Abstract:
A circuit for a variable gain amplifier is disclosed that uses two differential gain stages with independently adjustable bias currents. By changing the bias currents of the gain stages, the overall gain and phase of the amplifier can be adjusted over a wide range. Neither in-series nor in-parallel circuitry is required to implement or perform gain control. In addition to minimal part requirements for mechanization, the present invention features low power supply requirements while maintaining a wide operational bandwidth.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application claims priority from the following U.S. provisional Patent Application, the disclosure of which is incorporated by reference in its entirety for all purposes: U.S. Provisional Patent Application Ser. No. 60/290,463, Cheh-Ming Jeff Liu and Kai-Chun Chang entitled, “WIDEBAND VARIABLE GAIN AMPLIFIER WITH LOW POWER SUPPLY VOLTAGE,” filed May 11, 2001. 
    
    
     BACKGROUND—FIELD OF THE INVENTION 
     The present invention relates to transistorized variable gain amplifiers particularly suited for adjustable gain control and automatic gain control. 
     BACKGROUND—Description of Prior Art 
     Typically, amplifiers achieve variable gain control by coupling a pair of gain stages where their outputs are connected oppositely and both gain stages share the same bias current. The bias current of both gain stages are then differentially controlled by a pair of in-series transistors. This topology is termed a Gilbert cell. Amplifiers exploiting a Gilbert cell can provide wide variations in gain. However, because of in-series control circuitry, the minimal rail-to-rail voltage is higher for the typical variable gain amplifier when compared to a conventional fixed-gain amplifier. Variable gain amplifiers exhibiting this higher rail-to-rail voltage inherently consume more power than the similarly sized fixed-gain amplifiers. Not surprisingly, the power consumption produces a practical need for an extra or enhanced power supply beyond that nominally required for conventional fixed-gain amplifiers. 
     U.S. Pat. No. 5,418,494, issued May 23, 1995, to G. Betti, et al., and assigned to SGS-Thomson Microelectronics, S.r.l., discloses a variable gain amplifier with low power supply. FIG. 1 shows a variable gain amplifier with a fixed-gain amplifier  10 , a variable gain amplifier  20 , a gain control and stabilizing variable current generator  30 , and a current-to-voltage converter  40 . Due to the parallel configuration of fixed-gain amplifier  10  and variable-gain amplifier  20 , the amplifier can be operated with a low power supply. However, in order to compensate DC current variation, the amplifier employs an in-parallel gain control and stabilizing variable current generator  30 . This compensation circuitry can introduce additional parasitic effects that result in limiting the operational bandwidth. Moreover, due to the coupling of the three circuitries  10 ,  20  and  30 , the isolation between the voltage inputs V S  and control voltage V REF1  is degraded. This mediocre isolation can cause serious stability problems when the variable amplifier is used in an automatic-gain-control (AGC) loop. 
     SUMMARY OF THE INVENTION 
     The present invention is embodied as a circuit for a variable gain amplifier that uses two differential gain stages with independently adjustable bias currents. By changing the bias currents of the gain stages, the overall gain and phase of the amplifier can be adjusted over a wide range. Neither in-series nor in-parallel circuitry is required to implement or perform gain control and thus the present invention obviates the need for relatively high rail-to-rail voltage. In addition to minimal part requirements for mechanization, the present invention features low power supply requirements while maintaining a wide operational bandwidth. 
     Because gain control in the present invention is performed by directly adjusting the bias currents of the gain modules with a minimal number of components required, this economy also eliminates the introduction of parasitic components to the gain modules. By the economy of components, particularly parasitic components, the bandwidth of the overall amplifier is not adversely affected by the direct gain control of the present invention. Moreover, without any extra DC compensation circuitry, the variable-gain amplifier of the present invention provides excellent isolation between the RF signals and the DC control signal thus increasing the system stability as used in an automatic-gain-control (AGC) loop. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     For a more complete understanding of the present invention and for further features and advantages, reference is now made to the following description taken in conjunction with the accompanying drawings, in which: 
     FIG. 1 illustrates prior art of a variable-gain amplifier with low supply voltage. 
