Abstract:
An augmented common-base transistor amplifier circuit is described, including an input terminal and an output terminal. The amplifier circuit also includes an augmentation circuit, connected from the emitter to the base of the amplifying transistor, which detects an error voltage at the transistor emitter, amplifies and inverts the error voltage, and then applies the amplified error voltage to the base of the amplifying transistor, for the purpose of reducing the emitter error voltage and thus linearizing the common-base amplifier. According to a further embodiment, a second transistor is used for amplifying the emitter error voltage, and a further embodiment uses a transformer for augmentation. Circuits are described for single-ended, push-pull, and complementary amplifiers.

Description:
BACKGROUND OF THE INVENTION 
     Common-base amplifier circuits have long been recognized for their ability to deliver higher frequency response, higher collector voltage swings, and higher linearity than do their common-emitter counterparts under identical conditions of bias and loading for the same transistor device. Common-base amplifiers are readily adaptable to wide-band RF applications from HF, through VHF, and well into the UHF region of frequencies, and are easily designed with a minimum of effort devoted to input and output matching networks. In addition, common-base amplifiers achieve a higher degree of reverse isolation than do their common-emitter counterparts, thus leading to a greater degree of stability. All of these factors are desirable characteristics in amplifier design. In a conventional common-base amplifier, as shown in FIG. 1, the input resistance is the sum of a fixed resistance and the nonlinear emitter resistance of the transistor, the latter of which is a primary cause of both harmonic and intermodulation distortion. Traditional design techniques reduce this nonlinearity either directly by increasing the fixed input resistance or indirectly by decreasing the nonlinear emitter resistance by increasing the transistor bias current. The former technique is unsuitable for power amplifier applications, and the latter technique reduces the overall power efficiency of the amplifier. 
     SUMMARY OF THE INVENTION 
     An augmented common-base transistor amplifier circuit with improved intermoduiation (IM) performance is described which includes a common base transistor amplifier, consisting of an input emitter resistance, a common-base transistor, and an output load resistance. The augmented common-base transistor amplifier circuit further includes an augmentation circuit which detects an error voltage at the emitter of the common-base transistor, and which then inverts and amplifies the detected error voltage as a voltage to be applied at the base of the common-base transistor, thereby reducing the error voltage at the emitter of the common-base transistor and in turn improving the linearity and Fivl performance of the common-base transistor amplifier circuit. In a further embodiment the augmentation is accomplished by the addition of a common-emitter transistor amplifier circuit. In a further embodiment suitable for higher frequency and higher power applications, a transformer is used to perform the augmentation, and a tap is later added to the transformer for the purpose of providing additional current gain to the augmented common-base transistor amplifier circuit for applications requiring additional power gain. In a further embodiment suitable for medium- and high-power applications, a pair of augmented common base transistor amplifiers are arranged as a push-pull amplifier. In a further embodiment suitable for applications requiring higher linearity, a complementary pair of augmented common-base transistor amplifiers are arranged as a complementary amplifier. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention is described in the schematics of FIGS. 1 to  7 , in which: 
     FIG. 1 schematically illustrates the existing prior art, commonly referred to as a common-base transistor amplifier; 
     FIG. 2 schematically illustrates an inverting amplifier in a common-base transistor amplifier in accordance with the present invention; 
     FIG. 3 schematically illustrates a common-emitter transistor amplifier in a common-base transistor amplifier in accordance with the present invention; 
     FIG. 4 schematically illustrates a transformer in a common-base transistor amplifier in accordance with the present invention; 
     FIG. 5 schematically illustrates the addition of a tap to the primary winding of the transformer in the augmented common-base transistor amplifier of FIG. 4; 
     FIG. 6 schematically illustrates a push-pull amplifier incorporating a pair of the augmented common-base transistor amplifiers of FIG. 5; and 
     FIG. 7 schematically illustrates a complementary amplifier incorporating a complementary pair of the augmented common-base transistor amplifiers of FIG.  5 . 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Designers of linear amplifiers, for applications ranging from audio, through HF, VHF, UHF frequencies and beyond, are concerned with elements of system performance which include, but are not limited to, harmonic distortion, intermodulation distortion (IM), power consumption and efficiency, and environmental conditions, particularly temperature. Historically, the IM performance of linear amplifiers is improved by application of feedback or feedforward methods. The former of these is impractical for high frequency power amplifiers due to excessive phase shift in the active device or the feedback network (and very often a combination of both), which causes instability. The latter method is expensive to manufacture, excessively power inefficient, and is generally limited to narrow-band operation. This invention now presents a linear amplifier circuit which achieves a markedly improved IM performance over a wide bandwidth without introducing any instabilities or complex circuitry, and which is power efficient. 
     Referring to FIG. 1, a common-base transistor amplifier circuit  100  is shown in its most basic form. Here, a transistor  105  has its base connected to ground, hence the term common-base. A resistance  103  (illustrated as a fixed resistance R E  for convenience), is connected from a signal voltage source  101 , having an amplitude A and a frequency of f s , to an emitter  104  of transistor  105 . A collector of transistor  105  is connected through a load resistance  107  (illustrated as a fixed resistance R L  for convenience) to a common point, such as ground. An output voltage  106  is described by the equation: 
     
