage of the stage1202may be regulated based on the control voltage206. The control voltage206drives the base terminal of an NPN transistor302. In one implementation, an N-channel MOSFET can be used in place of the NPN transistor (in P-Channnel for a PNP transistor). An advantage of using a emitter follower is that no feedback to an operational amplifier is required, as the emitter follows the base voltage Ube˜0.7V (for Silicon) which can be generated by the current over a simple resistor. Alternatively, a voltage control loop can be implemented which is more accurate but has the disadvantage of every regulation, or a possible loop instability, slower response (i.e., swing in time), and noise. The emitter terminal of the transistor302can be connected to a transistor in the stage1202. The collector terminal of the transistor302is connected to a DC source304for biasing of the transistor302. The operating voltage at the stage1202is regulated with the use of the transistor302.

In another example, in systems where transistors in stage1202operate in the linear region, such as in the case of EDGE systems, output power can be controlled by using a VGA/PGA. In such systems, a variable gain amplifier306can be used. The output power of the stage1202is then dependent on the gain of the variable gain amplifier306. The gain of the variable gain amplifier306can be adjusted as needed to obtain the desired output power. The variable gain amplifier306can be implemented using transistors, operational amplifiers, and the like. The output of the stage1202may also depend on the load impedance of the stage1202. The isolating device208ensures the consistency of the output by maintaining load impedance at a constant value, as described above. The RF signal108is sent to the stage2210and subsequently to the stage3214.

At stage2210, the power control may be performed by current regulation. Stage2210, which includes electronic components such as transistors, receives a variable bias current via the variable current source212-1. Control voltage206dictates the output of the variable current source212-1. Depending upon the control voltage206, the variable current source212-1regulates the bias current for the stage2210. The RF signal108may again be amplified and sent to the stage3214of the power amplifier300for further amplification in the similar manner as performed in the stage2210.

FIG. 4illustrates an exemplary circuit diagram of the power amplifier110. The circuit diagram intends to provide a basic conceptual description of the power amplifier110and does not limit the number of components present in the power amplifier110. In the following description, the components common toFIG. 2andFIG. 3have been referred to by the same names and reference numbers.

The power amplifier110receives the RF signal108, which goes through three stages of amplification. The power amplifier110can be implemented using various electronic components such as transistors, resistors, capacitors, current sources, power sources, etc. At stage1-202, the power amplifier110receives the RF signal108via a capacitor402-1. The capacitor402-1can be a coupling capacitor that blocks DC signals that can be introduced in the path of the RF signal108. The stage1202can include an NPN transistor302for providing amplification to the RF signal108. In an implementation, an N-channel MOSFET can be used in place of the NPN transistor.

The transistor in the stage1202may operate in the saturated mode. The stage1202receives a constant current via a constant current source406. The constant current source406is energized by a current source or battery408included in the power amplifier110. The stage1202controls the output power by regulating the operating voltage of the transistor.

The NPN transistor302regulates the operating voltage in the stage1-202via the control voltage206. In an implementation, an N-channel MOSFET with low source-drain impedance can be used in place of the NPN transistor. The operating voltage may also depend on a load impedance404-1of the transistor in the stage1202.

The load impedance404-1may be susceptible to variations due to retroactivity of other stages of the power amplifier110. To avoid this, the isolating device208is introduced in the signal path between the stage1202and the stage2210. The isolating device208allows the RF signal108to pass through to the stage2210, but blocks any signal or noise from other stages to reach the stage2210. Therefore, the isolating device208ensures that the load impedance404-1remains at a constant value, thereby avoiding any problem in voltage regulation. The RF signal108may be amplified at the stage1202and sent to the stage2210and stage3214for further amplification.

The stage2210receives the RF signal108by a capacitor402-2. The capacitor402-2can be a coupling capacitor that blocks DC signals and isolates DC bias settings of the stage1202and stage2210. The stage2210includes an NPN transistor with a load impedance404-2for amplification. In an implementation, an N-channel MOSFET can be used in place of the NPN transistor.

The stage2210can control the output power by current regulation. The variable current source212regulates the bias current of the transistor in stage2210in accordance with control voltage206. After the amplification at the stage2210, the resulting RF signal108is sent to the stage3214.

Stage3214receives the RF signal108from the stage2210by a capacitor402-3. The capacitor402-3can be a coupling capacitor that blocks DC signals and isolates DC bias settings of the stage2210and the stage3214. The stage3214includes an NPN transistor with a load impedance404-3for amplification. In an implementation, an N-channel MOSFET can be used in place of the NPN transistor. The stage3214controls the output power by current regulation. The variable current source212regulates the bias current of the transistor in stage3214in accordance with the control voltage206. After amplification, the amplified RF signal112is transmitted by the antenna114.

FIG. 5illustrates an exemplary variable current source212. The stage2210and the stage3214of the power amplifier110control the output power by current regulation. To this end, the variable current source212provides a variable bias current per operating requirements, for the stage2210and the stage3214. The stage2210and the stage3214can either derive the variable bias current from two different variable bias current circuits, or from a single variable bias current circuit. In the following description, the variable current source212is described in reference to the stage2210of the power amplifier110; however, it will be understood that a similar circuit can be used at the stage3214of the power amplifier110.

