Amplifier

In one embodiment an amplifier circuit is disclosed. The amplifier circuit comprises an amplifying device configured to amplify a radiofrequency signal, the amplifying device having an output dynamic range; a supply modulator configured to modulate a supply voltage supplied to the amplifying device when an output of the amplifying device is within a first region of the output dynamic range; a tuneable matching network coupled to an output of the amplifying device; and a load controller configured to control the tuneable matching network, when the output of the amplifying device is within a second region of the output dynamic range, and thereby modulate the load to which the output of the amplifying device is applied.

FIELD

Embodiments described herein relate generally to amplifier circuits having load modulation and supply modulation

BACKGROUND

Communication signals such as Long Term Evolution (LTE) data communications, Digital Television (DTV) transmissions and LTE-Advanced communications transmissions possess large peak to average power ratios (PAPR). This causes the efficiency of power amplifiers (PA) to be very low. Increasing PA efficiency is very important for both for mobile devices and stationary base station transmitters to reduce power consumption.

DETAILED DESCRIPTION

In one embodiment an amplifier circuit is disclosed. The amplifier circuit comprises an amplifying device configured to amplify a radiofrequency signal, the amplifying device having an output dynamic range; a supply modulator configured to modulate a supply voltage supplied to the amplifying device when an output of the amplifying device is within a first region of the output dynamic range; a tuneable matching network coupled to an output of the amplifying device; and a load controller configured to control the tuneable matching network, when the output of the amplifying device is within a second region of the output dynamic range, and thereby modulate the load to which the output of the amplifying device is applied.

In an embodiment, the supply modulator comprises a charge pump circuit.

In an embodiment, the first region and the second region do not overlap.

In an embodiment, the first region comprises a higher output power than the second region.

In an embodiment the amplifier circuit further comprises a supply modulator bypass circuit configured to selectively bypass the supply modulator.

In an embodiment, the supply modulator bypass circuit is configured to bypass the supply modulator when the peak-to-average power ratio of the radiofrequency signal is below a threshold.

In an embodiment, the supply modulator bypass circuit is configured to bypass the supply modulator when the supply modulator fails to operate.

In an embodiment the amplifier circuit further comprises a load controller bypass circuit configured to selectively bypass the load controller.

In an embodiment, the load controller bypass circuit is configured to bypass the load controller when the peak-to-average power ratio of the radiofrequency signal is below a threshold.

In an embodiment, the load controller bypass circuit is configured to bypass the load controller when the load controller fails to operate.

FIG. 1shows an amplifier circuit100. The amplifier circuit100comprises a baseband processing module110and a radiofrequency power amplifier (RFPA)120. The baseband processing module110outputs a baseband signal112which is up-converted by an up-conversion module114using a local oscillator (LO)116. The up-converted signal is a radiofrequency (RF) signal118which is amplified by the RFPA120. The RFPA120comprises an amplifying element122. The amplifying element122is supplied by a fixed supply voltage124. A matching network (MN)126is connected across the output of the amplifying element122. The output of the RFPA120is an amplified RF signal130. A coupler132couples a feedback path to the amplified RF signal130. The feedback path carries a RF feedback signal134. The RF feedback signal134is down-converted by a down-conversion module136using the LO116. This provides a baseband feedback signal138which is fed into the baseband processing module110.

FIG. 2shows the gain and efficiency against output power for the amplifier circuit shown inFIG. 1. As shown inFIG. 2, the gain rises from approximately 5 dB for a normalised output power of −20 dB to approximately 15 dB for a normalised output power of −10 dB. The gain then levels off at approximately 15 dB for normalised output powers between −10 dB and −2 dB. The gain drops to approximately 10 dB for a normalised output power of 0 dB. The efficiency rises from approximately 2% for a normalised output power of −20 dB to 20% for a normalised output power of −10 dB. The efficiency then increases more rapidly with output power and reaches a peak of 60% for a normalised output power of −1 dB.

As shown inFIG. 2, amplifiers such as the one shown inFIG. 1suffer from low efficiency. Even the combination with digital pre-distortion will still lead to average efficiency of less than 30% when amplifying and OFDM type of signal. This is shown inFIG. 2where under a signal with 8 dB peak-to-average power ratio the amplifier is expected to have efficiency well below 30%.