       
         V 106 =I c ×R L   (1) 
       
     
     where I C  is the instantaneous collector current of transistor  105 . This collector current is related to the input emitter current I E  by the equation:                I   C     =         I   E     ×     h   fe           h   fe     +   1               (   2   )                                
     where h fe  is the signal current gain of transistor  105 . The input emitter current I E  is a result of the input signal voltage at  102  and the input resistance R I , which is approximately described by:                R   I     =       R   E     +     r   e     +       r   bb         h   fe     +   1                 (   3   )                                
     where r bb  is the base spreading resistance and r e  is the nonlinear incremental emitter resistance of transistor  105 , the latter of which is described by:                r   e     =         V   BE       I   E       =       V   BE         I   O     ×            q                   V   BE       kT                     (   4   )                                
     where L O  is the saturation current and V BE  is the base-emitter voltage of transistor  105 , the latter of which is equal to −V 104 . This voltage constitutes an error voltage  104  at the emitter of transistor  105 , which can be described as:                V   104     =       V   102     ×       (       r   e     +       r   bb         h   fe     +   1         )         R   E     +     r   e     +       r   bb         h   fe     +   1                     (   5   )                                
     By combining EQ. 3 and EQ. 5, the input resistance R I  can be determined by:                R   I     =         R   E     ×         V   102     +     V   104         V   102         =       R   E     ×         V   102     +     V   BE         V   102                   (   6   )                                
     Examination of EQ. 3 shows that as the second and third terms are reduced, the input resistance R I  more closely approximates the fixed emitter resistance RE. This, in turn, reduces the emitter error voltage V 104 , thus linearizing the input resistance R E  and the input current I E , which results in a linear collector current and thus linearizes the amplifier. Similarly, reducing the emitter error voltage V 104 , creating a virtual ground at the emitter terminal of transistor  105 , has the same effect. 
     Turning now to FIG. 2, an augmented common-base transistor amplifier circuit  200  in accordance with the present invention is illustrated. Circuit  200  includes an input signal source  201 , supplying an input signal voltage  202 , which is coupled through a resistance  203  (illustrated as a fixed resistance R E  for convenience) to the emitter of a transistor  205 . An augmentation circuit including an inverting voltage amplifier  206  has an input connected to the emitter of transistor  205  and an output connected to the base of transistor  205 . The collector of transistor  205  produces an output voltage  208  across a load resistance  209  (illustrated as a fixed resistance R L  for convenience), the opposite end of which is connected to a common point, such as ground. It will of course be understood that in accordance with common practice the input signal source  201  and the load resistance  209  represent any convenient input and output apparatus, respectively. The augmentation amplifier  206  has an inverting voltage gain factor of −A V , making an amplified error voltage  207 , which is applied to the base of transistor  205 . This voltage is described as: 
     
       
         V 207 =−A V ×V 204   (7) 
       
     
     where V 204  is the is the emitter voltage  204 . The resulting base-emitter voltage V BE  of transistor  205  becomes: 
     
       
         V BE =V 207 −V 204 =−A V ×V 207 −V 204 =−V 204 ×(A V +1)  (8) 
       
