GB2369734A

GB2369734A - Improving the linearity and efficiency of an amplifier by adjusting a bias current in accordance with leakage to an adjacent channel

Info

Publication number: GB2369734A
Application number: GB0029419A
Authority: GB
Inventors: John Domokos
Original assignee: Roke Manor Research Ltd
Current assignee: Roke Manor Research Ltd
Priority date: 2000-12-02
Filing date: 2000-12-02
Publication date: 2002-06-05
Also published as: GB0029419D0

Abstract

The invention relates to power amplifier control circuits; especially such circuits as are intended to maintain substantially linear operation, with good efficiency, of RF amplifiers used in mobile communications systems utilising linear modulation schemes. It addresses the difficulties associated with, on the one hand, operating the amplifier with a highly linear characteristic, necessary to conform with standards laid down for mobile communications systems and which specify a minimum acceptable value of Adjacent Channel Power Ratio (ACPR) to avoid causing interference to other users, whilst, on the other hand, avoiding the use of a large bias current which adversely affects battery lifetime. Accordingly, the invention provides, for a power amplifier subject to a prescribed value of Adjacent Channel Power ratio (ACPR), a closed loop control system in which the output ACPR is measured and the bias current adjusted to maintain a predetermined ACPR value, thereby permitting the power amplifier to operate at the lower limit of the acceptable ACPR.

Description

POWER AMPLIFIER CONTROL CIRCUIT This invention relates to power amplifier control circuits, and it relates especially to such circuits as are intended to maintain substantially linear operation with good efficiency.

Power amplifiers used in various operating environments, for example RF amplifiers used in mobile communications systems utilising linear modulation schemes, are required to exhibit linear response characteristics. The linearity of such amplifiers, however, is known to depend upon the amount of bias current applied to the active devices and thus, in order to ensure conformity to a highly linear characteristic, it is traditional to use a large bias current. This expedient, however, tends to render operation of such amplifiers inefficient in terms of power usage.

In circumstances where power usage needs to be minimised, e. g. in mobile telephone handsets and other mobile terminals, the use of high bias currents cannot be tolerated, as the lifetime of the terminal's battery is adversely affected. On the other hand, if the required linearity of operation of the power amplifier is not achieved, the terminal can cause interference to other users of the communications system.

Operational standards which have been laid down for mobile communications systems specify a minimum acceptable value for a criterion known as the Adjacent Channel Power Ratio (ACPR) which is directly related to the linearity, and hence the efficiency, of the power amplifier. If the ACPR of a terminal falls below the minimum acceptable value, the terminal will cause interference to other users.

On the other hand, it is highly desirable, for reasons of efficiency and

thus battery life, to operate the power amplifier at the lower limit of the acceptable ACPR.

Existing efforts to achieve the required operational balance between linearity and power usage require the bias current of the power amplifier to be controlled in direct proportion to the required transmitted power. This, however, does not achieve optimal balance, partly because the relationship between the ACPR and the bias current varies from terminal to terminal, and also with temperature, and partly because the relationship can be influenced by certain properties of the transmitted signal (e. g. peak to mean ratio; CCPDF; bandwidth, etc.).

It is therefore an object of this invention to provide a power amplifier control circuit in which the operational balance between linearity and power usage is addressed with greater precision.

According to the invention there is provided a control circuit for a power amplifier subject to a prescribed value of Adjacent Channel Power ratio (ACPR), the circuit including means for monitoring prevailing operational values of the ACPR associated with said amplifier, and means for developing, for said amplifier, a bias current influenced by the monitored values.

By this means, a closed loop control system is provided, in which the output ACPR is measured and the bias current adjusted to maintain a predetermined ACPR value.

Preferably, the operational level of the ACPR is monitored by converting the output signal of the power amplifier down to a suitable frequency at which actual measurements of total power and adjacent channel power are made.

By this means, measuring operations free of the handling and transmission constraints associated with RF operation can be achieved.

The invention is also intended to encompass mobile terminal devices including such circuits, and intended for use in communications systems such as cellular telephone systems.

In order that the invention may be clearly understood and readily carried into effect, certain embodiments thereof will now be described, by way of example only, with reference to the accompanying drawings, of which: Figure 1 shows, in schematic block diagrammatic form, a power amplifier control circuit in accordance with one example of the invention, and Figures 2 to 4 respectively show, in similar form to Figure 1, alternative arrangements for certain operational functions of the circuit.

In all figures of the drawings, identical components carry the same reference numbers.

