Abstract:
A system reduces non-linear distortions in an output signal of an amplifier stage that is configured according to the feed forward principle. An amplifier stage input signal arrives at a main branch of an amplifier. The output signal of the amplifier, which is distorted in a non-linear manner, arrives at an adder. An output of the adder forms the amplifier stage output signal. The output signal which is distorted in a non-linear manner and the amplifier stage input signal are fed to a secondary branch that comprises an error signal device. The error signal device generates an error signal from the delayed amplifier stage input signal and the output signal of the amplifier, which is distorted in a non-linear manner. The error signal is fed to the adder in order to reduce distortions in the amplifier stage output signal. The error signal device comprises at least one transmission device that is provided with a negative group delay time.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
   This application is based on and hereby claims priority to PCT Application No. PCT/DE03/00643 filed on Feb. 27, 2003, German Application No. 102 11 537.0 filed on Mar. 15, 2002 and European Application No. 02006023.2 filed on Mar. 15, 2002, the contents of which are hereby incorporated by reference. 
   BACKGROUND OF THE INVENTION 
   The invention relates to a system for reducing non-linear distortions for an amplifier stage output signal of an amplifier stage. 
   To compensate for non-liner distortions amplifier stages are embodied in accordance with what is known as the “feed-forward-principle”. Here in a main branch of the amplifier stage an amplifier stage input signal is routed via a non-ideal amplifier of which the non-linearly distorted output signal is routed with a delay to an adder and on the other side is routed to an auxiliary branch. 
   The amplifier stage input signal is also routed to the auxiliary branch where an error signal is obtained from the non-runtime-delayed amplifier stage input signal and from the non-linearly distorted output signal of the amplifier which is routed to the adder for distortion compensation. From the error signal and the non-linearly distorted output signal of the amplifier the added forms the amplifier stage output signal, in which case the error signal compensates for the non-linear distortions of the amplifier. 
   With the “feed-forward principle” the non-linearly distorted output signal of the amplifier must be correspondingly delayed in accordance with a group delay time needed to determine the error signal in the auxiliary branch. A delay of this type is generally realized using a delay line with finite electrical quality. The delay line exhibits electrical losses which in their turn make the efficiency of the amplifier stage worse. 
   To reduce the losses a correspondingly complex and expensive implementation of the delay line is necessary in which further attenuations are caused by the delay line. 
   SUMMARY OF THE INVENTION 
   One possible object of the present invention relates to improving the efficiency of an amplifier stage embodied according to the “feed-forward principle”. 
   The inventors propose an amplifier that significantly reduces the manufacturing effort involved. 
   Because of the main branch proposed by the inventors, depending on the application, the amplifier stage may be simple to implement in microstrip technology, the volume required for the “feed-forward” amplifier stage is reduced. 
   The efficiency of the amplifier stage is improved since losses are reduced within the main branch. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     These and other objects and advantages of the present invention will become more apparent and more readily appreciated from the following description of the preferred embodiments, taken in conjunction with the accompanying drawings of which: 
       FIG. 1  is a basic circuit diagram of an amplifier stage embodied in accordance with the feed forward principle, as per the related art, 
       FIG. 2  is, by comparison with  FIG. 1 , a basic circuit diagram of an amplifier stage in accordance with one embodiment of the invention, 
       FIG. 3  is a basic circuit diagram of a further amplifier stage according to one embodiment of the invention, 
       FIG. 4  is an exemplary embodiment for a transmission device envisaged in the system shown in  FIG. 3 , and 
       FIG. 5  to  FIG. 7  are transmission graphs of the analog filter shown in  FIG. 4 . 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
   Reference will now be made in detail to the preferred embodiments of the present invention, examples of which are illustrated in the accompanying drawings, wherein like reference numerals refer to like elements throughout. 
     FIG. 1  shows a basic circuit diagram of an amplifier stage VS 0  embodied in accordance with the feed forward principle in accordance with the related art. 
   An amplifier stage input signal u 0  arrives the amplifier stage VS 0  which is connected to both a main branch HZ and also to the auxiliary branch NZ of amplifier stage VS 0  as an input signal. An amplifier stage output signal u 5  is generated by amplifier stage VS 0  in which non-linear distortions are reduced with the aid of an error signal fs formed by auxiliary branch NZ. 
