GB2337384A

GB2337384A - RF amplifier with switching between a high efficiency mode and a low distortion mode

Info

Publication number: GB2337384A
Application number: GB9910003A
Authority: GB
Inventors: Anthony Peter Hulbert; John Domokos
Original assignee: Roke Manor Research Ltd
Current assignee: Roke Manor Research Ltd
Priority date: 1998-05-01
Filing date: 1999-04-30
Publication date: 1999-11-17
Anticipated expiration: 2019-04-30
Also published as: GB9910003D0; GB2337384B

Abstract

Described herein is a dual mode linear RF power amplifier circuit in which the amplifier can be switched between a mode in which it has high efficiency and a mode in which it has high linearity. The power amplifier circuit comprises an RF power amplifier (12) for amplifying an input signal (10) and producing an output signal (14). Coupling devices (16, 22) are positioned to provide the input and output signals (10, 14) to respective ones of first and second peak-to-mean determination devices (18, 24) which provide first and second signals indicative of the measured peak-to-mean ratios of the input and output signals (10, 14). A mode command signal (30) controls the gain compression ratio output by the gain compression estimation device (28) for combining with one of the first and second signals, the combined signal and the other of the first and second being fed to the error amplifier (20) to provide a bias current for the amplifier (12).

Description

DUAL MODE LINEAR RF POWER AMPLIFIER 2337384 The present invention relates

to a dual mode linear radio frequency (RF) power amplifier, and is more particularly, although not exclusively, concerned with such an amplifier in a mobile terminal for use in a telecommunications network.

Mobile terminals (or handsets) suffer from interference due to the presence of other signals in the same frequency band and also signals present in adjacent frequency bands. The tolerance of the terminal to such interference depends on the loading of the telecommunications cell of which they currently form a part, and may be time variant or location variant. A telecommunications cell comprises at least one base station and a plurality of mobile terminals which communicate with the base station(s) from within a particular area surrounding the base station(s). The boundaries of the cell are not rigidly set and can 'breathe' (expand and contract) in accordance with the loading of adjacent cells.

However, the linear power amplification of amplitude variant signals having high peak to mean ratios tends to be inefficient because the amplifier often needs to be 'backed off from its full power capability to achieve the required linearity. This means that a large bias current needs to flow in the power device to avoid unacceptable gain compression and spectral re- growth around the RF carrier.

In third generation Universal Mobile Telecommunication Systems (UMTS) for example, a wideband code division multiple access (WCDMA) air interface is proposed with a quaternary phase shift keying (QPSK) modulation scheme in the terminal equipment. The power amplifier in the terminal therefore must have very good linearity to avoid re-growth of the transmitted spectrum in the adjacent channel which would otherwise interfere with other users. In order to achieve the required adjacent channel power ratio, the power amplifier must operate at high current which in turn leads to poor efficiency and therefore short battery life in the terminal.

Under certain circumstances, higher adjacent channel power can be tolerated and therefore the linearity and the gain compression of the power amplifier is not so critical. In these cases the power amplifier could operate a more efficient mode at the expense of linearity.

It is therefore an object of the present invention to provide a linear RF power amplifier having two modes of operation.

According to the present invention, there is provided a linear RF power amplifier circuit comprising:- a linear RF power amplifier for amplifying an input signal to form an output signal; first measuring means for measuring a peak-to-mean power ratio of the input signal and providing a first signal indicative thereof; second measuring means for measuring a peak-to-mean power ratio of the output signal and providing a second signal indicative thereof; gain compression estimation means for providing a gain compression ratio in accordance with a mode command signal applied thereto; combining means for combining the gain compression ratio with one of said first or second signals to provide an error signal; and an error amplifier for receiving said error signal and the other of said first and second signal and for providing an output current for controlling the mode of operation of the linear power amplifier.

In one embodiment of the present invention, the combining means combines the second signal with the gain compression ratio. In another embodiment. the combining means combines the first signal with the gain compression ratio.

The first and second signals may comprise logarithmic signals and said combining means may comprise summing means. Alternatively, the first and second signal may comprise ratio signals and said combining means may comprise multiplying means.

The gain compression estimation means may comprise a look-up table having values relating gain compression ratios to the mode.

It is preferred that the first and second measuring means determine peak and mean power values over a first interval, and the peak-to-mean ratios are averaged over a second interval, the second interval being longer than the first interval.

