Patent Publication Number: US-6703892-B1

Title: Mechanism for coupling a wideband current signal between two different potentials

Description:
FIELD OF THE INVENTION 
     The present invention relates to a coupling mechanism for coupling a signal current including a high-frequency signal component and a low-frequency signal component between two different potentials. 
     BACKGROUND OF THE INVENTION 
     A consequence of the progression to shorter channel lengths and thinner gate oxides in MOS technology is that supply voltages become progressively lower. This is due primarily to lower breakdown voltage of thinner gate oxides. In analog signal processing circuits, lower supply voltages present a number of serious difficulties. 
     One such difficulty, sometimes referred to as the “stacking problem”, can be understood by considering the example of a Gilbert-Cell multiplier  10 , shown in FIG.  1 . In the Gilbert-Cell multiplier  10 , a differential input voltage V in  is converted to a differential current i a  using a transconductance stage comprising transistors  11  and  12 , resistors  14  and  16 , and current source  18 . The relationship between i a  and V in  is approximately 
     
       
         
           i 
           a 
           =g 
           m  
           V 
           in 
         
       
     
     The current i a  is then steered using transistors  20 ,  22 ,  24  and  26  in such a way that a fraction k of the current goes through the left path and a fraction l-k goes through the right path. This produces an output current i o  which is proportional to the product of V in  and k, 
     
       
         
           i 
           o 
           =g 
           m  
           k V 
           in 
         
       
     
     thus performing a multiplication operation. 
     The problem is that because the transistors are “stacked” in such a way that the same current signal flows through both a bottom transistor ( 11 - 12 ) and a top transistor ( 20 ,  22 ,  24  and  26 ), the supply voltage must be large enough to accommodate the collector-emitter drops for both transistors. The problem can be solved in some cases by “unstacking” the Gilbert-Cell into two separate stages and capacitively coupling the two stages together, as shown in FIG.  2 . Capacitors  50  and  52  provide DC isolation, allowing the common mode voltage to be different in the two stages, while also providing AC connectivity, passing the signal current between the stages. Current sources  46  and  48  provide a DC path for the current generated in the transconductance stage. 
     The primary problem with the capacitive coupling method of “unstacking” is that the coupling capacitor limits the band of frequencies the circuit can accommodate. The circuit no longer passes DC signals at all, and low frequency signals are highly attenuated. For many applications, such as radio, this is not a significant problem, because the signals do not extend all the way down to DC. In the case where the signal represents baseband data, for example in optical networking, the inability to pass DC and low frequency signals is a very significant problem, however. 
     In some cases, the current sources  48  and  46  can be implemented with resistors. This is because the input to the second stage has a low impedance, so that the voltage swing at the output of the first stage is small. 
     In some cases, the capacitive coupling method requires a common-mode control circuit (not shown in FIG. 2) to set the DC common mode voltage level of the first stage. 
     Thus, there exists a need in analog signal processing for a coupling mechanism between two different potentials that transmits DC signals as well as AC signals between the two different potentials even in a situation where the supply voltage is low. 
     SUMMARY OF THE INVENTION 
     In one embodiment, the invention is an apparatus for coupling a wideband current signal between two different potentials. The apparatus incorporates a capacitor for providing a signal path for a high frequency signal from a first potential to a second potential. The apparatus further incorporates a current mirror for providing a signal path to a low frequency signal from the first potential to the second potential. 
     In an alternate embodiment, the invention is a method for coupling a signal current between a first potential and a second potential. The method includes receiving a high frequency signal from the first potential. The method also includes receiving a low frequency signal from the first potential. The method further includes transmitting the high frequency signal via a high frequency signal path to the second potential. Moreover, the method includes transmitting the low frequency signal via a low frequency signal path to the second potential. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention is illustrated by way of example, and not limitation, in the figures of the accompanying drawings in which like reference numerals indicate similar elements and in which: 
     FIG. 1 is a block diagram illustrating a Gilbert cell multiplier. 
     FIG. 2 is a block diagram illustrating an unstacked Gilbert cell multiplier. 
     FIG. 3 is a block diagram illustrating one embodiment of a coupling apparatus  58  in accordance with the present invention. 
     FIG. 4 is a block diagram illustrating an alternative embodiment of the coupling apparatus in accordance with the present invention. 
     FIG. 5 is a block diagram illustrating an embodiment of the coupling apparatus configured in accordance with the present invention and incorporated into a Gilbert-Cell multiplier. 
     FIG. 6 is a block diagram a conventional current mirror. 
     FIG. 7 is a block diagram of an improved current mirror circuit utilized by the present invention. 
     FIG. 8 is a block diagram illustrating the application of the invention in a 3.3-volt supply 10 GBit/s variable gain amplifier (VGA) stage for optical networking applications. 
    