     FIG. 2 is a functional diagram of a variable-gain amplifier. 
     FIGS. 3A and 3B show normalized gain and phase characteristics respectively, each as functions of bias current difference. 
     FIG. 4 is a circuit drawing representing the preferred embodiment of the present variable gain amplifier invention. 
     FIG. 5 is a circuit drawing illustrating the variable gain amplifier of an alternative embodiment of the instant invention with common-emitter gain modules and a common-base combining module. 
     FIG. 6 shows a functional diagram of a variable gain amplifier of the instant invention with gain control logic. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Illustrated in FIG. 2 is a functional diagram of a variable gain amplifier  100  representing the preferred embodiment of the present invention. The amplifier  100  is composed of two differential gain modules  101  and  102 , in which the bias currents I B1    105  and I B2    106  are adjusted independently by control signals V c1    107  and V c2    108  respectively. The input signals V in +  109  and V in −  110  are coupled to the inputs of gain modules  101  and  102  through a distributing module  103 . The outputs of both gain modules  101  and  102  are combined by an optional combining module  104  and coupled to the output V o +  111  and V o −  112  in an opposite-phase configuration with respect to the configuration of the input coupling. 
     FIGS. 3A and 3B show normalized gain and phase characteristics versus differences in bias currents I B1    105  and I B2    106  of differential gain modules  101  and  102 . Referring to FIG. 2 briefly, the first gain path is defined as being from the input of the amplifier  100  through the first gain module  101  to the output of the amplifier  100 . Similarly, the second gain path is defined as being from the input of the amplifier  100 , through the second gain module  102 , and to the output of the amplifier  100 . The relative phase between the first and second gains paths is out-phased, i.e., 180 degrees out of phase from one another. As illustrated generally in FIG. 3A, if I B1    105  grows larger in magnitude than I B2    106 , the bias current difference (i.e., I B1 −I B2    201 ) increases positively and thus the overall gain  202  of the amplifier  100  increases monotonically. If I B1    105  grows smaller in magnitude than I B2    106 , then the magnitude of the bias current difference  201  increases causing the overall gain  202  of amplifier  100  to increase monotonically in the other direction. As illustrated in FIG. 3B, the phase  203  of the amplifier  100  changes by 180 degrees  204  as the bias current difference  201  changes sign. 
     FIG. 4 illustrates a circuit  300  representing the preferred embodiment of the present variable gain amplifier invention. The circuit  300  comprises of a distributing module  103 , a first differential gain module  101  and a second differential gain module  102 . As the preferred alternative to a combining module  104 , the outputs of the two gain modules  101  and  102  are wired directly together. The distributing module  103  consists of two emitter followers  305  as direct current (DC) level shifters with diodes  353  and  354  and respective current sources  355  and  356 . Transistors  351  and  352  of the emitter followers  305  increase the input impedances of the amplifier and provide current gains for the differential gain modules  101  and  102  that follow. The first differential gain module  101  is comprised of common-emitter transistors  311  and  312 , emitter degeneration resistors  313  and  314 , a current generator  315 , loads  316  and  317  and a pair of transistors  318  and  319  with a common-base configuration. The emitter degeneration resistors  313  and  314  increase the linearity and operational bandwidth of the gain stage  101 . Transistors  318  and  319  reduce the capacitance of transistors  311  and  312  that would otherwise be present due to the Miller effect and by working this reduction, transistors  318  and  319  thereby increase the operational bandwidth of the gain stage  101 . Similarly, the second gain module  102  consists of common-emitter transistors  331  and  332 , emitter degeneration resistors  333  and  334 , a current generator  335 , loads  336  and  337  and a pair of transistors  338  and  339  with a common-base configuration. The bias currents I B1    105  and I B2    106  of the gain modules  101  and  102  are provided by transistors  320  and  340  with emitter resistors  321  and  341 , respectively. The bias currents I B1    105  and I B2    106  are controlled by two independent control signals V c1    107  and V c2    108 . Gain variation is achieved by changing the magnitudes of the bias currents I B1    105  and I B2    106  via the independent control signals V c1    107  and V c2    108  respectively. 