Referring now to Figure 1, a power amplifier 1 develops an output signal on a line referenced 2. A portion of the output signal on line 2 is coupled to a down-converter 3 which reduces the output signal's frequency to a level at which total power and adjacent channel power measurements can be conveniently and accurately made. Total power and adjacent power measurement circuits are shown schematically as respective blocks 4 and 5; output signals from the former being applied directly to one input of a comparator circuit 6. Output signals from the block 5, representing the measured ACPR value, are applied to a gain block 7 into which is programmed the required ACPR value, and the output of the gain block 7 is applied to the other input of the comparator 6, whereby the output of the gain block 7 is compared with the total power measured by circuit 4.

The comparator 7 produces a stream of output pulses, the duty cycle of which is proportional to the instantaneous ACPR times the required ACPR (i. e. gain). These pulses are integrated in an integrator circuit 8 and the resulting control signal is applied to the bias circuitry of the power amplifier 1, thereby completing a closed servo control loop in which the bias current is influenced by the measured values of power and ACPR.

The frequency down-conversion of the power amplifier's output signals and the measurements performed upon the down-converted signals can be achieved in various alternative ways, three of which are illustrated (by way of example only) in Figures 2,3 and 4.

In the arrangement shown in Figure 2, the down-converter, schematically represented by means of the block 3 in Figure 1, is illustrated as comprising a mixer 30, in which the signals coupled from line 2 are mixed with oscillations derived from an external local oscillator (not shown) fed to the mixer 30 on a line 31.

The down-converted signals from the mixer 30 are applied to a low-pass (roofing) filter 32 and thence, directly, to the total power measuring device 4 and via a high-pass filter 33 to the adjacent power measuring device 5; from which points the circuit is identical to that of Figure 1.

Referring now to Figure 3, a mixer 30 is again employed in the down-conversion arrangement but, in this configuration, the reference oscillations are derived, by integration in a circuit 34, from the signals coupled from line 2. The output signals from the mixer 30 are applied

to a full-wave rectifier circuit 35, and thence to a roofing filter 32. In this case, however, because of the action of full wave rectification implemented by the circuit 35, the output signals from the roofing filter 32 can be applied directly to one input of the comparator circuit 6. The adjacent channel power monitoring limb, however, still requires the provision of high-pass filter 33, the measuring circuit 5 and the gain block 7.

The arrangement shown in Figure 4 incorporates into the mixing operation associated with the frequency down-conversion a square-law device such as a Gilbert cell 36, with a linear local oscillator. The remainder of the circuit conforms to that shown in Figure 3.

Although the invention has been described with reference to certain specific embodiments, its scope is not intended to be limited by the details of such embodiments, all of which are described by way of example only.

Claims

CLAIMS : 1. A control circuit for a power amplifier subject to a prescribed value of Adjacent Channel Power ratio (ACPR), the circuit including means for monitoring prevailing operational values of the ACPR associated with said amplifier, and means for developing, for said amplifier, a bias current influenced by the monitored values.
2. A circuit according to Claim 1 wherein a closed loop control system is provided for said amplifier, the loop including means for measuring the output ACPR of the amplifier and means for adjusting the bias current of the amplifier to maintain a predetermined ACPR value.
3. A circuit according to Claim 1 or Claim 2 wherein the means for monitoring the operational level of the ACPR incorporates means for down-converting the frequency of the output signal of the power amplifier.
4. A circuit according to Claim 3 including respective means for measuring total power and adjacent channel power in the downconverted signal.
5. A circuit according to Claim 4 comprising a comparator circuit connected to receive, at comparison inputs thereof, signals derived from said total power measuring means and said adjacent channel power measuring means respectively.
6. A circuit according to Claim 5 wherein the comparator is configured to generate a stream of output pulses, the duty cycle of which is influenced by the relationship between the measured ACPR and the required ACPR.
7. A circuit according to Claim 6 including an integrator circuit for integrating said output pulses to generate a control signal for application to bias circuitry of the power amplifier.
8. A circuit according to any of claims 3 to 7, wherein the frequency down-conversion of output signals from the power amplifier comprises a mixer connected to mix signals derived from said output signals from the power amplifier with oscillations derived from an external local oscillator.
9. A circuit according to any of claims 3 to 7, wherein the frequency down-conversion of output signals from the power amplifier comprises a mixer connected to mix signals derived from said output signals from the power amplifier with reference oscillations derived by integrating the same said output signals.
10. A circuit according to Claim 8 or Claim 9 comprising a fullwave rectifier circuit connected to receive output signals from the mixer.
11. A circuit according to any of claims 3 to 7, wherein the frequency down-conversion of output signals from the power amplifier comprises a square-law device with a linear local oscillator.
12. A control circuit for a power amplifier subject to a prescribed value of Adjacent Channel Power ratio (ACPR), the circuit being substantially as herein described with reference to and/or as shown in any of the accompanying drawings.
13. A mobile terminal device for use in a communications system; the device including a circuit according to any preceding claim.