According to another aspect of the present invention, there is provided a mobile terminal comprising a linear RF power amplifier circuit as described above.

For a better understanding of the present invention, reference will now be made, by way of example only, to the accompanying drawings in which:

Figure 1 shows a block diagram of one embodiment of a dual mode linear RF power amplifier in accordance with the present invention; Figure 2 shows a block diagram of a second embodiment of a dual mode linear RF power amplifier in accordance with the present invention; and Figure 3 shows a circuit diagram of a peak-to-mean detector which can be used in the embodiments of Figures 1 and 2.

Although the present invention will be described with reference to a linear RF power amplifier for use in a mobile terminal, it will be appreciated that the invention is also applicable to other applications where a linear RF power amplifier is required to operate in two modes one having high efficiency and one having high linearity.

In Figure 1, an input signal 10 for a linear RF power amplifier 12 is shown,, the amplifier producing an output signal 14. Input signal 10 is sampled through a first coupling device 16 and passed to a first peak-tomean determination device 18 where the ratio of the peak envelope power to mean envelope power for the input signal is determined. An arrangement for carrying out this determination is described later with reference to Figure 3. The output from the first peak-to-mean determination device 18 is passed to an error amplifier 20.

Similarly, the output signal 14 is sampled through a second coupling device 22 and passed to a second peak-to-mean determination device 24 where the peak envelope power to mean envelope power for the output signal is determined. It is preferred that the first and second peak-to- mean determination devices 18, 22 are identical. The output from the second peakto-mean determination device 22 is passed to a combining device 26 where it is combined with a signal from a gain compression estimator device 28. The output signal from the combining device 26 is then passed to the error amplifier 20. The combining device 26 and the error amplifier 20 comprise a bias control loop for the amplifier 12. The gain compression estimator device 28 is controlled by a mode command signal applied on line 30 as shown.

As shown in Figure 1, the first peak-to-mean determination device 18 produces a voltage signal V1 representing the input peak-to-mean power ratio. Similarly, the second peak-to-mean determination device 24 produces a voltage signal V2 representing the output peak-to-mean power ratio. If V I = V2. then the input and the output peak-to-mean ratios are the same and therefore no gain compression occurs.

The gain compression estimator device 28 receives a mode command signal to activate high or low efficiency operational modes for the amplifier 12. Depending on the properties of the RF waveform and the required adjacent channel power ratio, a voltage V3 is calculated by the gain compression estimator device 28 which represents the required gain compression ratio. This voltage V3 is injected into the bias control loop by means of the combining device 26. The appropriate bias currentId, for the required gain compression is then forced to the amplifier 12 by means of the error amplifier 20 and the bias control loop.

It will be appreciated that the bias current applied to the amplifier 12 comprises a dc current, and the greater the current the more linear the operation of the amplifier, and the lower the current the greater the efficiency of the amplifier.

Therefore, two modes of operation are possible. In Mode A, the power amplifier 12 has high linearity but poor efficiency, and in Mode B, the power amplifier 12 exhibits high efficiency but has poor linearity. In both modes, the mean output power delivered by the power amplifier 12 is the same.

Figure 2 is similar to Figure 1 and comprises identical components albeit arranged in a different way. The difference between Figure 2 and Figure 1 is that the bias control loop is located in the input side instead of the output side. In this case, the output from the first peak-to-mean determination device 18 is passed to the combining device 26. A mode command signal is provided on input 30 for gain compression estimator device 28 which forms the other input to the combining device 26. The output from the combining device 26 is passed to the error amplifier 20 as before. The output from the second peak-to-mean determination device 24 is passed directly to the error amplifier 20.

It will readily be appreciated that the combining device 26 may comprise a summing device or other suitable combining device. If the signals produced from the first and second peak-to-mean determination devices 18, 24 are voltage signals as described above with reference to Figure L then the combining device 26 may be a multiplying device. If the signals produced from the first and second peak-to-mean determination devices 18, 24 are dB signals, that is, logarithmic, then the combining device 26 may be a summing device.

A peak-to-mean determination device is illustrated in Figure 3. As discussed above, it is preferred that the first and second peak-to-mean determination devices 18, 24 are identical and therefore only one will be described.

