FIG. 1 depicts a communication device 1 arranged to transmit data in a series of time slots, such as is used in TDMA systems. The communication device 1 comprises a transmitter 2 that includes a power amplifier 3 and a power controller 4 arranged to control the power amplifier 3, together with a RX-TX switch 5 and an antenna 6. The power amplifier 3 supplies power to the antenna 7. A controller 20 is operable to control all the other components. The RX-TX switch 5 connects the antenna 6 to either a receiver (not shown) or the transmitter 2.
Recently, communication devices that are capable of transmitting data using more than one modulation scheme have become available. For example, the Nokia (RTM) 9500 Communicator is arranged to send data using EDGE and using GMSK (Gaussian Minimum Shift Keying) modulation. FIG. 2 depicts part of a prior amplifier arrangement 2 suitable for use in such a communication device. FIG. 2 shows a multimode power amplifier 3 and components of the power controller 4.
When modulated signals with a high degree of amplitude modulation, such as EDGE signals, are to be transmitted, the power amplifier 3 is operated in a linear mode. The power amplifier 3 is controlled using a first control loop 21, comprising a differential amplifier 7a, having a feedback loop including a capacitor 8a, and a sample and hold circuit comprising a capacitor 9 and differential amplifier 10. The first control loop 21 controls the output power of the power amplifier 3 by altering an input signal fed to the power amplifier 3, by adjusting the gain of a variable gain amplifier 11 preceding the power amplifier 3. The power amplifier 3 and the variable gain amplifier 11 cooperate to constitute together a power amplifier.
A switch 12 between the amplifier 7a and the amplifier 10 is closed during the beginning and end of a time slot used for linear transmission, to allow the output power of the power amplifier to be ramped up or down under the control of an input ramp signal TXC. During the intervening portion of the time slot, which is used for data transmission, the switch 12 is open, thereby disconnecting the differential amplifier 7a and its feedback loop. The variable gain amplifier 11 is then controlled by an output of the sample and hold circuit 9, 10, which applies a constant control voltage to the variable gain amplifier.
At the end of a time slot, the transmitter 2 is ramped down. The power controller 4 can then switch between the two control loops 21,22 by means of switches 13a, 13b, 13c, before the transmitter 2 is ramped up at the beginning of the next time slot. The ramping is performed using a ramp signal TXC that is input to the relevant control loop in order to control the output power of the power amplifier 3.
For modulated signals with little or no amplitude modulation, such as GMSK modulated signals, the power amplifier 3 is operated in a non-linear mode in order to improve its power efficiency. When the transmitter 2 is provided in a communication device 1 such as a mobile telephone, this increased efficiency may result in longer talktime. A second control loop 22 is provided, which comprises a differential amplifier 7b and a feedback loop including a capacitor 8b but does not include a sample and hold circuit. The second control loop 22 controls the power amplifier 3 by altering a voltage applied to a power control pin Vpctrl.
At the start and end of a time slot in which GMSK or similar modulated signals are to be transmitted, the output power of the power amplifier 3 is ramped up or down under the control of the input ramp signal TXC. The second control loop remains closed during the beginning, the intervening portion and end of the time slot. This means that, unlike the first control loop 21, the second control loop 22 remains connected to a diode power detector 14 that forms part of and monitors the output power of the power amplifier 3, and adjusts the control voltage accordingly throughout the time slot.
During a time slot in which the output power of the power amplifier 3 used for data transmission is high, the temperature of the components of the power amplifier 3 increases. Heat from the components is transferred to the power detector 14 and may result in a decrease in the accuracy of its measurements. The resulting measurement error may be of the order of 2 mV per degree centigrade, which may result in measurement results that are erroneously low.
If, then, during the next time slot, the power amplifier 3 is to be operated in a non-linear mode to provide an output power that is substantially lower than in the preceding time slot, the components of the power amplifier 3 and the power detector 14 will then cool down during this time slot. This temperature drift causes a drift on the power detector voltage during the time slot and results in the control voltage and, therefore, the output power of the power amplifier 3, decreasing during the intervening portion of the time slot. This is undesirable.
FIG. 3 is a graph showing the output of the power detector 14 during two successive time slots t1, t2. In the first time slot, the output power is high and is above the upper limit of the y-axis of the graph. During the second time slot, the temperature of the power detector 14 is high, due to heat generated by the components of the power amplifier 3 during the first time slot t1, and so the output of the power detector 14 starts at a relatively low value, compared with the actual output power of the power amplifier 3. The output of the power detector 14 then drifts upwards during the second time slot t2 as its diode cools down.
For GSM900, the output power in any given time slot must fall within a range of 5 to 33 dBm, with an accuracy of +/−1 dB during the useful part of a burst. The temperature drift between time slots is most significant when the output power is 33 dBm during a first time slot and 5 dBm during the succeeding time slot. FIG. 4 is a graph of the output power over two GSM 1900 time slots, also labelled t1, t2, and clearly shows a downwards drift in the level of the output power during the second time slot t2.
In some prior amplifier arrangements, this problem has been addressed by providing temperature compensation means for the power detector 14. However, this requires the provision of extra components. Furthermore, the provision of temperature compensation means that are capable of responding to very rapid temperature changes is not straightforward. The power detector 14 may also be located away from the power amplifier 3, for example, as in the arrangement disclosed in U.S. Pat. No. 6,369,635, so that it is not significantly affected by heat from the power amplifier components. However, this requires the provision of external components and may increase the size of the amplifier arrangement 2.