     According to the present invention, the amplifier gain can be adjusted between zero and the gains of the gain modules  101  and  102 . Furthermore, the phase of the amplifier  300  can be modulated by the sign of the difference between two adjustable bias currents I B1    105  and I B2    106  of the gain modules  101  and  102 . In comparing the present invention with prior approaches, neither additional in-series, nor additional in-parallel, gain-control transistors are required. This feature eliminates the need for a higher rail-to-rail voltage supply when compared with the typical Gilbert cell implementation. Due to the absence of the need for additional DC compensation transistors (FIG. 1) the bandwidth of the variable-gain amplifier  300 , in accordance with the present invention, approaches that of the bandwidth of a fixed-gain amplifier. The present invention makes it practical to use the same power supply as a fixed-gain amplifier and thereby reduces the complexity and power consumption of an entire system availing itself of the herein described variable gain amplifier. 
     DETAILED DESCRIPTION OF AN ALTERNATIVE EMBODIMENT 
     FIG. 5 illustrates an alternative embodiment of the present invention. The first gain module  101  is comprised of common-emitter transistors  411  and  412 , a current generator  415  and a pair of emitter degeneration resistors  413  and  414 . Similarly, the second gain module  102  is comprised of common-emitter transistors  431  and  432 , a current generator  435 , and emitter degeneration resistors  433  and  434 . The bias currents I B1    105  and I B2    106  of the gain modules  101  and  102  are provided by transistors  420  and  440  with emitter resistors  421  and  441 , respectively. The combining module is comprised of a pair of common-base transistors  441  and  442  with resistive loads  443  and  444 . 
     Each bias current of the gain modules  101  and  102  can be adjusted independently according to the present variable gain amplifier invention. Since the overall gain of the invented variable gain amplifier is determined by the difference of controlled bias current I B1  and I B2 , it is possible to achieve gain variation while maintaining the sum of the I B1  and I B2  as a constant. FIG. 6 shows an extension of the present invention with a gain control logic  600  providing the gain variation and maintaining same DC level at the outputs. As opposed to the prior art, in the gain modules  102  and  103 , there is no extra control circuitry introduced to compensate the output DC level. Therefore, as compared to the prior art, the present invention can provide wider bandwidth and better isolation between the RF signals and the control signals, such as those used in an automatic-gain-control (AGC) loop. 
     Many alterations and modifications may be made by those having ordinary skill in the art without departing from the spirit and scope of the invention. Therefore, it must be understood that the illustrated embodiment has been set forth only for the purposes of example and that it should not be taken as limiting the invention as defined by the following claims. 
     The words used in this specification to describe the invention and its various embodiments are to be understood not only in the sense of their commonly defined meanings, but to include by special definition in this specification structure, material or acts beyond the scope of the commonly defined meanings. Thus if an element can be understood in the context of this specification as including more than one meaning, then its use in a claim must be understood as being generic to all possible meanings supported by the specification and by the word itself. 
     The definitions of the words or elements of the following claims are, therefore, defined in this specification to include not only the combination of elements which are literally set forth, but all equivalent structure, material or acts for performing substantially the same function in substantially the same way to obtain substantially the same result. 
     In addition to the equivalents of the claimed elements, obvious substitutions now or later known to one with ordinary skill in the art are defined to be within the scope of the defined elements. 
     The claims are thus to be understood to include what is specifically illustrated and described above, what is conceptually equivalent, what can be obviously substituted and also what essentially incorporates the essential idea of the invention.