WO2012076924A1

WO2012076924A1 - Rf amplifier circuit and electronic system comprising such a circuit

Info

Publication number: WO2012076924A1
Application number: PCT/IB2010/003412
Authority: WO
Inventors: Jean Jacques Bouny
Original assignee: Freescale Semiconductors, Inc.
Priority date: 2010-12-09
Filing date: 2010-12-09
Publication date: 2012-06-14

Abstract

A radiofrequency, RF, amplifier circuit comprises an input for receiving an, RF, input signal and a circuit output for outputting an, amplified RF, output signal to a load. A carrier amplifier path comprises a carrier amplifier having a carrier amplifier input connected to the input for receiving the input signal and a carrier amplifier output for outputting an amplified carrier signal in both a low input power mode of the RF amplifier circuit and a high power input mode of the RF amplifier circuit. A first peak amplifier path comprises a first peak amplifier connected to the input, for receiving the input signal and outputting a first amplified peak signal, the first peak amplifier being inactive in the low input power mode. A second peak amplifier path comprises a second peak amplifier connected to the input, for receiving the input signal and outputting a second amplified peak signal, the second peak amplifier being inactive in the low input power mode. The first peak amplifier path and the second peak amplifier path have shared sub-paths in common with the carrier amplifier path. An output stage connects the paths to the circuit output and comprises the shared sub-paths, the shared sub-paths presenting a non-zero impedance to the carrier amplifier, both in the low input power mode and the high input power mode.

Description

Title : RF amplifier circuit and electronic system comprising such a circuit

Description Field of the invention

This invention relates to an RF amplifier circuit and an RF communication system comprising such a circuit.

Background of the invention

In the wireless telecommunications world, and particularly RF (Radio Frequency) and microwave amplifiers with the increased complexity of modulation (2.5G, 3G, WiMax, LTE, MC- GSM, etc ...), the Peak to Average Ratio (PAR) of the signals to be amplified is high and the efficiency of the RF amplifiers is going down if regular class AB amplifiers are used. To overcome this efficiency problem, high efficiency amplifier topologies like the so called "Doherty" amplifier are now widely deployed in current base-stations.

Several Doherty amplifiers with multiple auxiliary or peak amplifiers have already been described and used. Most of them are three way amplifiers, which provide a good trade-off between cost complexity, and performance improvement. In a three way Doherty, 2 auxiliary or peak amplifiers and 1 main or carrier amplifier are used.

However, the known systems exhibit several problems, for example the Q factor of the output matching networks tends to reduce the maximum bandwidth, and makes it more difficult to maintain a good matching in both low power mode and high power mode. Also, the known systems introduce phase distortion also called AM-PM conversion, and the shape of the phase distortion varies with frequency, which is a problem for pre-distortion based linearizers.

Summary of the invention

The present invention provides an RF amplifier and an RF communication system as described in the accompanying claims.

Specific embodiments of the invention are set forth in the dependent claims.

These and other aspects of the invention will be apparent from and elucidated with reference to the embodiments described hereinafter.

Brief description of the drawings

Further details, aspects and embodiments of the invention will be described, by way of example only, with reference to the drawings. In the drawings, like reference numbers are used to identify like or functionally similar elements. Elements in the figures are illustrated for simplicity and clarity and have not necessarily been drawn to scale.

Figure 1 schematically shows circuit diagram an example of an embodiment of an RF amplifier circuit Figure 2 shows a circuit diagram of an equivalent circuit for the example of FIG. 1 in the high input power mode.

Figure 3 shows a circuit diagram of an equivalent circuit for the example of FIG. 1 in the low input power mode.

Figure 4 shows a block diagram of an example of an embodiment of an RF communication system.

Figure 5, shows a graph of output current versus input signal voltage for the amplifies in the example of FIG. 1. Detailed description of the preferred embodiments

Because the illustrated embodiments of the present invention may for the most part, be implemented using electronic components and circuits known to those skilled in the art, details will not be explained in any greater extent than that considered necessary for the understanding and appreciation of the underlying concepts of the present invention and in order not to obfuscate or distract from the teachings of the present invention.

In the present document, the expression "RF amplifier" has its usual meaning and, in particular, denotes an amplifier designed to operate in a radiofrequency, i.e. a frequency band with a centre frequency of at least a few MHz, such as equal to, or less than, several GHz.