FIG. 3shows a supply modulated amplifier circuit300. Common reference numerals are used for elements also shown inFIG. 1. The supply modulated amplifier circuit300comprises a baseband processing module310which in addition to providing a baseband signal112also provides a supply modulator control signal342. A supply modulator340modulates the supply voltage324supplied to the amplifying device122using the supply modulator control signal342. The remaining features of the supply modulated amplifier circuit300are as described above with reference toFIG. 1.

FIG. 4shows the operating regions of the supply modulated amplifier circuit shown inFIG. 3. As shown inFIG. 4, supply modulation is implemented for high output powers. However, for low output powers, the device gain drops.

FIG. 5shows the gain and efficiency against output power for the amplifier circuit shown inFIG. 3. As shown inFIG. 5, the efficiency is approximately 10% for a normalised output power of −20 dB, this rises to approximately 62% for a normalised output power of −6 dB and then drops to approximately 60% for normalised output powers in the range −4 dB to −1 dB. The gain rises with normalised output power from approximately 7 dB for a normalised output power of −20 dB to approximately 12 dB for a normalised output power of −2 dB.

FIG. 6shows the normalised supply voltage for the amplifier circuit shown inFIG. 3. The curves inFIGS. 5 and 6result from measurements performed on a prototype of the circuit shown inFIG. 3. All the combinations of input power Pin and supply voltage were applied while measuring output power efficiency and gain. The single lines plotted for the gain, input power and supply voltage in the figures correspond to the values that lead to the maximum efficiency at each output power level. This is the curve of efficiency given inFIG. 5. As shown inFIG. 6, the normalised supply voltage is set to approximately 0.35V for normalised output powers in the range −20 dB to −10 dB. The normalised supply voltage is increased to 0.5V for normalised output powers of −7 dB to −6 dB. The normalised supply voltage is increased to 0.85V for a normalised output power of −2 dB and increased to 1V for a normalised output power of −1 dB.FIG. 6also shows a load control signal which is has the value of 1 for all normalised output powers. This is to illustrate that the matching network126has a fixed impedance.

It can be seen that firstly the efficiency of the amplifier circuit300at the −8 dB output power region has been dramatically increased with regards to the amplifier circuit shown inFIG. 2. Secondly, the signals that control the input of the amplifier and supply voltage are monotonically increasing signals with increasing power so the frequency content of the signals driving the amplifier are not significantly increased.

The gain of the amplifier is a strong function of the supply voltage, and in general drops for low supply voltages. This is the reason also that the supply voltage is not controlled for output power lower than −10 dB. Due to the fact that the device gain depends on the supply voltage which is here not fixed, the input power has to be co-controlled with the supply voltage to maximise efficiency and linearity. This leads to the non-smooth curves of Pin versus output power.

FIG. 7shows an amplifier circuit700with dynamic load modulation. Common reference numerals are used for elements also shown inFIG. 1. The dynamic load modulated amplifier circuit700comprises a baseband processing module710which in addition to providing a baseband signal112also provides a load modulation signal752which is received by a load controller750. The load controller750controls the impedance of a tuneable matching network (TMN)726coupled to the output of the amplifying element122. The impedance of the TMN726is controlled by a load control signal754. The load controller750is implemented as an amplifier which amplifies the load modulation signal752to provide the load control signal754. The remaining features of the amplifier circuit700are as described above with reference toFIG. 1.

FIG. 8shows the operating regions of the load modulated amplifier shown inFIG. 7. As shown inFIG. 8, at low output powers there is no efficiency enhancement from dynamic load modulation. For high output powers, dynamic load modulation is carried out.

FIG. 9shows the gain and efficiency of the dynamic load modulated amplifier circuit shown inFIG. 7. As shown inFIG. 9, the efficiency is approximately 4% for a normalised output power of −20 dB, this rises steadily to approximately 60% for a normalised output power of −1 dB. The gain rises with normalised output power from approximately 5 dB for a normalised output power of −20 dB to approximately 12 dB for a normalised output power of −2 dB.

