Abstract:
A broadband amplifier in which an amplifier output stage is part of a stepped attenuator where the amplifier output stage can be selectively replaced by, or bypassed by, an attenuator block to produce one step of the stepped attenuator.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims benefit of provisional application Ser. No. 61/895,868, filed Oct. 25, 2013, which application is incorporated herein by reference. 
    
    
     BACKGROUND 
     Programmable broadband gain amplifiers (PGA) are commonly implemented as indicated in  FIG. 1  with an input amplifier gain stage (IS)  10  that is followed by a programmable stepped attenuator (ATT)  20  that is followed by an output amplifier gain stage (OS)  80 . 
     Such a topology is quite useful. At low attenuation levels, this topology allows separate blocks to independently control the most important specifications of the system. If the system is properly designed, at low attenuation levels, the noise figure is set by the input stage IS; and linearity and output swing are set by the output stage OS. The gain is controlled by the attenuator ATT. 
     Problems arise at lower gain settings when the programmed attenuation level of the attenuator ATT is significantly higher than the gain of the input stage IS. 
     The first problem is related to the noise floor specification that many systems require to be maintained at a certain level at high attenuation. Assuming the attenuator ATT is implemented using a π, T or bridged T based resistive attenuator, then at high attenuation levels, the integrated noise power at the output of attenuator ATT is given by equation (1) and the integrated noise power at the output of the system is given by equation (2).
 
NP ATT =10*log( kTB )  (1)
 
NP OUT =10*log( kTB )+10*log(GAIN OS )+NF OS   (2)
 
where GAIN OS  is the gain of the output stage OS and NF OS  is a noise figure of the output stage OS. Since kTB is constant, the output noise power becomes mainly a function of the output stage OS parameters. This fact constrains output stage OS design flexibility.
 
     The second problem arises from the fact that broadband gain amplifiers are commonly used to transmit time division multiple access (TDMA) signals. To conserve power, the broadband gain amplifier is required to be able to turn the amplifiers off in the “Transmit Disable” mode and turn them back on in the “Transmit Enable” mode. Turning amplifiers off and on causes transient disturbances at the output of the amplifier. The specification on Transmit Enable/Disable transient spurious emissions is especially stringent at high attenuation levels. To achieve linearity typically requires the output stage OS is to be biased with a high current with the result that this is a major contributor to output spurious emission. 
     SUMMARY 
     This invention minimizes the above described problems by replacing the output stage OS or by turning the output stage off and bypassing it at high attenuation levels. This is achieved by use of an amplifier output stage OS that can be selectively replaced by, or bypassed by, an attenuator block having an attenuation selected to produce one of the attenuation steps. In addition, this offers the additional benefit of a significant reduction in power consumption at high attenuation levels. 
     Numerous variations may be made in the practice of the invention. 
    
    
     
       BRIEF DESCRIPTION OF DRAWING 
       These and other objects, features and advantages of the invention will be more readily apparent from the following Detailed Description in which: 
         FIG. 1  is a schematic diagram of a broadband gain amplifier; 
         FIG. 2  is a schematic diagram of a binary coded attenuator; 
         FIG. 3  is a schematic diagram of a thermometer coded attenuator; 
         FIG. 4  is a schematic diagram of a thermometer coded and binary coded attenuator; 
         FIG. 5  is a schematic diagram of an illustrative embodiment of a binary coded attenuator of the invention; 
         FIG. 6  is a schematic diagram of an illustrative embodiment of a thermometer coded attenuator of the invention; 
         FIG. 7  is a schematic diagram of an illustrative embodiment of a thermometer coded/binary coded attenuator of the invention; 
         FIG. 8  is a schematic diagram of another illustrative embodiment of a binary coded attenuator of the invention; 
         FIG. 9  is a schematic diagram of another illustrative embodiment of a thermometer coded attenuator of the invention; and 
         FIG. 10  is a schematic diagram of another illustrative embodiment of a thermometer coded/binary coded attenuator of the invention; 
     
    
    
     DETAILED DESCRIPTION 
     Broad band stepped attenuators are typically implemented as either binary coded attenuators, thermometer coded attenuators or a combination of both.  FIG. 2  depicts a binary coded attenuator comprising a plurality of attenuator stages (or steps)  230 .  231 , . . . ,  23 N that may be selectively connected in series by appropriate pairs of switches  260 . The attenuator stages have attenuation levels that are binary multiples of a minimum attenuation step of AS. 
     For the binary coded attenuator of  FIG. 2 , the maximum attenuation level is computed using equation (3):
 
MAX(ATT BIN )=Σ i=0   n AS*2 i   (3)
 
where AS is the minimum attenuation step of the attenuator.
 