Referring to Figure 1 , the example of an RF amplifier circuit shown therein, comprises an circuit input 5, a carrier amplifier path 1 , a first peak amplifier path 2, a second peak amplifier path 3, an output stage 4 and a circuit output 6.

The paths 1-3 are connected to the circuit input 5. The carrier amplifier path 1 comprises a carrier amplifier 10 having a carrier amplifier input connected to the circuit input 5. The first peak amplifier path 2 comprises a first peak amplifier 20 connected to the circuit input 5. The second peak amplifier path 3 comprises a second peak amplifier 30 connected to the circuit input 5. As shown, the paths 1 -3 share an output stage 4 which connects the paths 1-3 to the circuit output 6. The parts of the peak amplifier paths 2-3 which are in common with the carrier amplifier path 1 are from hereon referred to as "shared sub-paths". Thus, in the shown example the output stage 4 comprises the shared sub-paths.

At the circuit input 5, an, RF, input signal may received. At the circuit output 6 the, amplified

RF, output signal can be outputted to a load. For sake of simplicity, the load is not shown in Fig. 1 however it will be apparent that any suitable load may be used, such as an antenna or otherwise.

The carrier amplifier can receive the input signal at its input and amplify the received input signal to an amplified carrier signal. The carrier amplifier has a carrier amplifier output for outputting the amplified carrier signal. The carrier amplifier 10 operates both when the power of the input signal is low, i.e. in a low power mode, and when the power of the input signal is high, i.e. a high power mode. More specific the carrier amplifier operates over the entire range from 0 to maximum power of the input signal. For example, when the input signal is small the carrier amplifier may operate in a non-saturated mode whereas the carrier amplifier may be operating in saturated mode The first peak amplifier 20 can receive the input signal and amplify the input signal to a first amplified peak signal. As shown, the peak amplifier has a first peak amplifier output for outputting the first amplified peak signal. The second peak amplifier 30 can receive the input signal, amplify the input signal to a second amplified peak signal. As shown, the peak amplifier has a second peak amplifier output for outputting the second amplified peak signal. The peak amplifiers 20,30 do not output an amplified peak signal in the low input power mode and are designed such that they only operate above the low input power mode and provide an signal in the (medium to) high power mode

In the output stage 4 the amplified signals are combined to obtain an amplified signal. As explained in more detail with reference to FIGs. 2 and 3, the shared sub-paths present a non-zero impedance to the carrier amplifier 10, both in the low input power mode and the high input power mode. Thus, the difference between the impedance in the low input power mode and high input power mode, also referred to as the load modulation is reduced. This, in turn, may help to reduce the phase distortion and the reduction in maximum bandwidth. More in particular, because the load modulation is reduced, a suitable design of the RF amplified circuit should usually be sufficient to overcome any remaining drawbacks of the load modulation.

As can be seen in Fig. 2, in the high power mode the carrier amplifier 10 and peak amplifiers 20,30 may be represented as current sources which provide respective currents I10.I20. I30- The impedance seen in the high power mode by the carrier amplifier 10 for the example of FIG. 1 thus comprises the peak amplifier paths 2,3, and in this example is formed by the parallel connections of those current sources and the shared sub-paths. In this example, the shared sub- paths respectively comprise the respective impedances of the components 41 ,42 which connect the carrier amplifier to the peak amplifier paths 2,3

It should be noted that, in this example, the components 41 ,42 are transmission lines which in the high power mode have a defined constant impedance, which in this example is constant, whereas in the low power mode the transmission lines exhibit no significant impedance themselves but transform the load impedance and more specific operate as impedance inverters which invert the load impedance and hence present an impedance to the carrier amplifier 10 in that mode.

