Abstract:
An amplifier circuit for current amplification. An input stage is adapted to receive an input signal. At least one current multiplication stage is connected to the input stage. The current multiplication stage is adapted to receive a current signal from the input stage and to produce a multiplied output current signal at an output of the amplifier circuit. The current multiplication stage includes at least two current multiplication circuits connected to each other. Each current multiplication circuit is adapted to produce an output current signal essentially equal to the current signal from the input stage, such that the output current signal at an output of the amplifier circuit includes a sum of the current signals received at each current multiplication circuit. A method of improving linearity in an amplification circuit.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is the national phase under 35 U.S.C. §371 of PCT/EP2008/064463 filed 24 Oct. 2008. 
     TECHNICAL FIELD 
     The present invention is related to amplifier circuits. More specifically it is related to linearity in amplifier circuits. 
     BACKGROUND ART 
     In today&#39;s radar systems there is a requirement of higher and higher linearity at more or less constant current consumption for an amplifier circuit. 
     If two signals are supplied to the two inputs of the amplifier, then the linearity of the amplifier may be defined as the ability of the amplifier to curb intermodulation products at the amplifier output per given signal power. 
     The difference between a useful signal and undesired intermodulation products can be written as IM 3 =2(IP 3 −I signal ) (expressed in dB). In this case IP 3  is related to every tone, but it may also be related to the sum of powers on each tone, i.e. I signal +3 or the vector sum of the tones, i.e. I signal +6. IP3 is often given in dBm, but it may equally be put in relation to a current or a voltage. We will choose to relate IP 3  to a current, since it is the current amplitude which influences the amplitude of the intermodulation products. 
     One common circuit for signal amplification is a cascode amplifier shown in  FIG. 1 . 
     The upper part of the circuit comprises a current follower with the transistors Q 1  and Q 2  whose function is to keep the output current i out  at the same level even if its output is loaded by a high impedance which would lead to a high power consumption. Thus i out  stays essentially the same even if R 1  is increased. 
     The lower part of the circuit transforms an input voltage signal into an output signal current. Both stages use the same bias current i bias  shown in the figure. 
       FIG. 7  shows the cascode amplifier together with the intermodulation products produced by the two stages of the amplifier. The diagrams to the left of the cascode amplifier display the signal amplitude expressed in dB as a function of frequency. By studying the diagrams in  FIG. 7  it may be seen that the amplitude of the intermodulation products before and after the current follower stage remains the same. Hence the entire contribution to the increase of the amplitude for the intermodulation products comes from the transconductance stage of the cascode amplifier. 
     Examining the linearity of a cascode amplifier one notices that it is theoretically possible to decrease the bias current in the current follower and to increase the bias current through the transconductance stage comprising the transistors Q 3  and Q 4  without sacrificing linearity. However, this is difficult to do, since both stages share the same bias current i bias . Moreover, if the current saving due to the bias current reduction in the current follower stage and the subsequent current increase through the cascode amplifier should be of any use, the increased current should be put to use. Otherwise power consumption is reduced in the current follower stage only to be wasted in the transconductance stage. 
     However, the problem with this solution is that the transconductance stage cannot handle as high a signal current as the current follower at a given bias current. The reason for that is that an increase of the signal current through the transconductance stage would increase the amplitude of the intermodulation products in the output signal from the transconductance stage. 
     One other alternative may be to increase the bias current i bias  in the entire cascode amplifier, i.e. in both the transconductance stage and the current follower stage. This solution however, wastes current and therefore power in the current follower where no amplification of the signal is achieved. 
     One other alternative may be increase the linearity of the amplifier circuit by adapting the transconductance stage to deliver a lower signal current than the current follower. This, however, would necessitate replacing the current follower by a current amplifier. Traditionally, current amplifiers are built with a transconductance amplifier, which in this case would not lead to a satisfactory solution. 
     SUMMARY OF THE INVENTION 
     The present invention aims at solving at least some of the problems related to known technology. 