     
     Substituting EQ. 8 into EQ. 4, we find that the apparent emitter resistance r e ′ becomes:                r   e   ′     =         V   204       I   E       =         V   204         I   O     ×            q                   V   204     ×     (       A   V     +   1     )       kT           =       V   BE         (       A   V     +   1     )     ×     I   O     ×            q                   V   BE       kT                       (   9   )                                
     and by substituting EQ. 8 into EQ. 6, we further find that the input resistance R I  is now:                R   I     =       R   E     +       r   e     ×         V   202     +       V   BE         A   V     +   1           V   202                   (   10   )                                
     From inspection of EQ. 9 and EQ. 10 it is obvious that the apparent emitter resistance r e ′ is greatly reduced as the voltage gain A V  of augmentation amplifier  206  is increased, and that the input resistance becomes more closely equal to the fixed input resistance R E  as the voltage gain is increased, thus showing that the addition of augmentation amplifier  206  has caused the emitter terminal of transistor  205  to appear as a virtual ground, thus achieving the necessary condition discussed earlier for linearizing a common-base transistor amplifier. 
     In some applications, particularly those at higher frequencies, the use of augmentation amplifier  206  as shown in FIG. 2 may be impractical. Referring specifically to FIG. 3, another embodiment of an augmented common-base transistor amplifier circuit in accordance with the present invention, designated  300 , is illustrated. Circuit  300  includes an input signal voltage source  301 , supplying an input signal voltage  302 , which is coupled through a resistance  303  (illustrated as a fixed resistance R E  for convenience) to the emitter of a transistor  305 . An augmentation circuit including a common-emitter transistor amplifier  306  has a base connected to the emitter of transistor  305 , a grounded or common emitter, and a collector connected to the base of transistor  305 , which produces a base voltage  307  of transistor  305 . The collector of transistor  305  produces an output voltage  308  across a load resistance  309  (illustrated as a fixed resistance R L  for convenience), the opposite end of which is connected to a common point, such as ground. in this case, the input current at the emitter of transistor  305  is described as:                      I   E   ′     =         I   E1     +     I   B2       =                      I   B1     ×     (       h   fe1     +   1     )       +       I   B1       h   fe2         =                   =                  (       h   fe1     +   1   +     1     h   fe2         )     ×     I   O2     ×            q                   V   BE       kT                       (   11   )                                
     where h fe1  is the signal current gain of transistor  305 , h fe2  is the signal current gain of transistor  306 , I O2  is the saturation current of transistor  306 , and V BE  is the base-emitter voltage of transistor  305 . Substituting EQ. 11 into EQ. 4, we find that the apparent emitter resistance r e ′ becomes approximately:                r   e   ′     =         V   304       I   E   ′       =       V   304         (       h   fe1     +   1   +     1     h   fe2         )     ×     I   O2     ×            q                   V   BE       kT                     (   12   )                                
     which is a considerable reduction in the nonlinear emitter resistance of the common-base transistor amplifier, and thus showing that the use of common-emitter transistor amplifier  306  is a suitable alternative for an augmentation circuit for linearizing a common-base transistor amplifier. 
     For power amplifiers at high frequencies, an augmentation circuit including common emitter transistor amplifier  306  may be impractical. For such applications, the use of a simple transformer can give sufficient voltage gain to provide augmentation. Referring specifically to FIG. 4, another embodiment of an augmented common-base transistor amplifier circuit in accordance with the present invention, designated  400 , is illustrated. Circuit  400  includes an input voltage source  401 , supplying an input signal voltage  402 , which is coupled through a resistance  403  (illustrated as a fixed resistance R E  for convenience) to the emitter of a transistor  405 . An augmentation circuit including a transformer  406  has a primary winding connected between the emitter of transistor  405  and a common point, such as ground. A secondary winding of transformer  406  is connected, in reverse phase relative to the primary, between the base of transistor  405  and the common or ground, producing a base voltage  407 . The collector of transistor  405  produces an output voltage  408  across a load resistance  409  (illustrated as a fixed resistance R L  for convenience), the opposite end of which is connected to a common point, such as ground. The base-emitter voltage V BE , being the difference between base voltage  407  and emitter voltage  404 , and the base current I B  for circuit  400  are, respectively: 
     