As seen in FIG.3, in the low power mode, the peak amplifiers 20, 30 do not provide any signal and may hence be regarded as open circuits. The impedance seen in the low power mode by the carrier amplifier 10 of the example of FIG. 1 is thus formed by the components of the shared sub-paths only and the peak amplifiers 20, 30 themselves are not a component of that impedance. Accordingly, in the example, the impedance is constituted in the low power mode by the impedances in the carrier amplifier path 1 between the output of the carrier amplifier 10 and the circuit output 6 whereas any impedances in the peak amplifier paths 2,3 upstreams (in a current flow direction) of the output stage 4, i.e in parallel with the carrier amplifier 10, do not contribute to the impedance seen by the carrier amplifier 10. As a result, the parallel branches of the peak amplifier paths 2,3, in parallel with the carrier amplifier 10, present an open circuit and thus do not present impedance to the carrier amplifier 10. The impedances seen by the carrier amplifier 10 in example the transmission lines 41 ,42 only, however it will be apparent that other circuitry may be present between the carrier amplifier 10 and the circuit output 6 which may present an impedance in the low power mode. As mentioned, in the low power mode the transmission lines exhibit no significant impedance themselves but transform the load impedance and more specific operate as impedance inverters which invert the load impedance. Thus, the impedance seen by the carrier amplifier is mainly the impedance of the load inverted 2 times by the transmission lines 41 and 42.

As shown in the example, the RF amplifier circuit may further comprise an input stage 7. As shown, the input stage has an input connected to the circuit input 5 and multiple outputs of which each is connected to a respective one of the paths 1-3. The input stage 7 splits the input RF signals into respective signals for the paths 1-3, and in this example is an N-way power splitter with N being an integer equivalent to or larger than 3. The input stage 7 further comprises phase shifting elements 71 ,72, which shift the phase of the input signal to compensate for a phase shift introduced by the output stage 4 in one of more of: the carrier amplifier path 1 , the first peak amplifier path 2, the second peak amplifier path 3. In this example, the input stage 7 includes a phase shifter 71 in the first peak amplifier path 2, upstream of the first peak amplifier 20, which delays the phase with 90 degrees. The input stage includes a phase shifter 72 in the second peak amplifier path 3, upstream of the second peak amplifier 30, which delays the phase with 180 degrees. In this example, the output stage 4 introduces a delay of 90 degrees in the amplified carrier signal relative to the first amplified peak signal and a delay of 180 degrees is introduced in the amplified carrier signal relative to the second amplified peak signal. A delay of 90 degrees is introduced in the first amplified peak signal relative to the second amplified peak signal. The input stage 7 thus compensates for the delays in the output stage 4.

The amplifiers 10,20 and 30 may be implemented in any manner suitable for the specific implementation. For example, the amplifying power of the first and second peak amplifiers may be the same and be smaller, equivalent or bigger than the amplifying power of the carrier amplifier, suitable ranges for the carrier amplifier to first peak amplifier to second peak amplifier power ratio

Pcarrier: Ppeak 1 P peak 2 have found to be:

For example, where the peak amplifiers are 2 times bigger than the carrier ("1 ;2;2" amplifier): in a conventional configuration the load modulation would have been 5:1 where it is only 1.8: 1 in the present example. If amplifier corresponds to "1 ;3;3", load modulation goes from 7: 1 in the known configuration to 2.4:1 in the present example and it goes from 9:1 in the known configuration to 3:1 in the present example for a "1 ;4;4" amplifier. Of course any other power ratio can be used and it is not necessary to have the 2 peak amplifiers having the same size.

In the shown example the carrier amplifier is a class A/B amplifier and the peak amplifiers are class C amplifiers. However, the amplifiers 10,20,30 may alternatively be of a different type. As illustrated in FIG. 6. The carrier amolifier 10 amolifies the inout sianal for anv voltaae V_in (and hence power) of the input signal, from zero (V_in= 0 V) up to a maximum (V_in=V_max). As shown, at a threshold voltage V_thr the carrier amplifier enters a saturated mode above which the current output remains more or less constant. The peak amplifiers 20,30 are configured such that they start amplifying when the input signal exceeds a threshold voltage V_thr (0 < V_thr < V_max). It should be noted, that the threshold voltage for the peak amplifiers depends on the size ratio between the carrier and peak amplifiers and that the peak amplifiers may have the same or different threshold voltages.

The output stage 4 may be implemented in any manner suitable for the specific implementation. In the shown example, the paths 1-3 provide a connection between the circuit input 5 and the circuit output 6, and overlap, at least partially, in the output stage 4. More in particular, in this example the paths 1-3 provide parallel connections between the circuit input 5 and the output stage 4 and the output stage 4 comprises a chain of transmission lines 41 , 42 in series. In the shown example, the amplified signals provided through the paths 1-3 are superimposed. The amplified signals are superimposed with suitable phase delays which are compensated, partially or completely, by a phase delay upstream of the amplifiers in the respective paths.

