Doherty amplifier with efficiency optimization

An amplifier comprises a main-amplifier circuit, an auxiliary-amplifier circuit and a signal-generating device. Output terminals of the main-amplifier circuit and of the auxiliary-amplifier circuit are connected according to the Doherty principle. The signal-generating device is configured to generate directly a main-amplifier signal as an input signal of the main-amplifier circuit and an auxiliary-amplifier signal as an input signal of the auxiliary-amplifier circuit.

The invention relates to an amplifier, especially a Doherty amplifier.

Doherty amplifiers conventionally comprise two amplifier branches, a main-amplifier and an auxiliary-amplifier. The two outputs of the main-amplifier and the auxiliary-amplifier are combined via a λ/4 line in the main-amplifier branch. Accordingly, the load impedance on the main-amplifier and the auxiliary-amplifier is transformed dynamically, which leads to an increase in efficiency. In this context, the input signal to be amplified is supplied via a signal splitter to the main-amplifier and the auxiliary-amplifier. The phase shifting of the divided input signals for the main-amplifier and auxiliary-amplifier is 90° in order to compensate the phase shifting in the main-amplifier branch through the necessary λ/4 line transformation disposed at the output.

Such a Doherty amplifier is known from European Patent EP 1 609 239 B1. The disadvantage with the known Doherty amplifier is that, as a result of the signal splitting and tolerances of the individual amplifiers, an optimal signal division between the main-amplifier and auxiliary-amplifier does not take place. This leads to reduced efficiency.

The invention is based upon the object of providing an amplifier which achieves a high efficiency.

The object is achieved according to the invention by an amplifier with the features of the independent claim1. Advantageous further developments form the subject matter of the dependent claims relating back to claim1.

An amplifier according to the invention comprises a main-amplifier circuit, an auxiliary-amplifier circuit and a signal-generating device. Output terminals of the main-amplifier circuit and the auxiliary-amplifier circuit are connected according to the Doherty principle. The signal-generating device is embodied to generate directly a main-amplifier signal as an input signal of the main-amplifier circuit and an auxiliary-amplifier signal as an input signal of the auxiliary-amplifier circuit. An amplifier with a very high efficiency is achieved in this manner.

The problem underlying the present invention will first be explained with reference toFIGS. 1 to 3. The structure and functioning of various exemplary embodiments of the amplifier according to the invention will then be shown with reference toFIGS. 4 to 7. The presentation and description of identical elements in similar drawings have not been repeated in some cases.

FIG. 1shows an exemplary Doherty amplifier. A signal splitter10is connected to a main-amplifier circuit11and an auxiliary-amplifier circuit12. The main-amplifier circuit11is further connected to a λ/4 line13. The auxiliary-amplifier circuit12and the λ/4 line13are connected to a further λ/4 line14.

An input signal to be amplified is supplied to the signal splitter10. The latter divides it into a main-amplifier signal and an auxiliary-amplifier signal. The main-amplifier signal and the auxiliary-amplifier signal here provide a 90° phase offset. The main-amplifier signal corresponds to the signal to be amplified. In this context, the auxiliary-amplifier signal corresponds to these signal peaks. The main-amplifier circuit11amplifies the main-amplifier signal, while the auxiliary-amplifier circuit12amplifies the auxiliary-amplifier signal.

The λ/4 line13in the main-amplifier path implements a line transformation. As a result, a dynamic transformation of the load impedance takes place in such a manner that the main-amplifier circuit11goes into saturation at a threshold value, conventionally 6 dB. With a further increase of power, the auxiliary-amplifier circuit also makes a contribution to the output power, which leads to the dynamic reduction of the load impedance for the main-amplifier circuit11. When driven at full power, each of the amplifiers supplies 50% of the total power.

The theoretical efficiency curve21shown inFIG. 2is obtained for the Doherty amplifier according toFIG. 1. With the adjusted threshold value20, the main-amplifier circuit goes into saturation. Up to this point, the auxiliary-amplifier circuit does not make a contribution to the output power. By way of comparison, the efficiency curve26of a class B amplifier is additionally shown here.

An optimal splitting of the signal between the main-amplifier circuit and the auxiliary-amplifier circuit cannot be achieved because of the signal splitting at the input, the tolerances of the individual amplifiers, different bias-current settings of the amplifiers etc. The auxiliary-amplifier circuit already supplies a part of the power, although the main-amplifier circuit is not yet in saturation.

Accordingly, the peak efficiency in the region of the threshold value is not achieved reliably. This is illustrated inFIG. 3. The efficiency curve31corresponds to the efficiency curve21fromFIG. 2. A real, releasable efficiency curve33is additionally shown. Furthermore, the efficiency curve36of a conventional class B amplifier is shown by way of comparison. However, by contrast with this class B amplifier, the efficiency curve33of the real Doherty amplifier still continues to achieve a significantly higher mean efficiency.

