Patent Application
20100184389
An embodiment of the present invention relates to bias circuits for linear power amplifiers. The invention is applicable to, but not limited to, an arrangement to improve the linearity of power amplifiers suitable for EDGE/3G applications.
A primary focus of the present invention is the field of radio frequency (RF) and microwave amplifiers capable of use in telecommunication applications. Continuing pressure on the limited spectrum available for radio communication systems is forcing the development of spectrally-efficient linear modulation schemes. Since the signal envelopes of a number of these linear modulation schemes fluctuate, intermodulation products can be generated in the non-linear power amplifier. Specifically, in this field, there has been a significant amount of research effort focused on developing highly efficient topologies using the ‘back-off’ (linear) region of the power amplifier.
However, in order for a communication unit to transmit non-constant envelope modulation signals, the amplified signals must not be distorted. Notably, such non-constant envelope transmissions are required from communication units supporting the known EDGE/3 G communication standard. Numerous techniques are continually being proposed to further minimise the distortion introduced by the power amplifier into a non-constant envelope transmission by such wireless communication units.
US2003/0139153 A1 by R. J. McMorrow and titled “EDGE Power detector/controller” describes a circuit with a power control loop that is suitable for use with the EDGE communication standard. The circuit uses a log detector to detect an absolute power level, which is subsequently used to control the PA output power. However, the use of a logarithmic detector is solely used to control the output power.
US 2004/0174212 A1 by K. Bummam and titled “Doherty Amplifier Using Adaptive Bias Control” describes a very specific circuit with an injection circuit for a Doherty amplifier, which contains two power amplifiers. The circuit includes an envelope detector and highly complex envelope shaping circuits that are used to set a gain of a Doherty amplifier.
U.S. Pat. No. 6,785,521 B2 by A. Hadjichristos and titled “System and method for current-mode amplitude modulation” describes a circuit with a controllable current source. However, U.S. Pat. No. 6,785,521 B2 fails to explain any mechanism as to how the modulation envelope may be sensed.
There exists a need to improve the linearity of a power amplifier arrangement, and method of operation therefor, for a given current consumption or, in the converse, decrease the given current consumption to maintain a particular linearity.
In accordance with aspects of the present invention, there is provided a wireless communication unit, integrated circuit and method of operation, as defined in the appended Claims.
Exemplary embodiments of the present invention will now be described, by way of example only, with reference to the accompanying drawings, in which:
An embodiment of the present invention will be described in terms of a wireless communication unit capable of operation in accordance with the EDGE or 3G standards, as defined by the third generation partnership project (3GPP). However, it will be appreciated by a skilled artisan that the embodiments herein described may be embodied in any type of radio frequency amplifier arrangement, and biasing circuit therefor, where improving the linearity and efficiency of the power amplifier stage is important.
Referring now to
The user interface 130 is operably coupled to a memory unit 116 and a timer 118 via the signal processing function 108 and/or a controller 114. The controller 114 is also coupled to the receiver front-end circuit 106 and the signal processing function 108. The controller 114 may therefore receive bit error rate (BER) or frame error rate (FER) data from recovered information. The controller 114 is coupled to the memory device 116 for storing operating regimes, such as decoding/encoding functions and the like. A timer 118 is typically coupled to the controller 114 to control the timing of operations (transmission or reception of time-dependent signals) within the wireless communication unit 100.
As regards the transmit chain, the input device is coupled to a transmitter/modulation circuit 122 via the signal processing function 108 (or function 128 if the transmit and receive portions of the signal processor were distinctly implemented). Thereafter, the transmit signal is passed through a power amplifier 124 to be radiated from the antenna 102. The transmitter/modulation circuit 122 and the power amplifier 124 are operationally responsive to the controller 114, with an output from the power amplifier 124 coupled to the duplex filter or antenna switch 104. The transmitter/modulation circuitry 122 and receiver front-end circuitry 106 comprise frequency up-conversion and frequency down-conversion functions (not shown).
In summary, in accordance with an embodiment of the present invention, the power amplifier circuit 124, and particularly its associated bias circuit 126 has been adapted to provide a new envelope modulated-injection signal to apply to, and by, the bias circuit of the power amplifier in order to improve the power amplifier linearity. Injecting modulation into the biasing of the PA with the correct phase and amplitude reduces the output distortion signal at RF because the injected signal is folded around the RF and its harmonics by the non-linearity of the PA.
Notably, a voltage of the envelope modulated signal is ‘sensed’ by a logarithmic detector. In this context, the term ‘sensed’ encompasses a portion of the signal to be transmitted that is either input to the power amplifier or an amplified version output from the power amplifier.
In a first embodiment of the present invention, the detected voltage is phase shifted and is then converted into a current. This current is injected in the PA biasing circuit: via either the base/gate or the collector/drain of the RF power transistor. Thus, the envelope modulated signal is combined with the dc biasing signal that is applied by the bias control circuit to bias the RF PA.
In an alternative (second) embodiment of the present invention, the detected voltage is amplified and injected as a voltage in either the base/gate or the collector/drain of RF transistor. In this regard, an amplifier is used to ensure that the detected envelope modulated signal is applied at the correct amplitude (as well as phase, which is set by the phase shifter). The adaptation of the Power amplifier bias circuit 126 is further described below with respect to
It is within the contemplation of the present invention that the embodiments are equally applicable with any known biasing circuit, as would be appreciated by a skilled artisan.
