Patent Publication Number: US-10778148-B1

Title: Gain and sensitivity in a Gilbert switch stage

Description:
GOVERNMENT RIGHTS 
     N/A 
     BACKGROUND 
     As is well known, a Gilbert switch pair produces a differential output signal that is proportional to two input signals. These switches are often used for frequency conversion in, for example, a radio system, as they provide an output current that is an accurate multiplication of the differential base currents of the two input signals. Further, as used in a mixer, this balanced operation cancels out many unwanted mixing products and provides a more accurate output. 
     In many applications, it is desirable to increase the conversion gain of the active mixer. In one known approach, the overall current of the mixer and the LO (Local Oscillator) drive are increased. This approach, however, can make it more difficult for the switches to switch and may have to increase the current quite a bit. An increase in the LO swing, which can switch the Gilbert switch harder, increases the gain but at the cost of increased DC power consumption which is an undesirable side effect. 
     What is needed, however, is a better approach to improving the gain and, therefore, the sensitivity of a Gilbert switch. 
     SUMMARY 
     In one aspect of the present disclosure, there is an apparatus, comprising: a main transconductance (MT) stage having an MT differential input port and first and second MT output paths; a Gilbert switch (GS) stage, coupled to the first and second MT output paths, comprising a GS differential input port and a GS differential output port; a first current source having an output coupled to the first MT output path; and a second current source having an output coupled to the second MT output path. At least one of the first and second current sources is configured to: generate a cascoded trickle current into the respective first or second output path as a function of an output of a DAC. 
     Another aspect of the present disclosure is directed to an apparatus, comprising: a main transconductance (MT) stage having an MT differential input port and first and second MT output paths; a Gilbert switch (GS) stage, coupled to the first and second MT output paths, comprising a GS differential input port and a GS differential output port; a first current source having an output coupled to the first MT output path; and a second current source having an output coupled to the second MT output path. Each of the first and second current sources is configured to generate a respective first and second trickle current at the respective current source output and introduce the generated first and second trickle current into the respective first and second MT output path. Further, each of the first and second current sources comprises: first and second PMOS transistors coupled in series between a supply rail and the respective output; and a DAC having an output coupled to a node between the first and second transistors. 
     In yet another aspect of the present disclosure, a power detector comprises: an input differential amplifier having a first differential input port and first and second output paths; an output differential amplifier having first and second differential pairs of transistors wherein common sources of the differential pairs of transistors are coupled to the first and second output paths, inputs of the differential pairs of transistors are cross-coupled to form a second differential input port and outputs of the differential pairs of transistors are cross-coupled to form a first differential output port; a load coupled to the first differential output port; a first current source having an output coupled to the first output path; a second current source having an output coupled to the second output path. Each of the first and second current sources is configured to generate a respective first and second trickle current at the respective current source output and introduce the generated first and second trickle current into the respective first and second output path. Further, each of the first and second current sources comprises: first and second transistors coupled in series between a supply rail and the respective current source output; and a DAC having an output coupled to a node between the first and second transistors. 
     In the foregoing aspects, an output of the DAC can source or sink current. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Various aspects of the present disclosure are discussed below with reference to the accompanying figures. It will be appreciated that, for reasons of simplicity and clarity of illustration, elements shown in the drawings have not necessarily been drawn accurately or to scale. For example, the dimensions of some of the elements may be exaggerated relative to other elements for clarity or several physical components may be included in one functional block or element. Further, where considered appropriate, reference numerals may be repeated among the drawings to indicate corresponding or analogous elements. For purposes of clarity, not every component may be labeled in every drawing. The Figures are provided for the purposes of illustration and explanation and are not intended as a definition of the limits of the disclosure. In the Figures: 
         FIG. 1  is functional block diagram of a system in accordance with an aspect of the present disclosure; 
         FIG. 2  is a schematic diagram of the system of  FIG. 1 ; 
         FIG. 3  is a schematic diagram of a portion of the system of  FIG. 1 ; 
         FIG. 4  is another schematic diagram of the system of  FIG. 1 ; and 
         FIG. 5  is an alternate embodiment of a system in accordance with an aspect of the present disclosure. 