FIG. 4shows a first exemplary embodiment of the amplifier according to the invention. A signal-generating device40is connected to a main-amplifier circuit41and an auxiliary-amplifier circuit42. The main-amplifier circuit41is further connected to a λ/4 line43. The λ/4 line43and the auxiliary-amplifier circuit42are connected to a further λ/4 line44. The signal-generating device40is further connected to the control device46.

Instead of splitting an analog signal to be amplified into a main-amplifier signal and an auxiliary-amplifier signal, as implemented with the signal splitter10fromFIG. 1, in this context, a main-amplifier signal and an auxiliary-amplifier signal are generated by the signal-generating device40. Accordingly, the signals can be generated from digital signals to be amplified. The signal generation is advantageously implemented directly here. The signal-generating device is advantageously a modulation device or a control transmitter. The main-amplifier signal and the auxiliary-amplifier signal here are already generated by the signal-generating device40with a 90° phase offset.

The control device46controls the signal-generating device40. Accordingly, it adjusts the threshold value, at which the signal-generating device40splits the signal to be amplified into the main-amplifier signal and the auxiliary-amplifier signal. Dependent upon this threshold value, the control device46can therefore adjust different efficiency curves. This will be described in greater detail on the basis ofFIG. 5.

FIG. 5shows several efficiency curves50,51,52resulting from several different threshold values53,54,55. By adjusting these threshold values53,54,55, the control device46fromFIG. 4also simultaneously adjusts the respective efficiency curves50,51,52. The efficiency curve56of a conventional class B amplifier is additionally plotted here for comparison.

By dispensing with an analog signal splitter, a significant improvement in efficiency is therefore achieved. The attainable efficiency curves50,51,52are disposed close to the maximum theoretically attainable efficiency curve. Moreover, adjusting the threshold value also allows an optimisation of the signal currently to be amplified. Accordingly, an efficiency curve50could be adjusted for signals with very high peak values by comparison with the mean-signal value, for example, 9-12 dB, preferably 10 dB. In the case of signals with high peak values by comparison with the mean signal value, for example, 7-9 dB, preferably 8 dB, an efficiency curve51could be adjusted. In the case of signals with lower peak values by comparison with the mean signal value, for example, 5-7 dB, preferably 6 dB, an efficiency curve52could be adjusted.

FIG. 6shows a second exemplary embodiment of the amplifier according to the invention. The illustrated amplifier corresponds largely to the amplifier fromFIG. 4. The reference numbers60-64and66correspond to the reference numbers40-44and46fromFIG. 4. Additionally, the amplifier shown here contains a bias-current generating device65. The latter is connected to the control device66, the main-amplifier circuit61and the auxiliary-amplifier circuit62. In this context, the bias-current generating device65generates the bias current bias_main of the main-amplifier circuit61and the bias current bias_aux of the auxiliary-amplifier circuit62. The generation of the bias currents bias_main and bias_aux is implemented here dependent upon the required threshold value for the signal splitting. The determination of the bias currents bias_main and bias_aux to be generated is also implemented by the control device66. A further approximation of the efficiency curve to the maximum theoretically attainable efficiency curve is achieved in this manner.

The amplifier shown inFIG. 4andFIG. 6is particularly advantageous for the amplification of signals with digital modulation (for example, QAM-OFDM, COFDM, etc.). These digital modulation types are used in many applications. DVB-T, DVB-H, DVB-T2, MediaFlo, ISDB-T, ATSC, CMMB, CDMA, WCDMA, GSM etc. are mentioned here by way of example. These modulation types provide different crest factors or respectively different signal statistics. The signal statistic of these signals is either known or can be measured or calculated in the case of a signal generation. The main-amplifier signal and the auxiliary-amplifier signal can now be calculated in such a manner that the point with the highest peak efficiency is tuned optimally to the signal statistic, in order to achieve a maximal overall efficiency of the amplifier arrangement in this manner. This is shown inFIG. 7. Accordingly, the curve71shows a statistical amplitude distribution of a modulation method used. The efficiency curve70, which is adjusted by the control device46or respectively66fromFIG. 4orFIG. 6maximises the average overall efficiency of the transmission symbols to be broadcast.

A slight further improvement of the average efficiency can be achieved through a continuous monitoring of the statistic of the signal to be broadcast and a matching of the efficiency curve to be adjusted.

The invention is not restricted to the exemplary embodiment illustrated. Alongside the modulation methods already mentioned, other transmission methods are also conceivable. All of the features described above or illustrated in the figures can be advantageously combined with one another as required within the framework of the invention.