Abstract:
The present invention describes a device for detecting the power of a first signal at a given frequency. The device comprises a first circuit means for detecting the envelope of the first signal and a second circuit means coupled to the first circuit means and suitable for generating a second signal proportional to the average of the envelope detected. The second signal is proportional to the square root of the average power of the first signal.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
   This application is based upon and claims priority from prior European Patent Application No. 04425041.3, filed on Jan. 26, 2004 the entire disclosure of which is herein incorporated by reference. 
   FIELD OF THE INVENTION 
   The present invention generally relates to a device for detecting the power of a signal and more specifically to a circuit with amplifier and related power detection device. 
   BACKGROUND OF THE INVENTION 
   In radio frequency circuits, such as in the radio frequency transmitters, power amplifiers are commonly used in a circuit block that precedes the antenna. Radio frequency transmitters typically implement, in accordance with the power radiated by the antenna, a mechanism of gain control or of energy saving that acts on the whole transmission chain. An estimate of the power at the output of the power amplifier enables the above-mentioned gain control procedures to be undertaken. To reach a high spectral efficiency in the more recent generation of transmission standards, such as the standard IEEE802.11a/g, CDMA2000, IS95, WCDMA and more, a combination of modulation in amplitude and phase is used. The modulated signal that results is at variable envelope. This category of signals is typically characterised by the ratio between instantaneous peak power and average power. The above-mentioned gain control on the transmission chain operates in accordance with the average power rather than the instantaneous peak power. For these above-mentioned gain controls the use of a detector of peak power is excluded and is not used. Further, it is unnecessary to use a detector for time integrated power or average power. 
   One known detector of the power of an amplifier in radio frequency is disclosed in the WO 0180421 patent. This patent describes a power amplifier circuit to amplify an input signal. The power amplifier circuit includes an amplification transistor and a circuit for detecting the power. The circuit for detecting power generates a signal that is directly proportional to the level of the power of the amplification transistor. More specifically, the circuit for detecting power generates a direct voltage proportional to the square of a current, where the current is the sum of the radio frequency current and the direct current. This current flows in the amplification transistor and produces an average of the voltage which indicates an accurate power level in the amplification transistor. 
   Although this power detecting circuit is useful, it is not without its problems and shortcomings. One problem is the power detecting device of an amplifier is very sensitive to temperature. Further this temperature dependency is highly dependent on the idle current of the power amplifier. 
   Accordingly, a need exists to overcome these problems and shortcomings of the prior art and to provide a device for detecting the power that is not sensitive to temperature and a device that does not depend on the idle current of the power amplifier. 
   SUMMARY OF THE INVENTION 
   Briefly, in accordance with the present invention, a device is provided for detecting the power of a signal that overcomes the above-mentioned problems and shortcomings. 
   A power detecting device of a first signal at a given frequency is disclosed. The power detecting device comprises a first circuit means for detecting the envelope of the first signal. The power detecting device further includes a second circuit means coupled to the first circuit means and suitable for generating a second signal proportional to the average of the envelope detected. The second signal is proportional to the square root of the average power of the first signal. 
   Still in accordance with the present invention an integrated circuit using the power detecting device is disclosed. 
   The present invention provides a power detecting device for detecting the power of a signal which is less sensitive to the temperature and to the supply voltage. 
   Further, the present invention is applied to an output signal from a power amplifier, and is not dependent on the idle current of the power amplifier. 
   The foregoing and other features and advantages of the present invention will be apparent from the following more particular description of the preferred embodiments of the invention, as illustrated in the accompanying drawings. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The subject matter, which is regarded as the invention, is particularly pointed out and distinctly claimed in the claims at the conclusion of the specification. The foregoing and other features, and advantages of the invention will be apparent from the following detailed description and non-limiting examples taken in conjunction with the accompanying drawings in which: 
       FIG. 1  is a diagram of a device for detecting the power of a signal in output from an amplifier and of the same amplifier according to the present invention; 
       FIG. 2  is a circuit diagram of the device for detecting the power of a signal of  FIG. 1 ; and 
       FIG. 3  is a graph of the voltage detected by the device in function of the average power of the amplifier. 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   It should be understood that these embodiments are only examples of the many advantageous uses of the innovative teachings herein. In general, statements made in the specification of the present application do not necessarily limit any of the various claimed inventions. Moreover, some statements may apply to some inventive features but not to others. In general, unless otherwise indicated, singular elements may be in the plural and vice versa with no loss of generality. 
     FIG. 1  shows a diagram of a device  10  for detecting the power of a signal in output from an amplifier  20 , preferably a power amplifier, and of the same amplifier according to the present invention. The amplifier  20  has an input signal Vin at a given frequency, preferably a modulated radio frequency signal (for example modulated by an audio signal with a known amplitude modulation) or simply a radio frequency signal. The amplifier  20  has an output terminal  21  on which there is a radio frequency modulated or simply radio frequency output signal Vout, connected to an antenna  25 . The device  10  has an input terminal  11  connected to the output terminal  21  of the power amplifier  20  and is suited to detecting a voltage Vdet proportional to the square root of the average power Poutm of the same amplifier  20 , that is the average power relative to the output signal Vout of the amplifier  20 . The signal Vdet therefore has the same time course as the average power Poutm. 
   The device  10  comprises an envelope detector  1  suited to extracting the envelope of the modulated signal Vout coming out of the amplifier  20 , in this case a signal at radio frequency. 
   Block  2  is an integrator suited to producing a voltage signal Vm given by the average of the envelope of the signal Vout. 
   The device  10  also comprises preferably a buffer  3  suited to decoupling the device  10  from the load. The signal Vdet in output from the buffer  3  is the voltage signal Vm. 