The variable current source212includes a current mirror realized between an NPN transistor502-1, which may be diode connected, and the transistor in the stage2210. In an implementation, the threshold voltage of the diode-connected transistor502-1is higher than that of the current mirror. The current mirror is implemented by NPN transistors502-2and502-3. In an implementation, the transistors502-1,502-2, and502-3can be N-channel MOSFETs.

In another implementation, when the stage2210receives the RF signal108from the stage1202via the coupling capacitor402-2, a current flows from the transistor502-2of the current mirror to the stage2-210. The current introduced by the current mirror is relatively small compared to the RF signal108. A RF block resistor504-1preceding the stage2210is small enough to allow the stage2210to obtain optimal breakthrough voltage conditions. The RF block resistor504-1blocks the RF signal108to avoid electric diffusion of the RF signal108in the current mirror. If the voltage associated with the RF signal108is higher than the voltage of the variable current source212, the output power of the stage2210may be determined by the RF signal108. A resistor504-2in the emitter terminal of the diode-connected transistor502-1, may be used to achieve a correct current mirror factor and correlates with the resistor504-1and the beta of the transistors. The transistor502-3in the circuit may grounded via a “de-charge” resistor506.

Exemplary Method

FIG. 6illustrates an exemplary method600for output power control in a power amplifier. The order in which the method is described is not intended to be construed as a limitation, and any number of the described method blocks can be combined in any order to implement the method, or alternate method. Additionally, individual blocks may be deleted from the method without departing from the spirit and scope of the subject matter described herein.

The method introduced may, but need not, be implemented at least partially in architecture(s) such as shown inFIGS. 1-5. In addition, it is to be appreciated that certain acts in the methods need not be performed in the order described, may be modified, and/or may be omitted entirely. Furthermore, the method can be implemented in any suitable hardware, software, firmware, or a combination thereof, without departing from the scope of the invention.

At block602, an RF signal is received as input. For example, the power amplifier110in a mobile communication device such as a cell phone may receive the input signal102for amplification, which is modulated into the RF signal108and amplified as per operating requirement. For example, GSM specifications list nominal output power levels, and accepted tolerances of GSM mobile transmitters under various operating conditions. The power amplifier110can be responsible for providing power efficiency to the RF signal108. Therefore, output power of the power amplifier110may be controlled to meet the operating requirement.

At block604, operating voltage is set for power control. In one implementation, the power amplifier110includes multiple stages for amplification of the RF signal108. Each stage can include electronic components, such as transistors. The stage1202controls the output power by voltage regulation. The stage1202regulates the operating voltage as dictated by the control voltage206. The control voltage206drives the base terminal of the NPN transistor302. In one implementation, an N-channel MOSFET can be used in place of the NPN transistor302. The emitter terminal of the NPN transistor302can be connected to the transistor in the stage1202. The collector terminal of the NPN transistor302can be connected to a DC source304for biasing of the NPN transistor302. The operating voltage of the stage1202is regulated with the help of the transistor302. This approach is applicable to the systems, such as GSM, where the transistor in the stage1202operates in the saturated mode. In systems such as EDGE, the transistor may operate in the linear mode, and the approach for voltage regulation as described above may not be applicable. In one implementation, for systems that make use of operation of the transistor in the linear mode, the variable gain amplifier306can be used. The output power is dependent on the gain of the variable gain amplifier306. The gain of the variable gain amplifier306can be adjusted as required. The variable gain amplifier306can be implemented using transistors, operational amplifiers, and the like.

At block606, load impedance is maintained at a constant value. In one implementation, the output power of stage1202varies with a change in the load impedance of the transistor in stage1202. The load impedance may also vary if noise or interference is received from the antenna114. In an implementation, to avoid variations in the load impedance, the isolating device208may be introduced in the signal path from the stage1202to the next stage. The isolating device208is a directional coupler that couples the power from the stage1202to the next stage, but isolates the stage1202from the noise coming from the antenna114and other stages of the power amplifier110. Therefore, the isolating device208prevents any variation in the load impedance due to retroactivity. The isolating device208can be implemented with the help of passive electronic components such as inductors, transformers, 90° Hybrids, or an insulating material. In an alternate embodiment, forced adjustments can also be made in the load impedance to keep it at a constant value.

At block608, bias current is regulated for power control. In one implementation, at the stage2210and stage3214, the power control is performed by current regulation. Stage2210and stage3214can include electronic components such as transistors. The stage2210and the stage3214receive a variable bias current by the variable current sources212-1and212-2, respectively. In an implementation, the bias current can be obtained from a common variable current source212. In addition, the control voltage206dictates the output of the variable current source212. Depending upon the control voltage206, the variable current source212regulates the bias current for the stage2210and the stage3214.

At block610, the RF signal is amplified. In an implementation, the RF signal108gets amplified through various stages in the power amplifier110, with output power of the power amplifier110being controlled via voltage as well as current regulation. The amplified RF signal112is transmitted via the antenna114.

CONCLUSION

Although embodiments for power amplifier with output power control have been described in language specific to structural features and/or methods, it is to be understood that the appended claims are not necessarily limited to the specific features or methods described. Rather, the specific features and methods are disclosed as exemplary implementations for power amplifier with output power control.