   The main branch HZ contains connected in series a first transmission device H 1 , which features an attenuation a 1  and a group delay time τ 1 , a non-ideal first amplifier V 1  with an amplification g 1 , a delay element T 1  with a group delay time τ 5  and a first adder AD 1 . 
   The auxiliary branch NZ contains a series circuit comprising a delay T 2  with a group delay time τ 2 , a second adder AD 2  as well as a third transmission device H 3  with an attenuation a 4  and with a group delay time τ 4 -τ. The third transmission device H 3  has a second amplifier V 2  with an amplification g 4  and with a group delay time τ connected downstream from it. The auxiliary branch furthermore contains in a transverse branch a second transmission device H 2 , which is connected on one side at the output of the first amplifier V 1  in the main branch HZ and on the other side to a second adder AD 2 . 
   The third transmission device H 3  and the second amplifier V 2  will be grouped into what is known as an error signal device NP for which the output signal reaches the first adder AD 1  as an error signal. The error signal unit NP thus features a resulting group delay time τres composed of the group delay times of the third transmission device H 3  and of the second amplifier V 2 . 
   In the main branch HZ the amplifier stage input signal u 0  arrives via the first transmission device H 1  at the first amplifier V 1  assumed to be not ideal of which the non-linear distorted output signal u 1  features an error component y. This means that: u 1 =a 1 *g 1 *x+y with x=u 0 . 
   Group delay times caused by the first amplifier V 1  are taken into account by the group delay time τ 1  of the first transmission device H 1 . 
   The non-linear distorted output signal u 1  of the first amplifier V 1  on one side arrives via the delay element T 1  at the first adder AD 1  and on the other side via the second transmission device H 2  negated at a second input of the second adder AD 2 , in which case at a first input of the second adder the amplifier stage input signal u 0  delayed by the delay element T 2  is connected. 
   This means that for an output signal u 3  of the second adder AD 2  which arrives at the error signal device NP as an input signal: u 3 =−a 3 *y, with τ 2 =π 1  and with a 1 *g 1 = 1 /a 3 . 
   The input signal u 3  of the error signal device NP arrives via the third transmission device H 3  at the second amplifier V 2 , for which the output signal is error signal fs. 
   In this case the following applies: τ 5 =τ 4  and a 3 *a 4 *g 4 =1, which means that fs=y. 
   The non-linear distorted output signal u 1  of the first amplifier V 1  is connected to a first input of the first adder AD 1 . The first adder AD 1  forms the amplifier stage output voltage u 5  from this. In this case the following applies with the requirements given above:u 5 =a 1 *gl*x. 
   The branches and adders shown here are generally implemented as directional couplers. Phase reversals of the voltages are not taken into account individually here. 
   Since the second amplifier V 2  merely amplifies the error component Y it can be driven linearly so that only negligible non-linear distortions are created through it. 
   Delay element T 2  is for example embodied as a delay line and for frequency f exhibits an attenuation A, where the following applies: A=10 dB*log 10  (e) 2 πft S /Q=27.3 dB*fτ S /Q. 
   As a further embodiment of the delay element T 2  a filter is possible which also features the same attenuation A at the same quality Q. 
     FIG. 2  by comparison with  FIG. 1  shows a basic circuit diagram of an amplifier stage VS 1 . 
   By comparison with  FIG. 1  the starting point here is an ideal case in which in the main branch a delay element T 1 ′ with a group delay time τ 5 ′ is embodied such that the non-linear distorted output signal u 1  of the first amplifier V 1  essentially arrives with no delay at the first adder AD 1  in which case this ideal condition must be taken into consideration accordingly with the aid of the resulting group delay time τres of the error signal device NP. 
   The error signal device NP here includes two series circuits SS 1  and SS 2 , where each of these series circuits features a third transmission device H 31  or H 32  and the relevant transmission device features downstream amplifiers V 21  or V 22 . 
   In a further embodiment provision is made for arranging more than two series circuits in parallel with one another. 
   In this case it is also possible to connect a common amplifier downstream of the third transmission device. 
   However in this case it is always the case that the resulting group delay time τres of the error signal device NP is formed such that a group delay time τ 5 ′ occurring between the first amplifier V 1  and the first adder AD 1  is taken into account accordingly. 
   For the case described here “essentially delay-free” the resulting group delay time τres of the error signal device NP is to be selected as negative in the desired frequency range. 
   The error signal device NP here features a digital filter, in which case the two transmission devices H 31  or. H 32  feature  2 *a 4  and τ 4 -τ or. −a 4  and  2  τ 4 -τ as coefficients. 
   The output signals of the two amplifiers V 21  and V 22  are added with the aid of a further adder to error signal fs which again reaches the first adder AD 1 . 
   Under the condition a 3 *a 4 *g 4 =1 the following applies for the amplifier stage signal u 5 :
 