FIG. 10shows the load control signal for the amplifier circuit shown inFIG. 7. As shown inFIG. 10, the supply voltage takes a single value shown as a normalised supply voltage of 1V for all normalised output powers. The normalised load control signal has a value of approximately 0.125 of a normalised output power of −20 dB. This decreases to 0.1 for normalised output powers in the range −16 dB to −10 dB. The value of the load control signal then increases to approximately 0.13 for a normalised output power of −7 dB. The value of the load control signal then increases rapidly from approximately 0.2 to approximately 0.65 for normalised output powers of −3 dB to 0 dB.

As described above, for the dynamic load modulation architecture seen inFIG. 7, a tuneable matching network (TMN) is controlled by an additional amplifier. This is relatively easy to design compared to the supply modulator described before. This is because of the low drive current requirement which additionally doesn't have a large impact on system efficiency. The required current is as low as 1% of that from a supply modulator. This architecture still provides a large efficiency increase compared to the single stage system, but not as high as supply modulation. This is due to two reasons. First, because of the different loss mechanisms involved in the two systems which favour supply modulation and second due to the finite tuning range that practical TMN can provide—between 1:3 and 1:6—as shown inFIG. 10. Similarly, the drive signals are monotonically increasing functions which are appealing so that the signals will not show large frequency expansion.

FIG. 11shows an amplifier circuit1100according to an embodiment. The amplifier circuit1100uses a combination of supply modulation and dynamic load modulation to extend its high efficiency dynamic range.

The amplifier circuit1100comprises a baseband processing module1110and a radiofrequency power amplifier (RFPA)120. The baseband processing module1110is coupled to an amplifier750, which acts as a load controller, and a supply modulator340. The RFPA120comprises an amplifying element122. A tuneable matching network (TMN)726is coupled to the output of the amplifying element122. The load controller is configured to control the impedance of the TMN726. The supply modulator340is coupled to the amplifying element122. The output of the RFPA is coupled to a feedback path by a coupler130.

The baseband processing module1100outputs a baseband signal112, a supply modulator control signal342and a load modulation signal752. The baseband signal is up-converted by an up-conversion module114using a local oscillator (LO)116. The up-converted signal is a radiofrequency (RF) signal which is amplified by the RFPA120.

The RF signal118is supplied to the gate of the amplifying element122. A supply voltage324supplied to the drain of the amplifying element122is modulated by the supply modulator340. The impedance of the TMN726is controlled by a load control signal754. The load controller amplifier750amplifies the load modulation signal752to provide the load control signal754.

A coupler132couples a feedback path to the amplified RF signal130. The feedback path carries a RF feedback signal134. The RF feedback signal134is down-converted by a down-conversion module136using the LO116. This provides a baseband feedback signal138which is fed into the baseband processing module110.

The amplifier circuit shown inFIG. 11operates with supply and load modulation at each output power level as indicated inFIG. 12. These combinations can be determined using calibration and characterisation procedures.

FIG. 12shows an example of the control of the dynamic load modulation and supply modulation used with the amplifier circuit ofFIG. 11. As shown inFIG. 12, the dynamic load modulation and the supply modulation are used together across a range of output powers.

FIG. 13shows the shows the gain and efficiency against output power for the amplifier circuit shown inFIG. 11. As shown inFIG. 13, the efficiency is approximately 12% for a normalised output power of −20 dB, this rises to approximately 65% for a normalised output power of −6 dB and then drops to approximately 60% for normalised output powers in the range −4 dB to −1 dB. The gain rises with normalised output power from approximately 7 dB for a normalised output power of −20 dB to approximately 12 dB for a normalised output power of −2 dB.

FIG. 14shows an example of the normalised supply voltage and load control signal for the amplifier circuit shown inFIG. 11using dynamic load modulation and supply modulation together as described in relation toFIG. 12. The curves presented here are representative of amplifiers of the type described in relation toFIG. 12, however those of skill in the art will appreciate that the values will vary between different implementations. Those of skill in the art will also appreciate that this also applies to the other curves described.

