RF power amplifiers

Abstract: A radio frequency ( r.f.) amplifier power control system comprising an r.f. power amplifier for supplying r.f. transmission power to an antenna, a current sensing resistor via which operating current is fed to the power amplifier, a differential amplifier to a first input terminal of which a feed-back voltage derived from the sensing resistor indicative of r.f. transmission power is fed and to a second input terminal of which a reference voltage is fed thereby to produce in the presence of a voltage difference between the feed-back voltage and the reference voltage a control voltage in dependence upon which the output of the power amplifier is controlled in a sense such that the voltage difference tends to be nullified, whereby the transmission power is controllable in dependence upon the reference voltage, and a shunt which is fed with current from the sensing resistor the impedance of which shunt is arranged to vary responsively to the control voltage.

Description: This invention relates to radio frequency (r.f.) amplifier power control systems. More especially but not exclusively it relates to such systems for use in r.f. transmitters designed in accordance with the 'Groupe Speciale Mobile' (GSM) specification which is a mobile radio standard well known to those skilled in the art. Many radio transmitters including GSM mobile unit transmitters are required to be switched on and off in a controlled fashion. In known GSM systems, the switching on and off of a mobile transmitter to control the duty cycle and transmitted power may conveniently be effected using a power control loop wherein a power amplifier control signal is derived by comparing a signal indicative of transmitter output power with a reference voltage which is derived dynamically in dependence upon timing, power output and frequency spectrum requirements. One of the problems with presently known power control loops is that the dynamic control range tends to be insufficient with the result that if the loop characteristics are set to handle the highest transmission power envisaged then the loop operates unsatisfactorily at small transmitter output powers and vice-versa as will hereinafter be explained in greater detail. US Patent 5,412,341 discloses a closed loop power control system for an RF amplifier wherein a smooth turn-on is ensured by compensating for a threshold level of a control voltage of the amplifier. It is therefore an important object of the present invention to provide an r.f. power amplifier control system which operates satisfactorily at both ends of its power output range. The invention is defined by the appended claims. One embodiment of the invention will now be described by way of example only with reference to the accompanying drawings in which:- FIGURE 1 is a schematic block/circuit diagram of a known r.f. power amplifier control system, FIGURE 2 is a schematic block/circuit diagram of an r.f. power amplifier control system according to the present invention, FIGURE 3 is a graph showing output power characteristics appertaining to the systems shown in Figures 1 and 2, and FIGURE 4 is a further graph which illustrates operation of the system shown in Figure 2. Referring now to Figure 1, a power amplifier control system comprises a power amplifier 1 which is fed with current from a supply rail 2 via a sensing resistor 3. The sensing resistor 3 is coupled via a feed-back line 4 to one input terminal 5 of a differential amplifier 6. The other input terminal 7 of the differential amplifier 6 is fed from a reference current source 8 and a load resistor 9 which is coupled to the power supply rail 2, and across which a reference voltage is developed. A control voltage which is present on an output line 10 of the differential amplifier 6 is fed to the power amplifier 1. The power amplifier 1 is arranged to feed an antenna 11. The power fed to the antenna 11 is controlled in accordance with a reference voltage developed accross resistor 9. Thus in operation of the arrangement if the reference voltage developed accross resistor 9 is increased, the control voltage on the line 10 is increased so that the r.f. power output to the antenna 11 is increased accordingly whereby the voltage dropped across the sensing resistor 3 increases. The power output to the antenna 11 is thus ramped up/down until the voltages on the input terminals 5 and 7 of the differential amplifier 6 are equal whereby the control voltage on the line 10 tends to a steady state value. A problem with this known control system is that the sensing resistor 3 must be of a very small resistance so that the voltage dropped across it is not significant and thus at small power outputs a very small feed-back voltage is produced on the line 4. It will be appreciated therefore that the specification of the differential amplifier, which is used to compare the feed-back signal at the terminal 5 with the reference signal at the terminal 7, must accommodate a wide range of feed-back voltages at the terminal 5 as would correspond to the voltages present at maximum and minimum power outputs respectively. In practice these requirements cannot be satisfactorily met, since if the system is arranged to operate satisfactorily for high outputs then during conditions of low output the feed-back signals on the line 4 become too small and the differential amplifier is unable to provide sufficient gain-bandwidth. In order to overcome this problem, a system as shown in Figure 2 is provided wherein parts corresponding to those shown in Figure 1 bear as appropriate the same numerical designations. The arrangement of Figure 2 corresponds substantially to Figure 1 with the exception that the power amplifier 1 is shunted by a transistor 12 the collector of which is coupled via a resistor 13 to the line 4 and the emitter 14 of which is grounded via a resistor 15. The base of the transistor 12 is coupled to the control line 10 via a resistor 16. In operation of the system, current drawn by the transistor 12 is determined by the control voltage on the line 10. It will be appreciated that a current Ic which passes through the resistor 3 will be equal to a current Id which is drawn by the power amplifier 1 plus a current Ib which is drawn by the transistor 12. It will be appreciated therefore that the current supplied via the resistor 3 to the power amplifier is increased, due to operation of the transistor, from Id to Id + Ib. It will also be appreciated that at high output powers Id will be large whereas at low output powers Id will be relatively smaller whereby the transistor 12 has a proportionately greater effect at low powers whereby control at low powers is enhanced, such that the dynamic control range of the system is effectively increased. The effect of this is illustrated in Figure 3 wherein broken line 17 corresponds to the power output envelope for the circuit of Figure 1 and the line 18 represents the power output envelope for the circuit shown in Figure 2. The graph of Figure 4 shows the manner in which the currents Ib, Ic and Id vary with a control voltage Vc present on the line 10. It can be seen from Figure 3 that with a system which includes the circuit of Figure 1, the output power tends to overshoot and is slower to respond to a change of reference voltage at the input 7 to the differential amplifier 6 whereas with the improved system as shown in Figure 2, response is much quicker and smoother. As shown in Figure 4 as the system is turned on initially, current drawn through the resistor 3 is supplied primarily to the transistor 12. However when the control voltage Vc on the line 10 is high enough, the current Id starts to rise, and rises faster than Ib. The speed of operation of a control circuit comprising the differential amplifier 6 the resistors 3, 13, 15 and 16, the power amplifier 1 and the transistor 12 at any time is directly related to the slope of the Ic/Vc characteristic curve and it is apparent from Figure 4 that the slope of this characteristic curve increases around the point when the power amplifier starts to switch on. Without the additional current path provided by the transistor 12, the slope of the Ic/Vc characteristic would be approximately zero around this point thus causing slow operation of the loop. Various modifications may be made to the arrangement shown in Figure 2 without departing from the scope of the invention and for example although in the present example the differential amplifier 6 is a fast operational amplifier any other kind of suitable differential amplifier may be used. Also, although the shunt in the present example is a transistor 12, in alternative arrangements any other suitable type of controllable shunt may be used which would provide appropriate impedance changes as required.