       FIG. 3  depicts a thermometer coded attenuator comprising a plurality of attenuator stages (or steps)  351 ,  352 , . . . ,  35 N, any one of which may be selectively connected in the circuit by switch pair  370 . The attenuation stages have attenuation levels that increase linearly in steps where each step is AS. 
     For the thermometer coded attenuator of  FIG. 3 , the maximum attenuation level is computed using equation (4):
 
MAX(ATT THERM )=AS* n   (4)
 
       FIG. 4  depicts a thermometer coded and binary coded attenuator comprising a plurality of thermometer coded attenuator stages (or steps)  451 ,  452 , . . . ,  45 M and a plurality of binary coded stages (or steps)  431 ,  432 , . . . ,  43 N where the binary coded stages are connected in series with the thermometer coded stages. Any one of the stages of the thermometer coded attenuator may be selectively connected in the circuit by switch pair  470 ; and any one of the plurality of binary coded attenuator stages (or steps) may also be selectively connected in series by an appropriate pair of switches  460 . 
     For the attenuator of  FIG. 4 , maximum attenuation is found using equation (5):
 
MAX(ATT MIX )=AS* m+Σ   i=1   n BS*2 i   (5)
 
where the minimum attenuator step is equal to AS and the binary step is given by
 
BS=AS* m +1  (6)
 
     The main contributor to integrated noise power and transmit enable/disable spurious emissions on the output at low gain settings is the output gain stage. Bypassing the output gain stage when high gain is not needed avoids both issues. In accordance with the invention, this is done with each of the attenuator topologies depicted in  FIGS. 2, 3 , and  4 . 
       FIG. 5  depicts an illustrative implementation of the invention using a binary coded attenuator. Amplifier  500  comprises an input amplifier gain stage (IS)  510 , a plurality of binary coded attenuators  530 ,  531 , . . . ,  53 (N−1), selectively connected by appropriate pairs of switches  560 , an attenuator block ATT OS    540 , and an output amplifier stage (OS)  580 . Here, however, the output amplifier stage is a part of the binary weighted attenuator and together with attenuator block ATT OS    540  acts as one of the attenuation steps. When high gain is not needed and the attenuation level required is greater than AS*2 n , the output stage (OS)  580  is turned off and switched out while at the same time the attenuator block ATT OS    540  is switched in by switch pair  590 . The ATT OS  and Gain OS  are chosen such that they satisfy equation (7)
 
−dB(Gain)−dB(ATT OS )=AS*2 n  (dB)  (7).
 
     The maximum attenuation level is still given by equation (3) and is equal to Σ i=0   n AS*2 i . 
       FIG. 6  depicts an illustrative implementation of the invention using a thermometer coded attenuator. Amplifier  600  comprises an input amplifier gain stage (IS)  610 , a plurality of thermometer coded attenuators  651 ,  652 , . . . ,  65 M, selectively connected by switch pair  670 , an attenuator block ATT OS    640 , and an output amplifier stage (OS)  680 . Once again, when high gain is not needed and the attenuation level required is greater than AS*m, the output stage OS is turned off and switched out while at the same time the attenuator block ATT OS    640  is switched in by switch pair  690 . ATT OS  and Gain OS  are chosen such that they satisfy equation (8)
 
−dB(Gain OS )−dB(ATT OS )=AS*( m +1) (dB)  (8)
 
     The maximum attenuation level of the circuit of  FIG. 6  is given by:
 
MAX(ATT therm )=AS* m +AS( m +1)=AS*(2 *m +1)  (9)
 
     If we set 2*m+1=n then equation (8) becomes equal to equation (4). 
       FIG. 7  depicts an illustrative implementation of the invention using a combination of a thermometer coded attenuator and a binary coded attenuator shown in  FIG. 7 . Amplifier  700  comprises an input gain stage (IS)  710 , a first plurality of binary coded attenuators  731 , . . . ,  73 (N−1), selectively connected by appropriate pairs of switches  760 , a second plurality of thermometer coded switches  751 ,  752 ,  75 M, selectively connected by switch pair  770 , an attenuator block ATT OS    740 , and an output amplifier stage (OS)  780 . 
     Once again, when high gain is not needed and the required attenuation level is greater than BS*2 n , the output stage OS is turned off and switched out while at the same time the attenuator block ATT OS    740  is switched in by switch pair  790 . ATT OS  and Gain OS  are chosen such that they satisfy equation (10)
 
−dB(Gain OS )−dB(ATT OS )=BS*2 n  (dB)  (10)
 
     The maximum attenuation level is still given by equation (5) which is repeated here for convenience. 
     
       
         
           
             
               MAX 
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                 ( 
                 
                   ATT 
                   MIX 
                 
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             = 
             
               
                 AS 
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                   ∑ 
                   
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                     1 
                   
                   n 
                 
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                   BS 
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     The integrated output noise power for all three implementations (see  FIGS. 5, 6 , and  7 ) is given by equation (10).
 
NP OUT =10*log( kTB )  (11)
 
     As will be apparent, this is a significant reduction compared to the integrated noise power given in equation (2). Also, since output stage OS is turned off, no spurious emissions related to OS are present at the output of the PGA during transmit enable/disable transition. 
     Another big advantage of this implementation is significantly reduced DC power consumption since most of the PGA power dissipation is related to OS biasing. 
     Alternatively, one may choose not to switch out the output stage but rather just turn it off and bypass it.  FIGS. 8, 9 and 10  depict binary coded, thermometer coded and thermometer coded/binary coded circuits  800 ,  900 ,  1000  of this type. Except for switch pairs  895 ,  995 , and  1095 , the elements of  FIGS. 8, 9 and 10  are the same as those of  FIGS. 5, 6, and 7  and bear the same numbers increased by 300. Switch pairs  895 ,  995 , and  1095  enable the final attenuator block  840 ,  940 ,  1040  to be selectively switched into the amplifier circuit, thereby bypassing the output stage which is turned off. 
     As will be apparent to those skilled in the art, numerous variations may be practiced within the spirit and scope of the present invention.