       
         V BE =V 407 −V 404 =−N×V 404 −V 404 =−V 404 ×(N+1)  (13)                I   B     =       I   E       h   fe               (   14   )                               
      
     
     where N is the turns ratio of the secondary winding to the primary winding of transformer  406 . current I E ′ equal to:                I   E   ′     =         I   E     -       N   ×     I   E         h   fe         =       I   E     ×     (     1   -     N     h   fe         )                 (   15   )                                
     where                I   E     =         I   O     ×              q        (     1   +   N     )            V   404       kT         =       I   O     ×       [            q                   V   404       kT       ]       (     1   +   N     )                   (   16   )                                
     which allows the apparent emitter resistance r e ′ to be approximated as:                      r   e   ′     =         V   404       I   E   ′       =                    V   404         (     1   -     N     h   fe         )     ×     I   O     ×              q        (     N   +   1     )            V   404       kT           =                   =                  V   BE         (     N   +   1     )     ×     (     1   -     N     h   fe         )     ×     I   O     ×            q                   V   BE       kT                         (   17   )                                
     which, compared to EQ. 4, shows that the apparent emitter resistance r e ′ decreases dramatically as the turns ratio N of transformer  406  is increased, showing that transformer  406  does provide augmentation of a common-base transistor amplifier. 
     The transformer primary of the augmentation circuit of the augmented common-base transistor amplifier  400  can be tapped to provide current gain. Referring specifically to FIG. 5, another embodiment of an augmented common-base amplifier circuit in accordance with the present invention, designated  500 , is illustrated. Circuit  500  includes an input signal voltage source  501 , supplying an input signal voltage  502 , which is coupled through a resistance  503  (illustrated as a fixed resistance R E  for convenience) to one terminal of the primary winding of a transformer  507 , the opposite terminal of which is connected to a common point, such as ground. A tap of the primary winding (designated as M turns between the tap and terminal  504 ) is connected to the emitter of a transistor  506 , producing an emitter voltage  505 , the actual input voltage now being a voltage at terminal  504 . The base of transistor  506  is connected, in reverse phase relative to the primary, through an N turn secondary of transformer  507  to the common or ground, which produces a base voltage  508 . The collector of transistor  506  produces an output voltage  509  across a load resistance  510  (illustrated as a fixed resistance R L  for convenience), the opposite end of which is connected to a common point, such as ground. Here, EQ. 13, EQ. 15, EQ. 16, and EQ. 17 are modified, respectively, to describe the various voltages and currents as follows:                V   BE     =         V   508     -     V   505       =           -   N     ×     V   505       -     V   505       =       -     V   505       ×       N   +   1       M   +   1                     (   18   )                                              I   E   ′     =       I   E     ×       (     1   -     N     h   fe         )       M   +   1                 (   19   )                 I   E     =       I   O     ×       [            q                   V   504       kT       ]       (       1   +   N       1   +   M       )                 (   20   )                       r   e   ′     =         V   504       I   E   ′       =                      V   504     ×     (     M   +   1     )           (     1   -     N     h   fe         )     ×     I   O     ×              q        (     N   +   1     )            V   504       kT           =                   =                    V   BE     ×       (     M   +   1     )     2           (     N   +   1     )     ×     (     1   -     N     h   fe         )     ×     I   O     ×            q                   V   BE       kT                         (   21   )                                 
     Comparing EQ. 21 with EQ. 17 shows that the apparent emitter resistance r e  increases exponentially as the tapped transformer primary ratio M is increased, but in comparison with EQ. 4 this is still an improvement in the emitter resistance of the common-base transistor amplifier, provided that the turns ratios M and N are chosen properly. 
     The use of augmented common-base transistor amplifiers is not limited to single-ended applications. For instance, a pair of single-ended augmented common-base transistor amplifiers can be arranged in a push-pull configuration. Referring specifically to FIG. 6, another embodiment of an augmented common-base transistor amplifier circuit in accordance with the present invention, designated  600 , is illustrated. Circuit  600  includes an input signal voltage source  601 , supplying an input signal voltage  602 , which is coupled to one terminal of the primary winding of an input transformer  603 , the opposite terminal of which is connected to a common point such as ground. One terminal of a secondary winding of transformer  603 , producing a signal voltage  604 , is connected through a resistance  606  to one terminal of the primary winding of an augmentation transformer  609 , the opposite terminal of which is connected to the common point, such as ground. The opposite terminal of the secondary winding of input transformer  603 , producing a signal voltage  605 , is connected through a resistance  611  to one terminal of the primary winding of an augmentation transformer  614 , the opposite terminal of which is connected to the common point, such as ground. The secondary winding of input transformer  603  has a centre-tap connected to the common point or ground to provide a reference for the dual input signals. 
