Temperature compensation circuit for a power amplifier

Disclosed is a temperature compensation circuit for a power amplifier that is capable of stabilizing a variation in current of a bias circuit. The temperature compensation circuit comprises a bias voltage node for providing a bias voltage to the power amplifier; a regulated voltage node connected to a regulated voltage; a temperature sensor connected between the bias voltage node and a ground node, the temperature sensor having a resistance varying according to ambient temperature; a first resistor connected in parallel to the temperature sensor, for reducing a variation in resistance of the temperature sensor; and a second resistor connected between the regulated voltage node and the bias voltage node, for dividing the regulated voltage to generate the bias voltage.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT A preferred embodiment of the present invention will be described herein below with reference to the accompanying drawings. In the following description, well-known functions or constructions are not described in detail since they would obscure the invention in unnecessary detail. A thermistor, as a temperature sensor, is classified into an NTC Negative Temperature Coefficient) type having a low resistance at high temperatures and a PTC (Positive Temperature Coefficient) type having a high resistance at high temperatures. FIG. 6 illustrates an equivalent circuit of a temperature-compensated power amplifier according to an embodiment of the present invention, and FIG. 7 illustrates an equivalent circuit of a temperature-compensated power amplifier according to another embodiment of the present invention. Referring to FIG. 6 , Vref denotes a bias voltage node used to provide a bias voltage to a bias circuit of the power amplifier. The bias voltage is about 2.6-3.2 V according to the type of the power amplifier. Further, Vt denotes an output node of a voltage regulator, TH denotes a temperature sensor comprised of a thermistor, and C denotes a bypass capacitor. In addition, R 2 denotes a voltage-dividing resistor and R 1 denotes a resistor used to reduce a variation in resistance of the thermistor TH according to the ambient temperature (where R 1 >>R 2 ). The thermistor TH has a higher resistance at lower temperatures and a lower resistance at higher temperatures. In other words, the thermistor TH of FIG. 6 is preferably an NTC thermistor. The circuit shown in FIG. 6 is constructed using such a characteristic of the thermistor. In FIG. 6, a regulated voltage provided to the node Vt is divided by the resistors R 2 and R 1 and the thermistor TH in accordance with the following formula, and the divided voltage is provided to the bias voltage node Vref. Vref&equals;Vt *(( R 1 / TH )/( R 2 &plus;( R 1 / TH ))) As a result, the bias voltage is decreased at the high temperature, decreasing the bias current of the power amplifier. In contrast, the bias voltage is increased at the low temperature, increasing the bias current. Therefore, the power amplifier can maintain its constant characteristic regardless of the temperature variation. That is, the power amplifier has a temperature-compensated characteristic. The circuit shown in FIG. 7 also operates in the same manner. However, the circuit includes a PTC thermistor, which has a lower resistance at lower temperatures and a higher resistance at higher temperatures. A first resistor R 1 is connected in parallel to a thermistor TH, connected between a supply voltage node Vt and a bias voltage node Vref, in order to reduce a variation in resistance of the thermistor TH according to the ambient temperature. A second resistor R 2 is connected between the bias voltage node Vref and a ground node to divide the regulated supply voltage Vt. The divided voltage is determined by the following formula and provided to the bias voltage node Vref. Vref&equals;Vt *( R 2 /( R 2 &plus;( R 1 / TH ))) When actually realized, the circuits of FIGS. 6 and 7 may have somewhat different outcomes from their associated formulas because of an impedance of the bias voltage node Vref, a PCB (Printed Circuit Board) pattern loss, and errors of the resistors and the thermistor. All in all, however, the circuits will have virtually the same characteristics as their associated formulas. In use, the circuits of FIGS. 6 and 7 may be incorporated in a mobile phone. Although the circuits may additionally include a circuit for controlling the bias voltage and a circuit for improving the call efficiency of the mobile phone, the fundamental structure of the temperature compensation circuit with the thermistor remains the same. The invention can also be applied to a smart power amplifier to decrease an output current over an overall power range. This will be described with reference to FIG. 3 . The smart power amplifier shown in FIG. 3 operates in three operation modes, including high-power mode, intermediate-power mode and low-power mode. In the high-power mode, the smart power amplifier has a high gain and high current consumption. In the intermediate-power mode, the smart power amplifier has an intermediate gain and intermediate current consumption. Further, in the low-power mode, the smart power amplifier has a low gain and low current consumption. Therefore, it is possible to decrease current consumption of the communication terminal by allowing the power amplifier to operate in the low-power mode at an output power range between −55 dBm to −10 dBm. However, in the low-power mode, the power amplifier has an increased variation in the minimum power and, in the worst case, may be shut down at the low temperature (about −30° C.). Therefore, the power amplifier cannot normally operate in the low-power mode at the low temperature, without using a temperature compensation circuit with the thermistor. In this case, the power amplifier must operate in the high-power mode or the intermediate-power mode, or change (preferably increase) the number of operation modes according to temperature. FIG. 8 illustrates a current characteristic of the step gain power amplifier supporting the high-power mode and the intermediate-power mode. When supporting the two power modes, the step gain power amplifier operates in the intermediate-power mode instead of the low-power mode in the output power range between −55 dBm to −10 dBm. Therefore, the communication terminal consumes the increased current. However, when using the temperature compensation circuit with the thermistor according to an embodiment of the present invention, the step gain power amplifier can operate even in the low-power mode, since a variation in gain of each power mode according to the temperature is less. By applying this to the communication terminal, it is possible to drive the communication terminal with a decreased current at the output power range between −55 dBm to −10 dBm (this range can be varied according to the communication terminals). As described above, the present invention minimizes a variation in characteristic of the power amplifier according to ambient temperature by using a temperature compensation circuit. In addition, it is possible to decrease a call current by utilizing the characteristic of the power amplifier in the low-power mode. Therefore, it is also possible to maintain the same characteristic of the communication terminal even in a severe environment. In addition, the output power of the communication terminal increases at the low temperature, preventing attenuation of the output power, thereby making it possible to maintain the probability of transmission success. In addition, the temperature compensation circuit can be applied not only to the power amplifier for use in existing communication terminals but also to the power amplifier for use in future CDMA-2000 or IMT-2000 communication terminals. While the invention has been shown and described with reference to a certain preferred embodiment thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the invention as defined by the appended claims.