 u   5   =a   1   g   1   x+y[ 1−2exp( −j 2πfτ 4 )+exp( −j 2πfτ 4 )]
 
   Through a correspondingly small group delay time τ 4 -τ the group delay time τ 4  can be set so that a product f 0 τ 4  is a whole number, in which case f 0  here is a mid frequency of a working range of the amplifier V 21  or V 22 . For a storage frequency δf=f-f 0  the following then applies:
 
 u   5   =a   1   g   1   x+y[ 1−2exp( −j 2πδfτ 4 )+exp( −j 2πδf2τ4)]
 
   With a series development:
 
[1−2exp( −j 2πδfτ 4 )+exp( −j 2πδf2τ4)]
 
−( −j 2πδfτ 4 ) 2 +2( j 2πδfτ 4 ) 2 −⅓( j 2πδfτ 4 ) 3 + 4/3( j 2πδfτ 4 ) 3 
 
+. . .
 
=( j 2πδfτ 4 ) 2 +( j 2πδfτ 4 ) 3 +. . .
 
a suppression of non-linear distortions is produced for error component Y for small storage frequencies δfτ 4 &lt;&lt;1 in output signal u 5  by around −20 dB*log 10  ( 2 πδfτ 4 ).
 
   Unlike the ideal case, in reality with amplifier stage VS 1  in the main branch HZ between the output of the amplifier V 1  and the adder AD 1  the delay element T 1 ′ with a lower group delay time τ 5 ′ compared to the related art will be arranged. For this case the negative group delay times will be selected for the third transmission devices in such a way that the resulting group delay time τres of the error signal device NP compensates for the group delay time τ 5 ′, i.e. τres=τ 5 ′. 
     FIG. 3  shows a basic circuit diagram of an amplifier stage VS 2 . 
   Compared with  FIG. 2  an error signal device NP contains only a series circuit with a third transmission device H 33  and a second amplifier V 23 . 
   The third transmission device H 33  will be formed for example by a passive filter with a negative group delay time. The output signal of the second amplifier again arrives as the error signal fs at the first adder AD 1 . 
   A transmission function [1-exp(−j 2 πδfτ 4 )] n −1 of the error signal device NP will for large and whole number powers “n” only be reached in a good approximation with one amplifier V 23 , in which case the upstream filter approximates at least for small storage frequencies δfτ 4 &lt;&lt;1 to a filter transmission function h 4 (f)={[1−-exp(−j 2 πδfτ 4 )] n −1}* exp(+j 2 πfτ)/g 4 . This is possible in principle for τ 4 &gt;τ. 
   Since the third transmission device H 33  embodied as a filter only transmits low high-frequency powers the insertion loss of the filter is ignored. 
     FIG. 4  shows an exemplary embodiment for the third transmission device H 33  for which there is provision in the system according to  FIG. 3  which is embodied as an analog filter with negative group delay time. 
   A voltage source uE with a resistor R 1 =50 is connected at two reference points P 11  and P 12  of a first access port. between the reference points P 11  and P 12  a first and a second branch, Z 1  and Z 2 , are connected in parallel to each other, where the first branch Z 1  features a series circuit with a capacitor C 1 =6 pF, an inductor L 1 =1 nH and a resistor R 2 =1,5. 
   The second branch Z 2  features a series circuit with a capacitor C 2 =15 pF, an inductor L 1 =1,1 nH and a resistor R 3 =1,5. An output voltage can be tapped at a second access port, port  2  via two reference points P 21  and P 22  at a resistor R 4 =50. 
     FIG. 5  to  FIG. 7  show transmission graphs of the analog filter shown in  FIG. 4 . 
     FIG. 5  shows a frequency-dependent transmission graph, in which frequency in GHZ is plotted on the x axis and amplitude values in “dB” are plotted on the y axis. 
     FIG. 6  shows a frequency-dependent transmission graph, in which frequencies in GHz are plotted on the x axis and phases in “radians” are plotted on the y axis. 
   Finally  FIG. 7  shows a frequency-dependent graph in which frequencies in GHz are plotted on the x axis and group delay times in seconds are plotted on the y axis. 
   The invention has been described in detail with particular reference to preferred embodiments thereof and examples, but it will be understood that variations and modifications can be effected within the spirit and scope of the invention covered by the claims which may include the phrase “at least one of A, B and C” or a similar phrase as an alternative expression that means one or more of A, B and C may be used, contrary to the holding in Superguide v. DIRECTV, 69 USPQ2d 1865 (Fed. Cir. 2004).