An arrangement for eliminating offset errors in a power control circuit of a pulsed transmitter final amplifier

Abstract: A high frequency power amplifier (10) amplifies periodic signals in the form of HF bursts. The power output of the amplifier is adjusted by a control loop (20) which produces a control signal in response to a comparison made between a set-point value (BV) and the actual power output (Put) of the amplifier and applies the signal to the power amplifier. The control loop includes a differential amplifier (25). A correction circuit (40) measures the output voltage of the differential amplifier over a time period (t₁ to t₂ ) prior to each HF burst and stores in a memory (C) a magnitude proportional to this output voltage. During the following HF burst, this magnitude is used to introduce into the control loop a correction magnitude (Ukorr) for correction of offset voltages (Uoff) occurring in the control loop.

Description: TECHNICAL FIELD The present invention relates to an arrangement for eliminating offset errors in a power control circuit for a pulsed transmitter power amplifier particularly in mobile radio apparatus for TDMA-systems. BACKGROUND ART In a cellular mobile radio system, it is necessary to keep transmitter power to the lowest level possible without impairing signal quality, so as to reduce the risk of interference between neighbouring cells which transmit on mutually the same channels. Transmission quality is therefore measured continuously and the transmitter power is adjusted to one of a number of possible transmitter power levels (for instance 8). In known systems, for instance the system described in EP-A-0 403 247, an automatic gain control device for producing a gain controlled and d.c. offset removed signal is shown. In known mobile telephony systems, for instance the system described in U.S. 4,523,155, which relates to a mobile telephone of the FDMA-type, a base station measures signal quality and sends a signal to the mobile telephone, where the digital signal is converted to an analog signal which represents the set-point value of the desired signal level. A detector measures the power output of the transmitter stage and generates a voltage which represents said power output (the actual or real value). This voltage is compared with the set-point value and a transmitter stage control voltage is generated. Because of the wide dynamic range of the power output, the detector voltage will vary greatly and at the lowermost transmitter powers will be of the same order of magnitude as the offset voltage normally found in the differential amplifier used in the control circuit as a means for comparing the set-point value with the actual value. Consequently, the control circuit may have difficulty in distinguishing between offset voltage and the detector output signal at very low levels. DISCLOSURE OF THE INVENTION In TDMA-type mobile telephone systems in which the transmitter is switched on intermittently during an allotted time slot, it is particularly important to be able to control the power outputs efficiently down to very low levels, in order to prevent the transmitter from causing spectrum spreading when switched on and off. The error caused by the offset voltage at low levels can therefore not be tolerated and must be compensated for. Hitherto, this compensation has been achieved by circuit trimming in manufacture. Trimming, however, has been found insufficient owing to the fact that the offset voltage is temperature-dependent. Furthermore, trimming is time consuming. According to the invention, compensation can be achieved readily and in a simple manner at the pulsated transmitter final stage of class C. This is based on the fact that such a transmitter stage has a knee in the power output control voltage characteristic. Shortly before the transmitter is to start, a period for measuring and compensating the offset voltage is introduced. Because this compensation is repeated for each transmitter pulse, any effects caused by temperature are also compensated for. The characteristic features of the invention are set forth in the following Claims. BRIEF DESCRIPTION OF THE DRAWINGS The invention will now be described in more detail with reference to the accompanying drawings, in which Figure 1 is a block schematic of a known control system; Figure 2 is a diagram illustrating the principles of an inventive control system; Figure 3 is a voltage diagram which illustrates compensation of the offset voltage; and Figures 4 A-E are pulse diagrams illustrating a number of signals that can be used for controlling the transmitter part of a typical mobile telephone system according to the GSM standards. BEST MODE OF CARRYING OUT THE INVENTION Figure 1 is a block schematic which illustrates a known control stage for a transmitter final stage. Reference 10 identifies a standard power amplifier module of the kind generally used in mobile telephones. The module has an input 11 for a modulated HF-signal that has been generated in an earlier stage in the transmitter part of the mobile telephone, a current supply input 12, a power output control input 13, and an output 14 which is connected to an antenna circuit, not shown. Reference 20 indicates generally a regulator having an input 21 to which there is applied a set-point or control value UB generated in a digital/analog converter (not shown) as a function of a digital power control signal. A detector 23 is connected to the power amplifier for the purpose of generating an output signal as a function of the power output of the transmitter final stage and the output of the detector is connected to a second input 22 on the regulator. The regulator includes a differential amplifier 25 whose inputs are connected to the regulator inputs 21 and 22 and whose current source output is connected through a filter RlCl to a drive circuit 26, the output of which is connected to the regulator output 27. The regulator output is connected in turn to the power output control input 13 of the transmitter final stage. Drawn in the line between the detector output and the regulator input 22 is a voltage generator 28 which symbolizes the offset voltage Uoffs of the differential amplifier 25. It will be evident that the offset voltage Uoffs will be added to the detector voltage. At low power levels, the voltage output of the detector will be of the same order of magnitude as the offset voltage and consequently the power output is liable to deviate from a determined value to an impermissible extent. Figure 2 is a corresponding block schematic illustrating an inventive control stage. Corresponding details have been identified with corresponding references. In the Figure 2 embodiment, the current supply input 12 of the power module 10 is connected to the voltage source +Vce via a small resistance R2 and via the resistance R3 to one input 22 of the regulator 20. The resistance R2, which is relatively small (e.g. 0.1 Ohm) results in a voltage drop which is proportional to the collector current of the transmitter module, this collector current representing the power output Po (or more correctly a non-linear function of Po) with sufficient accuracy. The regulator 20 of the Figure 2 embodiment has a somewhat different construction than the regulator of the Figure 1 embodiment. In order to adapt the power control signal on the input 21 to the fact that the signal on the input 22 represents a non-linear function of Po, there is provided a non-linear circuit 30 which is connected to the plusinput of the differential amplifier 25 through a lowpass filter and an amplifier 32 with current output. In order to adapt the level of the signal on this input, the amplifier 32 is supplied from the same voltage source +Vce as the power module 10. In addition to being connected to the control input 13 on the power module 10, the output of the differential amplifier 25 is also connected to the input of a correction circuit 40. This correction circuit includes an amplifier TR1, TR2, R5 which is connected to a charging capacitor C2 via a control switch 33. The charging capacitor is connected in turn to the amplifier TR3, TR4, R6 to which current is supplied from +Vce via resistances R2 and R3. The contact 33 is controlled by a gate 34 having a non-inverting and an inverting input to which the signals TRON and ANTSW are respectively applied, as described in more detail herebelow. The manner in which the inventive arrangement operates will now be described in more detail with reference to Figure 2 and also with reference to the diagrams of Figures 3 and 4. Figure 4A shows the power output as a function of time. The transmitter starts at time point t 3 and then continues to supply the antenna during the time period of the allotted time slot t 3 to t 4 . In the GSM system, the time slot has a length of 580 µs. The next pulse in this channel arrives after about 4.6 ms. Prior to starting the actual transmitter, a logic signal TXON (Figure 4B) is generated at time t 1 and starts the oscillator in the transmitter, among other things. The final amplifier, however, is not started in this moment. Instead, the transmitter is activated by a logic signal ANTSW (Figure 4C) which appears a short time t 2 before the transmitter begins to deliver power. The set-point value BV (Figure 4D) which is intended to control the power output from the transmitter is also obtained at time point t 2 . At time point t 1 , the system is at rest between two transmitter pulses. The power control set-point value BV is then zero and since the power output from the final stage is also zero, the regulator output voltage also ought to be zero. However, because of the offset voltage in the regulator 20, the regulator will have a certain voltage on the output 27. This voltage is re-applied to the correction circuit 40 and controls charging of the capacitor C2 to a value Uc through the current switch 33. The switch 33 is closed when the logic signal TXON is on at the same time as the logic signal ANTSW is zero (the transmitter is inactive), but is opened immediately the logic signal ANTSW appears. The voltage Uc drives a current generator comprised of a Darlington switch TR3, TR4, R6 which draws sufficient current through the resistance R3 to provide a voltage Ukorr across R3 which is sufficient to compensate for the offset voltage Uoffs. The capacitor C2 is sufficiently large to maintain the voltage Uc over whole of the period from time t 3 to t 4 when the transmitter is active and the switch 33 is open. The current generator will therefore continue to generate Ukorr = Uoffs during the whole of the time slot in which the transmitter is active. In order for the correction to function satisfactorily, the transmitter shall not begin to deliver power at low control voltages Us, and the Us/Put-curve shall accordingly have a knee. This is illustrated in Figure 3, which shows Udet and Ukorr as a function of Us. No voltage Udet is obtained until the control voltage Us has reached the knee Uk. The point of equilibrium Uj for the control voltage that corresponds to compensation of offset must lie beneath the knee, since otherwise there is a risk of the transmitter final stage starting during the actual correction sequence. Uoffs must always have the same polarity. This can readily be achieved with the aid of appropriate leakage circuits when required. When testing an arrangement according to Figure 2 with a mobile telephone in the GMS system, it was possible to eliminate offset voltages as high as 5-10 mV. The regulator was able to adjust with good precision a power output whose peak value gave a detector voltage Udet of only 4 mV. Adjustment of the correction voltage could be achieved in about 10 µs. However, the regulator was able to regulate parts of millivolts during the up and down ramp of the power pulse Put.