     One solution according to the present invention is related to an amplifier circuit for current amplification comprising: an input stage adapted to receive an input signal; at least one current multiplication stage connected to the input stage, where the current multiplication stage is adapted to receive a current signal from the input stage and produce a multiplied output current signal at an output of the amplifier circuit, wherein the current multiplication stage comprises at least two current multiplication circuits connected to each other, each current multiplication circuit adapted to produce an output current signal essentially equal to the current signal from the input stage, such that the output current signal at an output of the amplifier circuit comprises a sum of the current signals received at each current multiplication circuit. 
     The advantage of amplifier circuit according to the present invention with respect to known technology is increased linearity and current amplification at the same time. The additional current resulting from the amplification can be put to use elsewhere in an electric circuit where the amplifier circuit according to the present invention is used. 
     Another aspect of the present invention comprises a current multiplication circuit comprising a first and a second transistor having their gate terminals connected to each other where the first transistor is adapted to receive an input signal current at its emitter terminal and to output an output current essentially identical to the input signal current on the collector terminal of the first transistor and the emitter terminal of the second transistor, where the current multiplication circuit is adapted to fulfil a function of the current multiplication stage in the amplifier circuit according to the invention mentioned earlier. 
     Moreover, another aspect of the present invention is related to a method for increasing linearity in an amplifier circuit comprising the steps of receiving a signal at an input stage of an amplifier circuit; receiving a signal current from the input stage at a current amplification stage, producing essentially the same signal current at the outputs of at least two current amplification circuits of the current amplification stage, combining the signal currents at the outputs of the at least two current amplification circuits to produce a sum of the signal currents at the output of the amplifier circuit. 
     These and other advantages of the present invention will become more apparent after studying the above detailed description together with the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  illustrates a cascode amplifier according to known technology. 
         FIG. 2  illustrates a common-base amplifier used as an active balun, hereafter denoted common-base balun. 
         FIG. 3  illustrates a modified cascode amplifier circuit according to one embodiment of the present invention. 
         FIG. 4  illustrates the modified cascode amplifier circuit according to another embodiment of the present invention. 
         FIG. 5  illustrates the modified cascode amplifier circuit according to yet another embodiment of the present invention. 
         FIG. 6  illustrates a current amplification circuit according to yet another embodiment of the present invention. 
         FIGS. 7-9  illustrate the amplification diagrams for intermodulation products in the circuits from  FIGS. 1 and 3 .  FIGS. 8-9  are essentially the same with different signal levels. 
     
    
    
     DETAILED DESCRIPTION 
     Before describing the components which are included in the embodiments of the present invention described in the figures below it is assumed that all transistors operate in the active mode, i.e. they act as amplifiers. Moreover, it should be pointed out that while the example embodiments of the present invention use bipolar transistors, this should not be construed as limiting the invention to the use of bipolar transistors only. Analogous solutions using Field Effect Transistors (FETs) or other solid state components are equally possible. Also, while the solutions in  FIGS. 3-6  utilize transistors in the form of discrete components, it should be pointed out that equivalent solutions using integrated components are equally possible. 
       FIG. 2  illustrates a common-base balun  200  being a basic component included into the embodiments of the present invention in  FIGS. 3-6 . In this version of the common-base balun, two bipolar transistors Q 5  and Q 6  share a common base, where the Q 5  transistor receives a signal current i signal  on its emitter terminal. This input signal current i signal  is reproduced as the output signal current at the collector terminal of the transistor Q 5 . 
     At the same time the current from the base terminal of transistor Q 5  is used as the input current i signal /β to the transistor Q 6 . Transistor Q 6  in turn produces the same output current i signal  on its collector terminal. Note however, that output currents from Q 5  and Q 6  have different polarity. R 1  and R 2  are the load resistances driven by the output current i signal . R 1  and R 2  may be either passive or active loads, the active loads for example being transistors. 