The first end of the chain is outside the peak amplifier paths 2-3, and connected to the output of the carrier amplifier 10. The node between the transmission lines 41 ,42 is part of both the carrier amplifier path 1 and the first peak amplifier path 2 and connected to the first peak amplifier 10. Through that node the amplified carrier signal is superimposed on the first amplified peak signal. The other end of the chain is part of both the carrier amplifier path 1 , the first peak amplifier path 2 and the second peak amplifier path 3 and connected to the second peak amplifier 20. At that end the combined amplified carrier signal and first amplified peak signal are superimposed on the second amplified peak signal.

The output stage 4 may, as shown in fig. 1 , for example comprise a first transmission line 41 and a second transmission line 42 in series between the carrier amplifier output and the second peak amplifier output. In the shown example, the carrier amplifier output is connected to the circuit output 6 through the first transmission line 41 and the second transmission line 42 and the first peak amplifier output is connected to a node between the first transmission line 41 and the second transmission line 42. The transmission lines 41 ,42 may for example be quarter wavelength transmission lines, with the wavelength λ corresponding to that of the carrier frequency of the RF signal

It should be noted that the quarter wavelength transmission lines may have a length that does not correspond exactly to λ/4, and for example have different lengths. For example a transmission line may have a length which introduces a 95 degrees phase shift, and another a transmission line may have a length which introduces a 85 degrees phase shift.

As explained above, and as can be seen in figure 3, at low power the peak amplifiers 20,30 are off and present an open circuit at their respective outputs. In that case, the load impedance R_L is transformed by the quarter wave length lines in a impedance R_m0d, higher than the load called load modulation. In the high power mode as can be seen in figure 2, by an appropriate choice of the impedances, both the carrier amplifier 10 and the peak amplifiers 20,30 can see their optimum impedance. Supposing that the carrier amplifier 10 has an optimum impedance R_opt_camer and the peak amplifier has an optimum impedance R_op_auxi , then the impedances Z₄₁ and Z₄₂ can be set to

and Z₄₂= R₀p_auxi//Ropt_camer- , and the load impedance at output node 6 is set to R_op_auxi Rop_auxi Ropt_camer- So that all amplifiers see their optimum impedance ( the sign "//" signifying "in parallel with")

Fig. 4 shows an example of a suitable implementation of the transmission lines 41 ,42. The example shown in FIG. 4, is a flat strip of metal, in this example a stripline micro-strip line, with a length of half a wavelength λ. The strip has a constant first width over a first half of the length, and a constant second width over a second half of the length. Thus, the strip is split in two sections, each of a length of λ/4, with different impedances and thus operates as two quarter wavelength transmission lines in series.

Referring to FIG. 4 FIG. 5, the example of an RF communication system shown therein comprises an antenna 102 for emitting RF signals, and a source 101 of RF signals. An RF amplifier circuit 100, for example as shown in FIG. 1 , is connected to the antenna and the source. More in particular the RF amplifier circuit is connected between the source 101 and the antenna, in order to receive the RF signals from the source 101 and output those to the antenna 102. It will be apparent that between the source 101 and the amplifier 100, and the amplifier 100 and the antenna 102 other components may be provided.

In the foregoing specification, the invention has been described with reference to specific examples of embodiments of the invention. It will, however, be evident that various modifications and changes may be made therein without departing from scope of the invention as set forth in the appended claims and that the claims should not be interpreted as being restricted to the shown examples.

For example, the RF amplifier may be implemented on any suitable type of semiconductor substrate, such as of any semiconductor material or combinations of materials, such as gallium arsenide, silicon germanium, silicon-on-insulator (SOI), silicon, monocrystalline silicon, gallium nitride, the like, and combinations of the above.

The connections as discussed herein may be any type of connection suitable to transfer signals from or to the respective nodes or components, for example via intermediate components. For instance between the amplifier 10,20,30 and the transmission lines 41 ,42 impedance matching or other elements may be present in the paths 1-3. Accordingly, unless implied or stated otherwise, the connections may for example be direct connections or indirect connections. The connections may be illustrated or described in reference to being a single connection, a plurality of connections, unidirectional connections, or bidirectional connections. However, different embodiments may vary the implementation of the connections. For example, separate unidirectional connections may be used rather than bidirectional connections and vice versa. Also, plurality of connections may be replaced with a single connection that transfers multiple signals serially or in a time multiplexed different connections carrying subsets of these signals. Therefore, many options exist for transferring signals.