As shown inFIG. 14, the supply voltage is set at 0.3V for normalised output powers of −20 dB to −11 dB, and then increases to 0.5V for normalised output powers of −11 dB to −6 dB. From normalised output powers −6 dB to −1 dB, the supply voltage is increased with output power from 0.5V to 1V. The normalised load control signal has a value of approximately 0.125 of a normalised output power of −20 dB. This increases to approximately 0.2 for a normalised output power of −14 dB, then increases more rapidly to 0.5 for normalised output power of −11 dB. The value of the load control signal then drops to approximately 0.22 for a normalised output power of −9 dB. The value of the load control signal increases from approximately 0.2 to approximately 0.65 for normalised output powers of −9 dB to −6 dB. The value of the load control signal drops from approximately 0.65 to 0.5 when the normalised output power changes from −6 dB to −4 dB. The value of the load control signal increases from 0.5 for a normalised output power of −3 dB to 1 for a normalised output power of 0 dB.

A first problem is that the very high complexity of the supply modulator will be present in this system. This is because the supply voltage of the RFPA has to be modulated at a fast speed to increase its efficiency. The problem occurs because the supply modulator has to provide a large current to the amplifier which has also has fast, that is high frequency components. It is difficult to achieve both of these conditions without having a very complex or power inefficient modulator. This is because a large size of transistors would be needed to provide the large current which makes them inefficient in high frequencies. Topologies exist to combat this which split the signal in a low and high frequency components and amplify the components separately before recombining them to feed the amplifier. However, due to the multiple paths that exist in such modulators their design is complex. As shown inFIG. 13, the efficiency and gain characteristics of the amplifier circuit are improved by using both load modulation and supply modulation. However, as shown inFIG. 14, the load control signal has problematic regions in which it is not a monotonically increasing function. The problematic regions are marked in the plot. These will introduce abrupt changes of the signal in the time domain, causing its frequency content to expand. This is problematic both for the digital to analogue converter driving the system input, and also for parts of the circuitry as the frequency response of the circuitry in that path will have to be able to accommodate the high frequency components.

FIG. 15shows an amplifier circuit1500according to an embodiment. The amplifier circuit1500uses a combination of supply modulation and dynamic load modulation to extend its high efficiency dynamic range. The amplifier circuit1500has a supply modulator bypass switch1541and a load controller bypass switch1551. The remainder of the amplifier circuit is as described above in relation toFIG. 11. The supply modulator bypass switch1541allows the supply modulator340to be bypassed. The load controller bypass switch1551allows the load controller750to be bypassed.

The amplifier circuit shown inFIG. 15can adapt its operation based on the characteristics of the input signal. If the peak-to-average power ratio (PAPR) is low, dynamic load modulation or supply modulation can be disabled by bypassing respectively, the load controller750or the supply modulator340.

Additionally, or alternatively, the supply modulator340or the load controller750can bypassed in case the associated circuitry is damaged and is malfunctioning. This allows the amplifier circuit to still operate if parts of it are damaged. As shown inFIG. 15, bypassing mechanisms are included so that if the supply modulator fails, a fixed supply voltage can be provided while dynamic load modulation will still provide some efficiency enhancement. In a similar fashion, if the TMN or the TMN amplifier fails and cannot modulate the load showing static impedance to the amplifier, the load control signal can be bypassed and only supply modulation will enhance the efficiency. The bypass switches may be controlled by the baseband processing module1510on the basis of the baseband feedback signal138.

Thus such embodiments provide a flexible amplifying circuit. Depending on the signal characteristics the amplifier circuit can adapt its operation for better performance. For example, if only a signal with 5-6 dB PAPR is amplified, the supply modulator and analogue to digital converter (ADC) driving it can be switched off reducing the power consumption while still providing sufficient efficiency enhancement for the PA. As soon as the PAPR of the signal increases, supply modulation can be re-introduced. Such control may be implemented by the base band processing module1310.

FIG. 16shows the operating regions of the amplifier circuit1500shown inFIG. 15. As shown inFIG. 16, the dynamic load modulation is operated in a low output power region and the supply modulation is operated in a high output power region. In the example shown inFIG. 16, the two operating regions do not overlap.