In Figure 3, an input signal is received on line 100 from either the first or second coupling device 16, 22, that is, the input signal for the peak-tomean determination device may be either an input RF signal 10 or an output RF signal 14 as shown in Figures 1 and 2. The input signal is supplied to a first circuit portion for determining the peak envelope power of the input signal and a second circuit portion for determining the mean envelope power. The first circuit portion provides a first input to a divider circuit 108 which produces an output signal 128 indicative of the peak-to-mean ratio,, and comprises resistors 102, 106, 122, diode 104, and capacitors 124, 126 connected as shown. The second circuit portion provides a second input to the divider circuit 108 and comprises resistors 110, 114, 118, diode 112, and capacitors 116, 120 connected as shown. Divider circuit 108 uses the first and second inputs to provide an output ratio as signal 128.

The values of the components in the first and second circuit portions are chosen so that appropriate time constants can be obtained. In UMTS, for example, block designated TO, that is, the resistance values of the resistors 102 1105 controls the attack time for the circuit response, and each may be typically I OOQ; block designated T1 controls the instantaneous measurements of the power envelope for the input signal typical resistance and capacitance values for the resistor 114 and capacitor 116 may be 1 kQ and I nF respectively; block designatedT2controls the time constant for peak measurement of the input signal - typical resistance and capacitance values for the resistor 122 and capacitor 124 may be 1 OkQ and 1 nF respectively; and blocks designated-C3 control the time constants for averaging of the peak and mean measurements - typical values of resistance and capacitance values for resistors 106. 118 and capacitors 120, 126 are 1 Mf? and 1 nF respectively.

It will readily be appreciated that other resistance and capacitance values may be chosen to provide the desired time constants Top TI,, T2andC3 in accordance with the particular application or system, the resistance and capacitance values given above specifically relating to UMTS.

Naturally, there may be other techniques known to persons skilled in the art which can be utilised to provide the required peak-to-mean determination. For example, it may not be necessary to calculate the peakto mean ratio for each of the input and output signals, that is, carrying out two division deten-ninations and then combining them. Instead, the peak input signal, the mean input signal, the peak output signal and the mean output signal can be combined to provide the same result in one dividing step.

The gain compression estimation device 28 may comprise a look-up table in which appropriate values are stored for each of the two modes, the appropriate value being selected in accordance with the mode command signal 30.

Although the present invention has been described as a dual mode RF amplifier, it will be appreciated that more than two modes can be utilised if required. In this case, the gain compression estimation device 28 will contain another set of values for each additional mode.

The mode command signal 30 is transmitted to the terminal in which the amplifier in accordance with the present invention is located from the base station in the telecommunications cell in which the terminal is currently located. Naturally, the mode command signal may be altered as the loading in the cell increases. Furthermore, as the terminal moves from one cell to another, the mode command signal may change in accordance with the particular loading of the cell to which it moves.

Claims

CLAIMS:

A linear RF power amplifier circuit comprising:a linear RF power amplifier for amplifying an input signal to form an output signal; first measuring means for measuring a peak-to-mean power ratio of the input signal and providing a first signal indicative thereof; second measuring means for measuring a peak-to-mean power ratio of the output signal and providing a second signal indicative thereof; gain compression estimation means for providing a gain compression ratio in accordance with a mode command signal applied thereto; combining means for combining the gain compression ratio with one of said first or second signals to provide an error signal; and an error amplifier for receiving said error signal and the other of said first and second signal and for providing an output current for controlling the mode of operation of the linear power amplifier.
2. A circuit according to claim 1, wherein the combining means combines the second signal with the gain compression ratio.
3. A circuit according to claim 2, wherein the combining means combines the first signal with the gain compression ratio.
4. A circuit according to any one of the preceding claims, wherein the first and second signals comprise logarithmic signals and said combining means comprises summing means.
5. A circuit according to any one of claims 1 to 3, wherein the first and second signal comprise ratio signals and said combining means comprises multiplying means.
6. A circuit according to any one of the preceding claims, wherein the gain compression estimation means comprises a look-up table having values relating gain compression ratios to the mode.
7. A circuit according to any one of the preceding claims, wherein the first and second measuring means determine peak and mean power values over a first interval, and the peak-to-mean ratios are averaged over a second interval, the second interval being longer than the first interval.
8. A mobile terminal comprising a linear RF power amplifier circuit according to any one of the preceding claims.
9. A linear R-F power amplifier circuit substantially as hereinbefore described with reference to Figures 1 and 2 of the accompanying drawings.