Temperature Compensated Automatic Output Control Circuitry for RF Signal Power Amplifiers With Wide Dynamic Range

The present invention is related generally to control circuits for RF signal power amplifiers, and more particularly to improved, temperature compensated, automatic output control circuitry for RF signal power amplifiers.

Prior art power amplifiers typically only have one output power level. Such power amplifiers were adjusted during manufacture to the desired output power level. In some applications, it was desirable to have two power settings, a low and a high setting. The RF signal power amplifier for such applications would have one setting for the high power level and another for the low power level.

More recently, it has become advantageous to reuse radio channels in some radio communications systems. Therefore, in order to avoid interference between two RF signal power amplifiers using the same frequency, it is desirable to have many different selectable output power levels. For example, four different power levels are provided for in the RF signal power amplifier in the mobile radiotelephone described in Motorola Instruction Manual No. 68P81039E25, entitled "Advanced Mobile Phone System", and published by Motorola Service Publications, 1301 E. Algonquin Road, Schaumburg, IL, 1979. The four power levels are produced by varying reference voltage which is applied to automatic output control circuitry. However, such automatic output control circuitry cannot accurately maintain a wide range of power settings in RF signal power amplifiers operable at several different RF signal frequencies and subjected to a wide range of temperatures.

Accordingly, it is an object of the present invention to provide, improved temperature compensated, automatic output control circuitry for RF signal power amplifiers that accurately maintains over a wide range of RF signal frequencies and temperatures output power levels selected from a wide range of power settings.

Briefly described, the present invention encompasses control circuitry for maintaining the magnitude of an RF signal from an RF signal amplifier at one of a plurality of levels selected by level control signals from a signal source. The amplification of the RF signal amplifier is proportional to a drive current signal provided by a current amplifier. The current amplifier varies the magnitude of the drive current signal in response to a current control signal. The control circuitry further includes circuitry for generating an output power signal that has a magnitude proportional to the magnitude of the RF signal from the RF signal amplifier; circuitry for compensating the output power signal for variations in temperature; circuitry for selecting, in response to the level control signals, one of a plurality of adjustment factors, such as amplification or attentuation factors circuitry for amplifying the output power signal by the selected amplification factor; and circuitry for varying the magnitude of the current control signal in response to the difference between the magnitude of the amplified output power signal and a predetermined magnitude.

FIG. 1 is a block diagram of an RF signal amplifier including the automatic output control circuitry of the present invention.

FIG. 2 is a detailed circuit diagram of the automatic output control circuitry in FIG. 1.

Referring to FIG. 1, there is illustrated an RF signal power amplifier that may advantageously utilize automatic output control (AOC) circuitry embodying the present invention. An RF signal power amplifier is typically part of a radio, such as that described in the aforementioned instruction manual number 68P81039E25, that includes an RF signal power amplifier for amplifying a modulated RF signal which is radiated by an antenna. The RF signal power amplifier in FIG. 1 includes two cascaded stages of amplification, drivers 102 and final amplifiers 104. The output of final amplifier 104 is coupled by way of isolator 106 and filter 108 to antenna 110. The final amplifiers 104 typically boost the output power level from approximately one-fourth watt to five or more watts. The output power level developed by final amplifiers 104 is proportional to the drive current applied thereto by AOC circuitry 112.

AOC circuitry 112 in FIG. 1 detects the magnitude of the RF signal from final amplifiers 104 and varies the drive current applied to final amplifiers 104 for maintaining the magnitude of the final amplifier output at one of several predetermined magnitudes selected by a plurality of AOC signals. In the preferred embodiment, three AOC signals are used to select one of eight different output power levels ranging from nine milliwatts to five watts, a dynamic range of twenty-eight dB. The AOC circuitry of the present invention maintains the output power level to within plus and minus one-half dB of the selected level over temperature. Furthermore, the output power level is also accurately controlled by the unique AOC circuitry over a wide range of RF signal frequencies.

Referring to FIG. 2, there is illustrated a detailed circuit diagram of AOC circuitry 112 in FIG. 1. Three AOC signals are coupled to analog multiplexer 202 which applies signal ground to one of eight different resistors 222 and 226 depending on the binary state of each of the AOC signals. The AOC signals can be provided by a signal source, such as the radio control circuitry described in the aforementioned instruction manual number 68P81039E25.