     
    
    
     DETAILED DESCRIPTION 
     Details are set forth in order to provide a thorough understanding of the aspects of the disclosure. It will be understood by those of ordinary skill in the art that these may be practiced without some of these specific details. In other instances, well-known methods, procedures, components and structures may not have been described in detail so as not to obscure the aspects of the disclosure. 
     Advantageously, aspects of the present disclosure provide an increase in the conversion gain of a Gilbert active mixer for a given LO swing for a very small increase in DC power. In addition to increasing the conversion gain, aspects of the present disclosure operate to mitigate the DC offsets that may be present in a main transconductance stage and/or a Gilbert switch stage of the Gilbert mixer resulting in enhancing the sensitivity of the active mixer. 
     The following description includes reference to mixer structures and Gilbert structures which are well understood by those of ordinary skill in the art. Accordingly, these structures will not be discussed in great detail. Reference is made to U.S. Pat. No. 5,859,559, which is incorporated by reference herein for all purposes, for background information. 
     Generally, and as will be described in more detail below, one aspect of the present disclosure is directed to, as a non-limiting example, an active Gilbert power detector system  100  as shown in  FIG. 1 . The system  100  includes an input differential amplifier, sometimes referred to as a main transconductance stage,  105  having an MT differential input port  110  and first and second output paths  112 ,  114 . A current source  113  is coupled to the main transconductance stage  105 . An output differential amplifier, i.e., a Gilbert switch stage,  115  is coupled to the main transconductance stage  105  via the first and second output paths  112 ,  114 . The Gilbert switch stage  115  includes a switch stage differential input port  120  and a switch stage differential output port  125 . A load  130  is coupled between the switch stage differential output port  125  and to a power supply VDD. 
     A first DAC-controlled (DACC) trickle current stage  135 - 1  has an output  136 - 1  coupled to the first output path  112  at a first node P and a second DACC trickle current stage  135 - 2  has an output  136 - 2  coupled to the second output path  114  at a second node S. As will be discussed in more detail below, each of the first and second DACC trickle current stages  135 - 1 ,  135 - 2  is configured to generate and introduce a respective first and second trickle current into the respective first and second output paths  112 ,  114 . 
     Referring to  FIG. 2 , the main transconductance stage  105  includes two FETs F 1 , F 2  with their sources coupled to the current source  113 . The gates of the FETs F 1 , F 2  comprise the MT differential input port  110 . The Gilbert switch stage  115  includes first and second differential pairs of transistors P 1 , P 2 . As is shown, the common sources of the first differential pair P 1  are coupled to the first output path  112  and the common sources of the second differential pair P 2  are coupled to the second output path  114 . The inputs of the differential pairs of transistors P 1 , P 2  are cross-coupled to form the switch stage differential input port  120  and the outputs, i.e., the drains, of the differential pairs of transistors P 1 , P 2  are cross-coupled to form the switch stage differential output port  125 . 
     Each DACC trickle current stage  135 - 1 ,  135 - 2  includes first and second PMOS FETs  305 - 1 ,  305 - 2  coupled in series, i.e., cascoded, with one another between the power supply VDD and the respective first and second output paths  112 ,  114 , as shown in  FIG. 3 . An N-bit DAC  310  has its output coupled to a node A between the two FETs  305 - 1 ,  305 - 2 . The DAC  310  is a two-way DAC in that it can both source and sink current at its output. Advantageously, as the DAC  310  is a single-line output device, it does not load down node A. In one non-limiting implementation, N=4, however, one of ordinary skill in the art will understand from this disclosure that the number of bits N can vary depending on the desired functionality. 
     All of the components of the system  100  are shown in  FIG. 4 . 
     Operation of the DACC stages  135 - 1 ,  135 - 2  will now be explained. Assuming, for explanatory purposes, that the DAC  310  is not present, the two FETs  305 - 1 ,  305 - 2  operate as a current mirror from a bandgap in another portion of the circuit (not shown and not germane to the present disclosure) to set a predetermined, i.e., a nominal amount of standing current. The DAC  310  is then operable to skew the current to be larger or smaller, i.e., each DAC  310  operates as an adjustment of the amount of current being injected or withdrawn as a function of its respective digital input. 
