THERMALLY CONTROLLED, LINEAR POWER REDUCTION CIRCUIT
BACKGROUND AND SUMMARY OF THE INVENTION
The present invention relates to amplifier protection circuits and, more particularly, to a thermally-controlled, linear power reduction circuit.
Amplifier protection circuits which detect temperature at or near a radio frequency (RF) amplifier and adjust the output power level of the RF amplifier upon detecting a temperature in excess of a predetermined value are known. See, for example, U.S. Patents 4,122,400 to Mendorf et al; 4,870,698 to Katsuyama et al; and 4,939,786 to McC allum et al. While the circuits in each of these patents is generally useful for reducing radio overheating to protect the radio against destruction and/or reduction in its useful life, each relies upon thermistors to detect temperature. Thermistors work on the principal that as temperature increases, the thermistor resistance decreases causing a change in thermistor output, i.e. a decrease in voltage drop across the thermistor or an increase in current through the thermistor. The thermistor output is typically compared in a comparator provided in a power control loop to a predetermined threshold level corresponding to a particular temperature. Once the threshold is reached, the output power of the RF power amplifier is abruptly dropped to a much lower power output level.
The problems with these prior art amplifier protection circuits are twofold. First, thermistors are very difficult to consistently manufacture. Specifically, it is extremely difficult to manufacture thermistors with consistent repeatability so that each thermistor component has substantially the same resistance-to-temperature ratio or slope characteristic. In other words, the specific resistance of one thermistor at a particular temperature may be a significantly different resistance when compared to a similar thermistor at that same temperature. As a result, when thermistors are used as temperature sensors in radios, these variances in the thermistor characteristics cause different radios to operate differently under the same temperature conditions. As a result, some radios may not meet various industry and governmental specifications for transmitter power to temperature requirements, e.g. the TIA EIA industry and FCC governmental standards.
Another drawback with these prior art circuits is that as temperature rises to the threshold point, the power output level is abruptly reduced either to an extremely low power output level or in multiple abrupt jumps to the low power level. Such abrupt changes in output power are often not warranted because temperature is usually only gradually increasing. In addition, abrupt jumps in power output are a source of irritation to the radio user with a large drop in power being audibly detected by the user. Moreover, a large drop in radio output power may result in the loss of the current communication.
The present invention overcomes the drawbacks of the prior art amplifier protection circuits by eliminating the unpredictable and unreliable thermistor and using instead a precision temperature sensor, such as the LM35 precision temperature available from National Semiconductor Corporation. Such a precision temperature sensor provides an extremely accurate and repeatable voltage for a predetermined temperature characteristic. For every change in temperature, e.g., one degree centigrade, the precision temperature sensor generates a consistent corresponding linear change in voltage, e.g., ten millivolts. The voltage output of the precision temperature sensor is connected to an operational amplifier comparator which compares the voltage output to a reference voltage. When the detected voltage exceeds the reference voltage, the comparator output decreases. As the temperature sensor output increases linearly, the comparator output decreases linearly so that when the detected output power of an RF power amplifier stage is compared to that linearly decreasing comparator output in a power control unit, the power control signal used to drive the RF power amplifier state is linearly reduced. Consequently, the transmitter output power gradually decreases as temperature continues to increase beyond a certain threshold level. As the temperature continues to increase beyond an upper temperature threshold maximum, the minimum power output is set.
These and other advantages and objects of the present invention as well as other advantages and objects will become apparent to those skilled in the art upon consideration of the accompanying specification, claims and drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
FIGURE 1 is a functional block diagram of the thermally controlled linear power reduction circuit according to the present invention;
FIGURE 2 is a schematic diagram of the thermally controlled linear power reduction circuit in accordance with the present invention; and FIGURE 3 is a graph of transmitter output power versus temperature describing exemplary performance of the thermally controlled power reduction circuit in accordance with the present invention.