     The main advantage of the common-base balun circuit  200  is that it produces the same output signal current i signal  on both of its output terminals from a single input signal current. 
       FIG. 3  illustrates an amplifier circuit  300  according to one embodiment of the present invention. In the embodiment in  FIG. 3 , the amplifier circuit  300  comprises a modified cascode amplifier, where the two cross-coupled common-base baluns from  FIG. 2  have replaced the transistors Q 1  and Q 2  of the current follower. The signal current received at the emitter of Q 5   a  is reproduced at the collector of Q 6   a  with different polarity. The same way the signal current received at the emitter of Q 5   b  is reproduced at the collector of Q 6   b  with opposite polarity. When combining the currents of the collectors of Q 5   a  with Q 6   b  and Q 5   b  with Q 6   a  an effectively doubling of the signal current is achieved compared to the signal current out from the transconductance stage Q 3  and Q 4 . This doubling can be done without increasing the intermodulation products signal levels relative the useful signals hence increasing linearity which will be explained more in detail in  FIG. 8 . 
     The output current I out  current is supplied to the loads R 1  and R 2  which may be purely passive in the form of resistive, capacitive, inductive or other similar loads, or comprise active components, such as additional amplifier circuits or transistors. 
     Hence the advantage of the present invention is increased linearity of the amplifier circuit by an increase in the current through the amplifier circuit. 
     At the same time the bias current i bias  from the two common-base balun can be used to bias other circuit components which may be connected to the amplifier circuit  300 . One way of utilizing the extra bias current i bias  from the two common-base baluns shown in  FIG. 4 . 
     It should be pointed out here that one may use more than two baluns from  FIG. 2  connected in series to replace the current follower in order to achieve an even higher increase of the output current. This is illustrated more in detail in  FIG. 5 . 
     In  FIG. 4  the amplifier circuit from  FIG. 3  has been modified, so that transistors Q 7  and Q 8  have been connected to the modified cascode circuit, such that they form an emitter follower feeding the modified cascode amplifier. The input signal is now present in the circuit in the form of an input voltage Vin over the base terminals of transistors Q 7  an Q 8 . At the same time, the extra bias current i bias  from the two common-base baluns has been used to bias the transistors Q 7  and Q 8 . 
       FIG. 5  illustrates an amplifier circuit  500  according to another embodiment of the present. The amplifier circuit is analogous to the amplifier circuit in  FIG. 4  with the difference that the current multiplication stage of the amplifier circuit  500  now comprises a first, second and third balun are connected to each other. 
     It should be mentioned that the first, second and third baluns comprising the transistors Q 5   a , Q 6   a  and Q 5   b , Q 6   b  and Q 5   c , Q 6   c  are essentially identical to the balun in  FIG. 2 . 
     The advantage of the amplifier circuit in  FIG. 5  is that the amplifier circuit offers higher linearity by producing a triple output current 3*i signal  with constant IM3 from an input voltage signal Vin present over the base terminals of Q 3  and Q 4 . 
     It may be mentioned here that theoretically up to N number of baluns may be connected to replacethe transistors Q 1  and Q 2  of the current follower. This may theoretically result in an output current I out  N times that of that in circuit  300 . 
       FIG. 6  illustrates a pure current multiplier without the transconductance amplification stage according to another embodiment of the present invention. Here, the amplification circuit  600  only comprises a current multiplier with the transistors Q 5   a , Q 6   a , Q 5   b  and Q 6   b . An input voltage Vin is present over the terminals depicted in  FIG. 6  resulting in a input signal current i signal  flowing into the emitter terminal of transistor Q 5   a . Comparing the circuits in  FIG. 3  and  FIG. 6  one notices that apart from the missing transconductance stage, the amplifier circuit  600  is analogous to the one in  FIG. 3 . Hence, the amplifier circuit  600  amplifies an input signal in the form of an input current i signal  to double the input current i signal , i.e. iout=2*i signal . Since the functioning principle of the upper part of the circuit in  FIG. 3  has been explained earlier and is identical to the circuit  600 , its functioning principle will not be repeated here. The embodiment of the present invention in  FIG. 6  merely serves to illustrate that the principle of current multiplication with increased linearity using two common-base baluns also holds in a circuit without a transcoductance amplifier. 