     Referring now to  FIG. 5 , in another embodiment of the present disclosure, a system can include a main transconductance stage  505  and a Gilbert switch stage  515  comprising NPN transistors in place of the transconductance stage  105  and the Gilbert stage  115 , respectively. Accordingly, the transconductance stage  505  comprises NPN transistors T 1 , T 2  and the Gilbert switch stage  515  is implemented with pairs of NPN transistors PN 1 , PN 2  similarly configured as described above with respect to the transconductance stage  105  and the Gilbert stage  115 . In addition, a node  520  is coupled to the coupled bases, as shown, to provide a bias voltage as is understood by one of ordinary skill in the art. 
     In operation, per an aspect of the present disclosure, by inserting a cascoded PMOS trickle current between the main transconductance stage  105 ,  505 , and the Gilbert switch  115 ,  515 , the effective voltage, Vgs−Vt of the Gilbert switch  115  or the ΔVbe of the Gilbert switch stage  515 , can be controlled. Accordingly, the sensitivity of the Gilbert switch stage  115 ,  515  can be increased such that it will fully switch in response to a much lower LO drive. 
     It is known, to those of ordinary skill in the art, that the gain Gm of the system  100  is due to the contribution of both the main transconductance stage  105 ,  505  and the Gilbert switch stage  115 ,  515  where, for example, Gm is given by 2I/(Vgs−Vt), for the Gilbert switch stage  515 . By adding a PMOS trickle current from the DACC stages  135 - 1 ,  135 - 2 , the Gm of the main transconductance stage  105 ,  505  remains constant, but since the current into the Gilbert switch stages  115 ,  515  decreases, the LO drive (Vgs−Vt) of the Gilbert switch stage  115 ,  515 , respectively, decreases. This decrease in the Vgs−Vt in the Gilbert switch stage  115 , for example, allows the given LO to fully switch all the current in the Gilbert switch stage  115  which will result in an increase in the overall gain and sensitivity of the system. 
     It has been observed that if current is not trickled away, the higher current (I) injected into the Gilbert switch stage  115  will increase the Vgs−Vt in the Gilbert switch stage  115 , resulting in a situation where a given LO swing might not fully switch all the current in the Gilbert switch stage  115 , resulting in a lower gain than when the trickle current is being applied. As is known, the Vbe or Vgs−Vt of the Gilbert switch stage  515 ,  115 , respectively, is integral in determining the gain and sensitivity of the Gilbert switch stage  515 ,  115 . In addition, optimum operation is achieved when the Gm of the main transconductance stage  105 ,  505  is kept constant. 
     The digitally programmable DACs  310  in each DACC stage  135 - x  can be independently operated to trim out any DC offsets in the main transconductance stage  105 ,  505 . Advantageously, by using the PMOS trickle current from the DACC stages  135 - 1 ,  135 - 2 , overall gain and sensitivity can be optimized as it is known that DC offsets can affect the sensitivity of the overall circuit. 
     Further, the setting of the DACs  310  can be chosen by a user to provide for increased sensitivity while sacrificing, to some extent, an operating range or providing an extended operating range at the expense of sensitivity. Advantageously, the DACC stages  135 - 1 ,  135 - 2  provide this design flexibility. Still further, in one aspect of the present disclosure, the DAC  310  in the first DACC stage  135 - 1  need not have the same input as the DAC  310  in the second DACC stage  135 - 2 . The ability to separately set the DACs  310  and, therefore, the respective trickle current, provides additional capability to compensate for any offsets that might be present in the system  100 . 
     It is to be understood that the disclosure is not limited in its application to the details of construction and the arrangement of the components set forth herein or illustrated in the drawings as it is capable of implementations or of being practiced or carried out in various ways. Also, it is to be understood that the phraseology and terminology employed herein are for the purpose of description only and should not be regarded as limiting. 
     Certain features, which are, for clarity, described in the context of separate implementations, may also be provided in combination in a single implementation. Conversely, various features, which are, for brevity, described in the context of a single implementation, may also be provided separately or in any suitable sub-combination. 
     The present disclosure is illustratively described in reference to the disclosed implementations. Various modifications and changes may be made to the disclosed implementations by persons skilled in the art without departing from the scope of the present disclosure as defined in the appended claims.