Claims: An RF amplifier power control system comprising: an RF power amplifier (1) for supplying RF transmission power to an antenna (11); a current-sensing impedance (3) arranged to supply operating current (Id) to the power amplifier; a differential amplifier (6) having first (5) and second (7) input terminals, wherein the first input terminal (5) is arranged to receive a feedback voltage (4) indicative of RF transmission power derived from the current-sensing impedance, and wherein the second input terminal (7) is arranged to receive a reference voltage (8,9), said differential amplifier (6) being arranged to produce a control voltage (Vc) in response to the presence of a voltage difference between the feedback voltage (5) and the reference voltage (7), said RF amplifier power control system being further arranged to receive said control voltage such that the transmission power of the RF power amplifier is controlled according to the control voltage in a sense such that the voltage difference tends to be nullified; characterised in that said system further comprises: a shunt (12,13,14,15) which also receives said control voltage, and which is fed with current (Ib) from the current-sensing impedance (3), the impedance of said shunt being arranged to vary responsively to the control voltage. A system according to claim 1, further arranged such that the current (I c ) flowing through the current-sensing impedance (3) is increased, by the operation of the shunt (12,13,14,15) thereby to increase the sensitivity of the feedback voltage to the control voltage. A system as claimed in claim 1 or claim 2 wherein the current sensing impedance (3) is a resistor. A system as claimed in claim 1 or claim 2 wherein the shunt (12,13,14,15) comprises a transistor (12) which is arranged effectively to shunt the power amplifier (1), the control voltage being applied to a base terminal of the transistor via a resistor (16). A system as claimed in claim 4 wherein a collector terminal of the transistor (12) is connected via a load component (13) to the sensing impedance (3) and an emitter terminal (14) of the transistor is coupled via a further resistor (15) to ground, the base of the transistor being coupled via a series resistor (16) to an output terminal (10) of the differential amplifier (6). A system according to any of claims 4-5 characterised in that the transistor (12) is arranged to conduct a current substantially larger than the current drawn by the power amplifier (1) in response to relatively low control voltages and to conduct a current substantially smaller than the current drawn by the power amplifier in response to relatively high control voltages. A system according to claim 6 characterised in that the transistor (12) is arranged to conduct relatively little current at relatively low control voltages; to conduct a relatively rapidly increasing current at increasing control voltage values and to conduct a relatively slowly increasing current at relatively high control voltages. A system as claimed in any preceding claim including a transmitter antenna (11) which is fed from the power amplifier (1). A system as claimed in any preceding claim forming a part of a mobile unit transmitter of a mobile radio system. A system as claimed in claim 9 wherein the mobile radio system is a GSM mobile radio system.

Inventors: DAVIS COLIN MICHAEL, WHITTLE MARTIN VICTOR, DAVIS, COLIN MICHAEL, WHITTLE, MARTIN VICTOR
Applicants: ROKE MANOR RESEARCH, ROKE MANOR RESEARCH LIMITED
Application year: 1996
Main classification: H03G3
IPC codes: H03G3