Claims: An arrangement comprising a power control circuit (20) for a pulsed transmitter final amplifier (10), for eliminating offset error in the power control circuit (20), said power control circuit (20) including a differential amplifier (25) having a first input (21) which is supplied with a level control signal (BV) occurring over the duration of the transmitter pulses for determining one of a number of possible levels of the power output of the transmitter final amplifier, a second input (22) which is supplied with a signal (Udet) derived from the power output, and an output which is connected to a power control input (13) on the transmitter final amplifier, characterized by a correction circuit (40) arranged to form a control loop between the output of the differential amplifier (25) and the second input (22) of the differential amplifier, said circuit (40) being arranged for measuring the differential amplifier output signal during the pauses between transmitter pulses for storing in a memory (C) a magnitude which represents this output signal and for generating during the transmitter pulse a correction voltage (Ukorr) which is controlled by the magnitude stored in said memory and which compensates the offset voltages (Uoff) in the control loop which produce an output signal from the differential amplifier (25) despite the absence of a level control signal (BV) and a signal (Udet) derived from the power output. An arrangement according to Claim 1, characterized in that the correction circuit (40) includes a charging circuit (TR1, TR2, R5) which is controlled by the differential amplifier output signal; and in that the memory circuit is comprised of a capacitor (C2) connected to the charging circuit through a switch (33) which is controlled to be closed during a short time period (from t 1 to t 2 ) prior to the transmitter pulses. An arrangement according to Claim 2, characterized by a current generator (TR3, TR4, R6) connected to the capacitor (C2) and supplying a resistance (R3) connected in the second input of the differential amplifier, and where a correction voltage (Ukorr) is generated across the resistance (R3) directed against the offset voltage (Uoff).

Inventors: CARLSSON JOHN TORSTEN, CARLSSON, JOHN TORSTEN
Applicants: ERICSSON GE MOBILE INC, ERICSSON INC.
Application year: 1993
Main classification: H03G3
IPC codes: H04B1, H03F1, H03G3, H04B7