DETAILED DESCRIPTION OF THE DRAWINGS
In the following description, for purposes of explanation and not limitation, specific details are set forth, such as particular circuits, circuit components, etc. in order to provide a thorough understanding of the present invention. However, it will be apparent to one skilled in the art that the present invention may be practiced in other embodiments that depart from these specific details. In other instances, detailed description of well known methods, devices, and circuits are omitted so as not to obscure the description of the present invention with unnecessary detail.
Figure 1 shows a function block diagram of the thermally controlled power reduction circuit in accordance with the present invention. A voltage supply 10 supplies DC voltage to a precision temperature sensor 12, which may be for example an LM35 precision temperature sensor available from National Semiconductor Corporation. Precision temperature sensor 12 generates an absolute voltage output corresponding to absolute detected temperature. For example, for every one degree centigrade increase in temperature, the voltage output of the precision temperature sensor increases by ten millivolts. In contrast with thermistors whose temperature-voltage relationship varies greatly depending upon the specific thermistor, the temperature-voltage characteristic of the precision temperature sensor employed in the present invention is both accurate and predictable for each manufactured sensor. This ensures that temperature detection and amplifier regulation in a particular model of radio can be implemented with accuracy and consistency which is important in order to meet industry specifications, e.g., the EIA TIA standard, and governmental requirements, e.g. FCC specifications. The precision temperature sensor 12 voltage output is received in comparator 16 along with a reference voltage provided by reference generator 14. The reference voltage corresponds to a predetermined threshold temperature where the output power of the radio transmitter is set to decrease. As shown for example in Figure 2, the reference signal may correspond to an ambient temperature at or just above ambient room temperature, i.e. 25°C. Up until this temperature threshold, the radio output may be set at full transmission power, shown in the graph for example at 40 watts. As the temperature exceeds 25°C, output power is gradually and linearly decreased until another reference voltage generated by generator 14 is reached.
This gradual and linear decrease in transmitter output power is accomplished by generating a linearly decreasing output from comparator 16 as the precision temperature sensor voltage output continues to linearly increase above the reference voltage provided by reference generator 14. The linearly reducing output from comparator 16 is provided to power control circuit 18 which compares in comparator 20 that decreasing output to a signal provided by output sensor 26 corresponding to the detected transmitter output power. Because the sensed output power is greater than the now decreasing signal level provided by comparator 16, the feedback output from comparator 20 to driver 22 decreases such that the driver output provided to RF power amplifier stage 24 also decreases. The net effect is that the RF power amplifier stage 24 reduces amplification of the RF input signal which reduces the transmitter output power generated at antenna 28.
As shown in the graph at Figure 2, the transmitter output power continues to decrease linearly until an upper threshold temperature such as 85°C is reached as determined by comparison of another reference voltage generated by reference generator 14 with the output from precision temperature sensor 12. At that upper threshold, the transmitter output power is changed to a minimum output power value shown for example as 15 watts in Figure 2. Obviously, the transmitter output power could be reduced to any ultimate output power level (including zero output) in order to protect the amplifier and other radio circuitry.
Thus, the thermally-controlled linear power reduction circuit of the present invention provides an accurate and repeatable measurement/monitoring of amplifier temperature which permits linear, gradual adjustment of the transmitter output power as a function of temperature. Although the present invention is particularly useful in protecting the radio from overheating at high temperatures, it is also useful in that if and when temperature decreases in the radio, the radio output power can be gradually (rather than abruptly) increased toward full output power.
A more detailed schematic drawing of the thermally-controlled linear power reduction circuit in accordance with the present invention will now be described in conjunction with Figure 3. A precision temperature sensor 50 such as the LM35 precision temperature sensor from National Semiconductor Corporation is connected to a power supply and to ground. For simplicity, DC power supplies are conventionally indicated as circles with plus (+) symbols next to those circles. A resistor 52 is connected between the output terminal of precision temperature sensor 50 and ground. The output of precision temperature sensor 50 is also connected to an input resistor 1^ 54 to the negative terminal of operational amplifier 60. A DC voltage is connected through a voltage divider consisting of resistors 56 and 58 to the positive terminal of operational amplifier
5 60. A feedback resistor Rf 62 connects the output of operational amplifier 60 to the negative input terminal of operational amplifier 60. DC power is connected to series resistors 66, 67, and 68 to ground with variable potentiometer 67. The output of operational amplifier 60 is connected to a node between resistors 66 and 67
10 through output resistor R0 64. The positive terminal of a second operational amplifier comparator 70 is connected to the potentiometer 67. Negative terminal of operational amplifier 70 is connected both to a feedback capacitor 72 and to the detected radio transmitter output power sensed by bidirectional coupler 84 which is converted to
15 a DC value via capacitor 85 and rectifying diode 86.