     FIG. 1  shows the device  10  according to the invention applied to the output signal of an amplifier  20 , nonetheless said device  10  could be applied to any signal at a given frequency such as a radio frequency signal or a radio frequency modulated signal. 
     FIG. 2  shows a circuit diagram of the device  10  of  FIG. 1 . The device  10  comprises a capacitor C 1  positioned between the input voltage Vout and the input of the envelope detector  1 ; the task of the capacitor C 1  is to block the continuous component of the signal Vout. 
   The envelope detector  1  comprises a transistor Q 1 , for example a bipolar transistor, whose input terminal, that coincides with the input terminal of the envelope detector  1 , is made up of the base terminal of the transistor Q 1 . The collector terminal of the transistor Q 1  is connected to the supply voltage Vcc while the emitter terminal is connected with a current generator  12  connected to ground. 
   The device  10  also comprises a series of a current generator I 1 , a diode D 1  and a resistor R 1 ; said series is positioned between a supply voltage Vcc and ground and the terminal in common of the generator I 1  and of the anode of the diode D 1  is connected to the input terminal of the envelope detector  1 . The current generator I 1  is obtained through any reference of known bandgap voltage; the current generated by I 1  together with the resistor R 1  and the diode D 1  determines the bias of the base of the transistor Q 1 . 
   The device  10  also comprises another series of a current generator  13 , a diode D 2  and a capacitor C 2 ; said series is positioned between a supply voltage Vcc and ground. The terminal in common of the generator  13  and the anode of the diode D 2  is connected to the input terminal of the block  2  while the terminal in common of the cathode D 2  and of a terminal of the capacitor C 2  is connected to the emitter terminal of the transistor Q 1 . 
   Block  2  comprises a series of a resistor R 2  and a capacitor C 3  connected between the input of the block  2  and ground. 
   The device  10  comprises preferably a block  3  made up of a common collector bipolar transistor Q 2  whose base terminal is the input of the block  3  that is connected to the common terminal of the resistor R 2  and of the capacitor C 3 , the collector terminal is connected to the supply voltage Vcc and the emitter terminal, in which the output voltage Vdet is taken, is connected to a terminal of a current generator  14  having the other terminal connected to ground. 
   The voltage signal Vout leaving the amplifier  20  drives the transistor Q 1 ; when the signal Vout is positive the transistor Q 1  conducts and enables the capacitor C 2  to be charged, while when it is negative the transistor Q 1  is cut-off enabling the capacitance C 2  to be discharged towards ground. If the signal Vout is a sinusoidal signal in radio frequency, during the rising phase of the sinusoidal waveform the transistor Q 1  enables the capacitor C 2  to be charged while during the descending phase the transistor Q 1  is cut-off enabling the capacitance C 2  to be discharged towards ground in a linear manner with a slew-rate given by (I 2 –I 3 )/C 2 . The choice of the slew-rate is based on a compromise between the capacity of detecting signals whose envelope varies rapidly and the rejection to the ripple due to the charging and discharging of the capacitor C 2 . Therefore at the leads of the capacitor C 2  there will be a voltage signal that reproduces the time course of the signal Vout. 
   Block  2  enables the signal detected by the block  1  to be integrated and enables the elimination of the ripple generated in the block  1  that represents the highest frequency component of the envelope Vi of the signal Vout. 
   Block  3  enables an output voltage Vdet to be obtained that does not depend on the load applied by the output terminal of the device  10  for a very big range of values. 
   The presence of the diodes D 1  and D 2  permits the compensation of the temperature variations of the base-emitter junctions of the respective transistors Q 1  and Q 2 . For this reason and because the voltage on the resistor R 1  is proportional to a bandgap voltage, it results that the output voltage Vdet is insensitive to the temperature. 
   The voltage signal Vdet is insensitive to the idle current of the amplifier  20  because it depends on the measurement of the envelope of the output signal Vout of the amplifier. 
   The circuit  10  is also insensitive to the supply voltage Vcc because it comprises the current generators I 1 –I 4 ; this comes about in the normal use conditions of the device  10  that is for amplitudes of the sinusoidal signal Vout that does not lead the current generators I 1 –I 4  to saturation. Nevertheless it is always possible to carry out a partition of the input signal Vout of the device  10  to prevent the saturation of the current generators I 1 –I 4 . 
   In addition said device has no limitations on the frequency bandwidth because the integration block  2  only comprises passive electric components. 
   In accordance with the invention an integrated circuit can be produced comprising the amplifier  20  and the device  10 . The integrated circuit comprises various pins like that for the output of the signal Vout, that for the input of the supply voltage Vcc, that for ground, the pin in which the voltage signal Vdet is present, and the pin on which the signal Vdet is present. 
     FIG. 3  is a graph of the detected voltage signal Vdet in function of the average output power Poutm in decibels relating to one milliwatt of the signal in output from the amplifier  20 . When the amplifier  20  operates in conditions of linearity that is in the typical operation of an amplifier in radio frequency, the detector  10  has a volume range of the output voltage of about 1 V. 
   The device  10  is particularly suitable for use with wide signals that is, when the signal Vout in input to the device  10  have amplitude that can be compared with the supply signal. 
   It is important to note that the device  10  can use other types of bipolar transistors, for example MOS transistors, with circuit modifications which are within the true scope and spirit to those of average skill in the art. 
   Although a specific embodiment of the invention has been disclosed, it will be understood by those having skill in the art that changes can be made to this specific embodiment without departing from the spirit and scope of the invention. The scope of the invention is not to be restricted, therefore, to the specific embodiment, and it is intended that the appended claims cover any and all such applications, modifications, and embodiments within the scope of the present invention.