Any arrangement of components to achieve the same functionality is effectively "associated" such that the desired functionality is achieved. Hence, any two components herein combined to achieve a particular functionality can be seen as "associated with" each other such that the desired functionality is achieved, irrespective of architectures or intermedial components. Likewise, any two components so associated can also be viewed as being "operably connected," or "operably coupled," to each other to achieve the desired functionality.

Also for example, the RF amplifier may be implemented as any number of separate integrated circuits or separate devices interconnected with each other in a suitable manner. For example, the amplifiers 1-3 may be provided as a single integrated circuit to which discrete components are connected to obtain the circuit shown in fig. 1. Alternatively, the RF amplifier circuit may be implemented as circuitry located on a single integrated circuit.

Also for example, the examples, or portions thereof, may implemented as soft or code representations of physical circuitry or of logical representations convertible into physical circuitry, such as in a hardware description language of any appropriate type.

However, other modifications, variations and alternatives are also possible. The specifications and drawings are, accordingly, to be regarded in an illustrative rather than in a restrictive sense.

In the claims, any reference signs placed between parentheses shall not be construed as limiting the claim. The word 'comprising' does not exclude the presence of other elements or steps then those listed in a claim. Furthermore, the terms "a" or "an," as used herein, are defined as one or more than one. Also, the use of introductory phrases such as "at least one" and "one or more" in the claims should not be construed to imply that the introduction of another claim element by the indefinite articles "a" or "an" limits any particular claim containing such introduced claim element to inventions containing only one such element, even when the same claim includes the introductory phrases "one or more" or "at least one" and indefinite articles such as "a" or "an." The same holds true for the use of definite articles. Unless stated otherwise, terms such as "first" and "second" are used to arbitrarily distinguish between the elements such terms describe. Thus, these terms are not necessarily intended to indicate temporal or other prioritization of such elements The mere fact that certain measures are recited in mutually different claims does not indicate that a combination of these measures cannot be used to advantage.

Claims

1 . A radiofrequency, RF, amplifier circuit, comprising:

an input for receiving an, RF, input signal;

a circuit output for outputting an, amplified RF, output signal to a load;

a carrier amplifier path comprising a carrier amplifier having a carrier amplifier input connected to the input for receiving the input signal and a carrier amplifier output for outputting an amplified carrier signal in both a low input power mode of the RF amplifier circuit and a high power input mode of the RF amplifier circuit,

a first peak amplifier path comprising a first peak amplifier connected to the input, for receiving the input signal and outputting a first amplified peak signal, said first peak amplifier being inactive in the low input power mode;

a second peak amplifier path comprising a second peak amplifier connected to the input, for receiving the input signal and outputting a second amplified peak signal, said second peak amplifier being inactive in the low input power mode;

said first peak amplifier path and said second peak amplifier path having shared sub-paths in common with said carrier amplifier path;

an output stage which connects said paths to said circuit output and comprising said shared sub-paths, said shared sub-paths presenting a non-zero impedance to the carrier amplifier, both in the low input power mode and the high input power mode.

2. An RF amplifier circuit as claimed in claim 1 , wherein:

the output stage comprises a first transmission line and a second transmission line in series between the carrier amplifier output and the second peak amplifier output; and

the carrier amplifier output is connected to the circuit output through the first transmission line and the second transmission line and the first peak amplifier output is connected to a node between the first transmission line and the second transmission line.

3. An RF amplifier circuit as claimed in any one of the preceding claims, wherein said first transmission line and the second transmission line are quarter wavelength transmission lines.

4. An RF amplifier circuit as claimed in any one of the preceding claims, where the amplifying power of the first and second peak amplifiers is bigger than the amplifying power of the carrier amplifier.

5. An RF amplifier circuit as claimed in any one of the preceding claims, wherein the first and second peak amplifiers have substantially the same amplifying power.

6. An RF amplifier circuit as claimed in any one of the preceding claims, further comprising an the output stage in one of more of: the carrier amplifier path, the first peak amplifier path, the second peak amplifier path.

7. An RF communication, comprising an antenna for emitting RF signals, an RF amplifier circuit as claimed in any one of the preceding claims connected to said antenna and a source of said RF signals connected to said RF amplifier circuit.