     Thus, it would also be possible to use more than two cross-coupled common-base baluns in the amplifier circuit  600  coupled into the amplifier circuit in the same fashion as in  FIG. 5 . 
       FIG. 7  illustrates the linearity of the cascode amplifier circuit from  FIG. 1  by means of diagrams where the amplitude of two close tones and their related intermodulation products are displayed as a function of frequency. It should be mentioned that the y-axis of the diagrams displays signal amplification expressed in dB, while the x-axis displays the frequency in GHz. 
     As can be seen from the figure, the lowermost amplification diagram illustrates the signal level of an input voltage signal V signal  which is present as the two frequencies f 1  and f 2  over the base terminals of transistors belonging to the transconductance stage of the cascode circuit in  FIG. 1 . 
     The original signal level at the two frequencies (which are around 1 GHz) in this example simulation was −40.91 dB. 
     After being converted into a current signal i signal  and amplified, the current signal will comprise intermodulation products at the frequencies  2   f   1 -f 2  and  2   f   2 -f 1 . As mentioned earlier, the ability of an amplifier circuit to suppress intermodulation products is a measure of its linearity. 
     We notice the amplitude of the signal current after the transconductance stage to be −58.02 dB. At the same time, the amplitude of the intermodulation products lies about 42 dB lower than the useful signals. From the middle diagram in  FIG. 7  it can be seen that the signal level of the useful signal IM3 is 42.2 dB after the transconductance amplification stage. 
     After passing the current follower stage, the signal level of the useful current signal is essentially unchanged. 
     In conclusion it can be seen that the entire contribution to the intermodulation products in the cascode circuit comes from the transconductance amplification stage. 
     Studying the simulation results in  FIG. 8 , where the amplifier circuit according to the embodiment from  FIG. 3  is shown, we notice that the input signal level has been amplified from −40.91 dB to −58.04 dB (in absolute terms) by the transconductance stage. The middle diagram shows that the difference between the useful signal and the intermodulation products is 42.26 dB. However, analyzing the uppermost amplification diagram in  FIG. 8  it is apparent that the difference between the useful signal and the intermodulation products is somewhat higher than in the case of the cascode amplifier. At the same time, the output current from the amplification circuit according to the embodiment in  FIG. 3  is doubled. Remembering IM 3 =2(IP 3 −I signal ) this means an increase in linearity of 6 dB. 
       FIG. 9  illustrates a situation where the same embodiment as in  FIG. 3  is used, but where the signal current out from the transconductance stage has been reduced by half. The output current from the circuit in  FIG. 9  is thus also reduced by half meaning that the amplification circuit has essentially the same output current as the cascode amplifier in  FIG. 1 . 
     However, the reduction of the current after the transconductance stage leads to an important effect. 
     As can be seen from the amplification diagrams on the left side of  FIG. 9 , the reduction of the current out from the transconductance stage results in a considerable suppression of intermodulation products and thus increased linearity of the amplifier circuit from  FIG. 3 . From the middle diagram it is evident that the difference between the useful signal and the intermodulation products has increased to 54.36 dB, thus an improvement by 12.16 dB. Analyzing the uppermost diagram in  FIG. 9  it can be seen that the useful signals has the same amplitude as in  FIG. 7  while the intermodulation products are reduced by 12 dB. 
     As will be apparent to the skilled person having studied the above circuit solutions for the modified cascode amplifier, other circuit solutions besides the common-base balun for the modified cascode circuit in  FIG. 3  may be possible. 
     Ultimately, the present invention is only limited by the scope of the accompanying patent claims.