    
     DETAILED DESCRIPTION 
     Described herein is a technique for coupling a wideband current signal between two different potentials. Specifically, according to the methods and apparatus of the present invention, a high-frequency path, implemented by a capacitor, and a low frequency path, implemented by a current mirror are provided for delivering both the AC and the DC signals. 
     Although the present invention will be discussed below with reference to presently preferred embodiments, it should be remembered that this discussion is not meant to limit the scope of the present invention to those specific embodiments. Rather, the discussion of these specific embodiments is being provided in order to describe, in a fashion intended for those of ordinary skill in the art, the presently known best mode for practicing the invention. Others having had the benefit of this disclosure may realize additional embodiments that represent insubstantial difference from those disclosed herein. Such embodiments are intended to fall within the ambit of the present invention. Therefore, when measuring the true scope of the present invention, readers should refer to the claims that follow this detailed description, and use this description only to enhance their understanding of those claims. 
     The present invention provides a mechanism for coupling a signal current between two different potentials. The mechanism includes the transmission of both AC and DC. 
     FIG. 3 is a block diagram illustrating one embodiment of a coupling apparatus  58  in accordance with the present invention. In this example, a differential current signal Iin is received from a first stage (not shown) and a differential current signal Iout is transmitted to a second stage (not shown). Two signal paths are provided to represent respectively the positive and negative polarity of the differential signal. Within each of these two signal paths, two subpaths are provided: a high-frequency subpath, implemented by a capacitor, and a low frequency subpath, implemented by a current mirror. The high frequency part of the input current is coupled directly to the output with the capacitor. The gain in this path is one (0 dB) by virtue of Kirchoff&#39;s current law. The low frequency path is therefore also designed to have a gain of one. This results in a transfer function for the coupling stage that maintains an essentially flat unity current gain from DC all the way out to tens of GHz. 
     FIG. 4 is a block diagram illustrating an alternative embodiment of the coupling apparatus  62  in accordance with the present invention. The low frequency path is designed to have a gain of −1, but the outputs of the current mirrors are “cross-coupled” between the differential paths, resulting in an effective gain of one in the low frequency path. The embodiment of FIG. 4 is preferred because it is more straightforward to design a current mirror with a gain of −1 than with a gain of +1, for implementation reasons that are well known in the art. 
     FIG. 5 is a block diagram illustrating an embodiment of the coupling apparatus configured in accordance with the present invention and incorporated into a Gilbert-Cell multiplier. High frequency coupling capacitors  88  and  90  pass high frequency components of the signal from stage  100  to stage  102 . Current mirrors  96  and  98  pass low frequency components of the signal from stage  100  to stage  102 . 
     The advantages of the circuit of FIG. 5 over that of FIG. 1 are: (1) the circuit can operate at a lower supply voltage since the transistors are “unstacked”; (2) different common-mode currents can be used in different stages to reduce power consumption. 
     The advantages of the circuit of FIG. 5 over that of FIG. 2 are: (1) the input signal bandwidth extends all the way down to DC; (2) a smaller coupling capacitor can be used, thereby saving area and reducing parasitics; (3) a common mode control circuit is not required because the current mirror stage has a low-impedance input that sets the bias point of the first stage. 
     FIG. 6 is a block diagram of a conventional current mirror  104 . In implementing the current mirror  104  for the low frequency path, an important consideration is that the current mirror  104  does not load down the high frequency path with parasitic capacitances. This requires careful attention to the input and output nodes of the mirror circuit. In particular, for very high bandwidth BiCMOS applications, it is necessary to use bipolar transistors at the input and output of the current mirror  104 . Internal to the current mirror  104  it is possible to use MOS transistors. One circuit that satisfies this design criterion is the current mirror  104  illustrated in FIG.  6 . The operation of this circuit is described in detail in P. R. Gray et al.,  Analysis and Design of Analog Integrated Circuits , Wiley, 1984, p. 237, and can be summarized as follows. By virtue of the fact that nominally identical transistors  108  and  110  have the same base-emitter voltage, they will generate approximately the same collector current. This results in a current gain of −1. Transistors  106  and  108  comprise a feedback loop that creates a low source impedance at the input of the current mirror  104 . The DC bias voltage at the input of the current mirror  104  is maintained at the ground potential plus twice the base-emitter voltage (2*Vbe). 
     The current mirror  104  of FIG. 6 suffers from some important disadvantages that have motivated the inventors to develop an improved current mirror  114  (see FIG.  7 ). Chief among these disadvantages are: (1) thermal noise from the base resistance of transistors  106 - 110  has a very high gain to the output, resulting in poor noise performance; (2) offset between transistors  108  and  110  results in a large error in the DC current; (3) the bias voltage of the input is set by the ground potential and the base-emitter voltage, whereas it is often desirable to have a bias voltage that depends in a different way on process and supply. 
     FIG. 7 is a block diagram of an improved current mirror  114  circuit utilized by the present invention. The improved current mirror circuit  114  in FIG. 7 addresses these three problems. Transistors  116  and  128 , used as emitter followers, in conjunction with operational amplifier  132 , amplify the difference between the input voltage and a reference voltage VREF. The amplified difference is fed to the bases of transistors  118  and  120 , which then generate a current proportional to the difference. Since degeneration resistors  124  and  126  are nominally identical and transistors  118  and  120  are nominally identical, the currents in the input and output path are nominally equal, resulting in a gain of −1. The input impedance is given approximately by R in =R/A v , where A v , is the voltage gain of the op amp. Because the gain A v  is large, large degeneration resistors  124  and  126  can be used to reduce the noise gain and the effect of transistor offsets without significantly increasing the input impedance. 
     FIG. 8 is a block diagram illustrating the application of the invention in a 3.3-volt supply 10 GBit/s variable gain amplifier (VGA) stage for optical networking applications. The ability to use such a low supply voltage results from the separation of the VGA stage into three substages. The substages are coupled together using the invention. The first substage  810  is a transconductance stage that converts the input voltage into a current. The second substage  820  is a variable attenuator stage that multiplies the signal current generated by substage  810  by a fraction k determined by the control voltage VX. The third substage is a transimpedance stage that converts the current back into a voltage.