The output from final drivers 104 in FIG. 2 is coupled by capacitor 204 to a half wave rectifier formed by diode 206 and capacitor 208. The voltage at the anode of diode 206 is biased to a predetermined voltage by means of resistor 210, diode 212 and resistor 214. According to a feature of the present invention, diode 212 also compensates for variations in the forward voltage drop across diode 206 due to temperature. Furthermore, forward biasing diode 206 allows for a lower value resistor 215, resulting in improved performance in high humidity environments. The voltage produced by the rectifier formed by diode 206 and capacitor 208 varies from a minimum of one-half volt when the final drivers 104 are operating at the lowest power setting to approximately fifteen volts when final drivers 104 are operating at the highest power setting.

The output power voltage from capacitor 206 in FIG. 2 is applied to the plus input of amplifier 220 by way of resistors 216 and 218. The gain of amplifier 220 is varied by grounding one of the eight resistors 222 and 226 selected by the AOC signals. When resistors 222 are selected, the gain of amplifiers 220 is two, and the output power voltage is attenuated. When resistors 226 are selected, the gain of amplifier 220 is varied, and the output power voltage is not attenuated. Analog multiplexer 202 applies ground to one of eight resistors 222 and 226 in response to one of eight binary states provided by the three AOC signals. Analog multiplexer 202 may be a Motorola type MC14051 CMOS integrated circuit. The values of resistors 222 and 226 are chosen so that the output of amplifier 220 will have substantially the same magnitude for each of the eight power level settings corresponding to output power voltages ranging from one-half volt to fifteen volts. Since the operating voltage of AOC circuitry 112 is eight volts in the preferred embodiment, high output power voltages from approximately two to fifteen volts corresponding to high power settings are selected by selectively grounding resistors 222. The attenuation is provided by a voltage divider formed by resistor 218 and the grounded resistor 222. Conversely, output power voltages from approximately one-half to two volts corresponding to low power settings are selected by selectively grounding resistors 226. The gain of amplifier 220 is determined by the feedback resistance provided by resistor 224 and the parallel resistance of resistor 228 and the grounded resistor 226. Although the gain of amplifier 220 has been varied by grounding resistors 222 and 226, the gain of amplifier 220 could also be varied by varying the resistance of resistors 224 and 228. Thus, according to an important feature of the present invention, amplifier 220 either attenuates or amplifies the output power voltage depending on which of resistors 222 and 226 is grounded.

The output of amplifier 220 in FIG. 2 is applied to the minus input of comparator 230. Comparator 230 controls current amplifier 234 by way of base drive transistor 236. The drive current signal from current amplifier 234 is coupled to final amplifiers 104 for varying the power output of final amplifiers 104. The power output of final amplifiers 104 is increased as the drive current signal increases, and vice versa.

The positive input of comparator 230 in FIG. 2 is a reference voltage 232 provided by two resistors and a potentiometer 238. During manufacture of the RF signal amplifier, potentiometer 238 is varied to produce the desired power output from final amplifiers for the highest power level setting. In the preferred embodiment, the electrical circuit component values were selected so that the reference voltage 232 is approximately four volts. In operation, comparator 230 varies the drive current signal from current amplifier 234 to maintain the magnitude of the output of amplifier 230 substantially the same as the magnitude of the reference voltage 238.

The loop response time of AOC circuitry 112 is on the order of one millisecond. The loop turn-on time is primarily determined by the circuitry coupled to the base of transistor 236. Power level switching time is primarily determined by the series-coupled resistor and capacitor 239 coupled to the negative input of amplifier 230. Thus, AOC circuitry 112 has both a fast turn-on time and a fast power level switching time.

In summary, improved AOC circuitry for RF signal power amplifiers has been described that provides a wide range of output power settings by varying amplification factors in developing the drive current signal applied to the amplifier. Furthermore, the AOC circuitry is temperature compensated so that the selected output power level is maintained over a wide range of temperatures. The improved AOC circuitry of the present invention can be advantageously utilized in any suitable application where it is desired to accurately maintain over temperature and frequency variations the output of an RF signal power amplifier at a selected one of several output power levels.