     A tap of the primary winding of augmentation transformer  609  is connected to the emitter of a transistor  608 , producing an emitter voltage  607 , and a similar tap of the primary winding of augmentation transformer  614  is connected to the emitter of a transistor  613 , producing an emitter voltage  612 , which is similar to the emitter voltage  607  of transistor  608 . The base of transistor  608  is connected, in reverse phase relative to the primary, through an N turn secondary winding of augmentation transformer  609  to the common point or ground, which produces a base voltage  610 , and the base of transistor  613  is connected, in reverse phase relative to the primary, through a secondary winding of augmentation transformer  614  to the common point or ground, which produces a base voltage  615 , similar to the base voltage  610  of transistor  608 . The collector of transistor  608  is connected to one terminal of the primary winding of an output transformer  616 , and the collector of transistor  613  is connected to the opposite terminal of the primary winding of output transformer  616 . The primary winding of output transformer  616  has a centre-tap connected to the common point or ground to provide a reference for the dual output signals. One terminal of a secondary winding of output transformer  616  is connected through a load resistance  618  (illustrated as a fixed resistance R L  for convenience) to the common point, such as ground, producing an output voltage  617 . The opposite terminal of the secondary winding of output transformer  616  is connected to the common or ground. 
     For applications that require a higher degree of linearity, a pair of single-ended augmented common-base transistor amplifiers, one using an NPN amplifying transistor and the other using a PNP amplifying transistor, may be arranged as a complementary, or symmetrical, amplifier. Referring specifically to FIG. 7, another embodiment of an augmented common-base transistor amplifier in accordance with the present invention, designated  700 , is illustrated. Circuit  700  includes an input signal voltage source  701 , supplying an input signal voltage  702 , which is coupled through a resistance  703  to one terminal of the primary winding of an augmentation transformer  706 , the opposite terminal of which is connected to a common point such as ground. The input signal voltage  702  is also coupled through a resistance  708  to an one terminal of the primary winding of an augmentation transformer  711 , the opposite terminal of which is connected to the common point or ground. 
     A tap of the primary winding of augmentation transformer  706  is connected to the emitter of a transistor  705  of the positive, or PNP polarity, producing an emitter voltage  704 , and a similar tap of the primary winding of augmentation transformer  711  is connected to the emitter of a transistor  710  of the negative, or NPN polarity, producing an emitter voltage  709 , which is similar to the emitter voltage  704  of transistor  705 . The base of transistor  705  is connected, in reverse phase relative to the primary, through a secondary winding of transformer  706  to the common or ground, producing a base voltage  707 . The base of transistor  710  is connected, in reverse phase relative to the primary, through a secondary winding of transformer  711  to the common point or ground, producing a base voltage  712 , which is similar to the base voltage  707  of transistor  705 . The collectors of transistors  705  and  710  are together connected through a load resistance  714  (illustrated as a fixed resistance R L  for convenience) to the common point or ground, producing an output voltage  713 . 
     Those familiar with the art will recognize that the two applications illustrated in FIGS. 6 and 7 may employ any of the augmented common-base transistor amplifiers described earlier. 
     Although detailed embodiments of the invention have been described, it should be appreciated that numerous modifications, variations, and adaptations may be made without departing from the scope of the invention as described in the claims. For example, those familiar with the art will recognize that the bipolar transistors shown in the embodiments may be alternatively replaced with field effect transistors. Further, while the terminals of the bipolar transistors described in the various embodiments are referred to as the emitter, base, and collector, it will be understood that these terminals will be the source, gate, and drain, respectively, when the transistors utilized are field effect transistors or other similar types and may be referred to as input, control, and output terminals, respectively, however the titles of the various components and terminals are only intended to enhance the understanding of the disclosure and are not intended to in any way limit the type of component utilized. In addition, it should be understood that the term “common-base transistor amplifier” used throughout this disclosure refers to a general type of amplifier and should not be limited in any way to prior concepts of common-base amplifiers. Also, the tapped two-winding transformers shown in the FIG. 5, FIG. 6, and FIG. 7 may be alternatively replaced with three-winding transformers.