FIG. 17shows the shows the gain and efficiency against output power for the amplifier circuit shown inFIG. 15operated with the operating regions shown inFIG. 16. As shown inFIG. 17, the efficiency is approximately 8% for a normalised output power of −20 dB, this rises to approximately 65% for a normalised output power of −6 dB and then drops to approximately 60% for normalised output powers in the range −4 dB to −1 dB. The gain rises with normalised output power from approximately 7 dB for a normalised output power of −20 dB to approximately 12 dB for a normalised output power of −2 dB.

From a comparison ofFIGS. 17 and 13, it can be seen that by separating the operating regions as described in relation ofFIG. 16, there is a small decrease in efficiency for low output powers.

FIG. 18shows the normalised supply voltage and load control signals for the amplifier circuit shown inFIG. 15using dynamic load modulation and supply modulation in separate regions as described in relation toFIG. 16. As shown inFIG. 18, the supply voltage is set at 0.5V for normalised output powers in the range of −20 dB to 6 dB, and then increases gradually to 1V over the range of normalised output powers of −6 dB to 0 dB.

The normalised load control signal has a value of approximately 0.125 of a normalised output power of −20 dB. This increases to approximately 0.25 for a normalised output power of −11 dB. The value of the load control signal then increases more rapidly with normalised output power to reach a value of 1 for a normalised output power of −4 dB. The load control signal remains at the value of 1 for all normalised output powers greater than −4 dB.

As shown inFIG. 18, the dependence of both the supply voltage and the load control signal on the normalised output power is monotonic. As avoids the introduction of high frequency components into the control signals and simplifies the circuit requirements. The separation of the modes also provides ease of practical implementation. If the two modes are not separated, driving the system becomes extremely challenging if not impossible, especially with broadband signals like LTE-Advanced.

In an embodiment, the supply modulator is implemented as a charge pump modulator. This provides a relatively simple and efficient modulator. Other modulator types could also be used.

FIG. 19shows a supply modulator implemented as a charge pump modulator in an embodiment. The supply modulator1900comprises a level shifting and biasing network1910, an envelope amplifier1920a diode D1and a capacitor C1.

The supply modulator1900provides a supply voltage324for the RFPA based in response to an envelope signal342. The envelope signal342is the supply modulator control signal342provided by the baseband processing module in the embodiments described above.

A modulator supply voltage V+ is provided to the envelope amplifier1920. The modulator supply voltage V+ is coupled to the supply voltage324of the RFPA by the diode D1. The envelope signal342is coupled to the level shifting and biasing network1910. The output from the level shifting and biasing network1910is coupled to the input of the envelope amplifier1920. The capacitor C1is connected between the output of the envelope amplifier1920and the supply voltage324of the RFPA.

The supply modulator1900is configured so that the envelope amplifier1920does not track the entire envelope of an input RF signal. When the desired output voltage is below a threshold, the output from the level shifting and biasing network1910is set to zero and therefore the output of the envelope amplifier1920is zero. In this operating regime, the modulator supply voltage V+ is coupled to the supply voltage324of the RFPA. In this operating regime, the capacitor C1is charged by a current flowing through the diode D1.

When the desired output voltage is above the threshold, the level shifting and biasing network1910provides an input signal to the envelope amplifier1920. This results a voltage at the output of the envelope amplifier1920. Since the capacitor C1is charged with a voltage of V+, the result is that the output voltage of the envelope amplifier1920is added to the voltage V+. Thus the capacitor C1acts a charge pump and the supply modulator1900can supply an output voltage324which is greater than the modulator supply voltage V+.

FIG. 20shows a supply modulator implemented as a charge pump modulator in an embodiment. The supply modulator2000shown inFIG. 20comprises a switched mode power supply (SMPS)2010which is controlled by a power control signal2020. The power control signal2020is provided by the baseband processing module described above. The remaining features of the supply modulator2000shown inFIG. 20are as described above with reference toFIG. 19. The addition of the SMPS2010in the supply modulator2000provides the advantage of achieving output power control which increases the overall efficiency of the circuit.