The output of comparator operational amplifier 70 drives the base of transistor Tl shown at 74, and the output of transistor Tl is provided to a driver transistor T2 shown at 76. The emitter of driver transistor T2 is connected to control the power output of RF power
20 amplifier 82. The RF input signal to be amplified by the RF power amplifier 82 may originate for example from microphone 78, the output of microphone 78 being converted to the RF input signal via conventional transmitter 80. The output of power amplifier 82 is fed to transmitter antenna 90 through conventional coupling capacitor
25 88.
The operation of the circuit shown in Figure 3 is now described. Precision temperature sensor 50 generates a voltage which is linearly proportional to the detected temperature. Parallel resistor 52, which may be for example on the order of 100 ohms,
30 improves the performance of the precision temperature sensor 50 by dissipating current that would otherwise be absorbed by sensor 50. Summing operational amplifier 60 is designed using feedback resistor Rf to have a finite gain to enable adjustment of the transmitter output power versus temperature graph shown in Figure 2. Specifically, the ratio of the feedback resistor Rf to the input resistor Rj controls the slope of the linearly decreasing portion of the graph shown in Figure 2.f and j are determined and selected based on the thermal packaging of the radio in which the thermally controlled linear power reduction circuit of the present invention is to be employed. For example, a ratio of approximately 4.5 was found to be optimal for at least one radio application with nominal values of 18,000 ohms for Rf and 4000 ohms for Rj.
The reference voltage Vref supplied to the positive terminal of operational amplifier 60 is determined by resistors 56 and 58. Assuming for example a DC supply voltage of eight volts, an exemplary value of resistor 56 is 4300 ohms and 1000 ohms for resistor 58. The reference voltage sets the point at which the voltage output begins to decrease, e.g. at 25°C in Figure 2.
Finite gain summing operational amplifier 60 therefore functions as a comparator whose output at cooler temperatures, i.e. below 25°, is clamped to its positive voltage supply. As temperature rises and the precision temperature sensor's increasing input voltage exceeds the reference voltage, the operational amplifier 60 responds to that increasing input voltage by generating a linearly decreasing output voltage. The output of operational amplifier 60 is connected in parallel with potentiometer 67 through output resistor R^,. Potentiometer 67 calibrates and sets the output power from the temperature sensing circuitry so that it can be compared in operational amplifier comparator 70 with the actual RF power amplifier 82 output detected via bidirectional coupler 84 and rectifying components 85 and 86. Feedback capacitor 72 stabilizes comparator operational amplifier 70 whose output represents the difference between the sensed RF transmitter output power and the commanded transmitted power generated at the output of comparator operational amplifier 60 via potentiometer 67. That difference causes transistor Tl to conduct which reduces the current supplied via transistor T2 to the RF power amplifier 82 thereby decreasing the transmitter output power seen at antenna 90.
As the temperature continues to increase to the upper temperature threshold, the operational amplifier clamps its output to the low power supply terminal so that the output power remains at a constant low output level.
While the invention has been described in connection with what is presently considered to be the most practical and preferred embodiment, it is to be understood that the invention is not to be limited to the disclosed embodiment, but on the contrary, is intended to cover various modifications and equivalent arrangements included within the spirit and scope of the appended claims. For example, while the power control circuitry is described using analog circuitry, digital circuits and/or a suitably programmed microprocessor could also be used to implement the present invention.