It will be appreciated that the various components and circuits within the wireless communication unit 100 can be arranged in any suitable functional topology, in order to utilise the embodiments of the present invention. Furthermore, the various components within the wireless communication unit 100 can be realised in discrete or integrated component form, with an ultimate structure therefore being merely an application-specific selection.
Advantageously, the first and second embodiments of the present invention improve the linearity of the power amplifier 124, particularly for EDGE/3G applications that use envelope modulation.
Referring now to
Referring now to
In this manner, the first and second embodiments of the present invention provide a new envelope-injection circuit that improves power amplifier linearity. In particular, the first and second embodiments of the present invention propose using the logarithmic detector in a novel manner; that is to detect the envelope modulation of the RF PA output (or input) signal rather than the amplified RF power level.
Referring now to
The sensed envelope modulation signal is then input to a phase shifter, where the envelope modulation signal is phase shifted and thereafter input, say (according to the first embodiment), to a voltage to current (V2I) converter 220.
The output of the V2I converter 220 is input to a direct current (DC) biasing circuit configured to supply a bias current 440 to the PA. Here, the DC input voltage 425 is summed with the V2I converter output. The V2I current is passed to a first reference transistor 420, whose base port is operably coupled to the base of the PA 435 to force the PA base voltage. A reference voltage, such as a battery voltage 405 is provided as a collector voltage to a second transistor 410, which is supplying the base current for the PA 435.
The summed current, i.e. the dc plus modulation current is provided to the collector of the first reference transistor. It is envisaged that this summation may be performed at any suitable location, such as before the V2I or immediately after the V2I or employing two V2I elements and combining their output.
In this manner, the V2I, in contrast to known uses of the V2I function, is now arranged to provide a bias current to the PA. Furthermore, the logarithmic detector 210, 310 can also be utilised within the loop that controls the output power delivered by the PA in the known manner.
In an enhanced embodiment of the present invention, the use of a logarithmic detector and V2I converter arrangement in the above manner is only used periodically or intermittently (any time desired), say during an Edge burst transition, i.e. during a ramp-up and/or a ramp-down process.
This invention can also be used in the known 3G (WCDMA or other) standard. In this regard, it is envisaged that the log detector can be used to control the PA output power during the periods where the modulation is constant (advantageously utilising such period of times) and used for re-injection during the real signal transmitting periods. In this enhanced embodiment, once a transmission burst is underway, the delivered output power is held constant, and the logarithmic detector, phase shifter and V2I converter arrangement is no longer used to bias the power amplifier. In this regard, the power-control loop is periodically opened and closed in alignment with the EDGE burst. In Edge mode, the log detector is used to control the PA output power in a closed-loop manner, during ramp-up and ramp down of the burst. During the burst itself, the output-power control loop is opened and the log detector is used for the re-injection process. Outside of the ramp-up or ramp-down process, it is envisaged that the V2I converter reverts to its normal operation.
Thus, the logarithmic detector senses the PA envelope modulation signal and re-injects this modulation signal as a low-frequency AC signal (without changing the nominal DC biasing value) into the PA biasing circuit in a dynamic process. In this manner, the logarithmic detector in the first and second embodiments of the present invention has been used to improve the linearity of the radio frequency power amplifier by ‘re-injecting’ an envelope modulation signal in the biasing control circuit, in contrast to its known use of detecting a PA output power level, and this detected level then being processed to determine an optimal bias setting of the bias control circuit.
Referring now to
Referring now to
The phase shifted envelope modulation signal is then either amplified (if a voltage bias mechanism is used) or V2I converted (if a current bias mechanism is used), as shown in step 630. Thereafter, the V2I converted or amplified envelope modulation signal is applied as a low-frequency current or voltage signal to the bias control circuit of the RF PA, as in step 635.
Advantageously, the proposed power amplifier bias circuit is self-adjusting for all output power levels, due to the intrinsic properties of the logarithmic detector. The log detector provides constant modulation envelope results, whatever the PA output power, for small modulation indexes (which is the case for Edge and 3G modulation standards), therefore providing a constant gain (versus output power) for the re-injection process.
A skilled artisan will appreciate that in other applications, alternative functions/circuits/devices and/or other techniques may be used that still apply the embodiments hereinbefore described.
It will be understood that the improved wireless communication unit, integrated circuit to provide bias control of a radio frequency power amplifier, and method of operation therefor, as described above, aims to provide at least one or more of the following advantages:
In particular, it is envisaged that the aforementioned embodiments can be applied by a semiconductor manufacturer to any front-end module. Furthermore, the embodiments can be applied to any power amplifier device or bias circuit therefor. It is further envisaged that, for example, a semiconductor manufacturer may employ the embodiments in a design of a stand-alone device, or application-specific integrated circuit (ASIC) and/or any other sub-system element.
Whilst the embodiments of the present invention are described above, it is clear that one skilled in the art could readily apply variations and modifications of such embodiments that fall within the concepts described.
Thus, an improved wireless communication unit, integrated circuit to provide a bias control of a radio frequency power amplifier, and method of operation therefor have been described, wherein the aforementioned disadvantages with prior art arrangements have been substantially alleviated.
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/EP2005/009963 | 8/9/2005 | WO | 00 | 4/5/2010 |
