Abstract:
Techniques for designing a high performance analog switch for use in electronic circuit applications. In one aspect, a variable bulk voltage generation module is provided to vary the bulk voltage of a transistor in the switch, such that the threshold voltage of the transistor is reduced during the on state. In another aspect, a pulling transistor is provided to pull a middle node of the switch to a DC voltage during the off state to further increase the isolation provided by the switch.

Description:
TECHNICAL FIELD 
     The disclosure relates to electronic circuits and, more particularly, to techniques for designing high-performance analog switches. 
     BACKGROUND 
     In the field of electronic circuit design, analog switches are used in a wide variety of applications, including, but not limited to, radio-frequency (RF) multiplexers, demultiplexers, passive mixers, switched-capacitor circuits, etc. Analog switches are designed to couple an analog signal at a first node to an analog signal at a second node. In an “on” state, the switch may directly couple an analog voltage or current at the first node to the second node, while in an “off” state, the switch may isolate the analog voltage or current at the first node from the second node. 
     In general, a high performance analog switch must meet several, possibly conflicting, design objectives. First, during the on state, the switch should provide low series impedance between the first and second nodes. Second, during the off state, the switch should provide a high series impedance between the first and second nodes. Furthermore, to minimize signal attenuation at high frequency, the switch should introduce minimal parasitic capacitance to the first and second nodes. 
     It would be desirable to provide novel and effective techniques to meet the objectives for designing a high performance analog switch. 
     SUMMARY 
     An aspect of the present disclosure provides an analog switch for coupling a first node to a second node during an on state, and for isolating the first node from the second node during an off state, the switch comprising first and second switch transistors, the first switch transistor coupling the first node to a middle node, the second switch transistor coupling the middle node to the second node, the switch further comprising: a bulk voltage generation module coupling the bulk node of at least one of the first and second switch transistors to a first bulk voltage during the on state, and to a second bulk voltage during the off state. 
     Another aspect of the present disclosure provides a method for coupling a first node to a second node during an on state, and for isolating the first node from the second node during an off state, the method comprising switching first and second switch transistors, the first switch transistor coupling the first node to a middle node, the second switch transistor coupling the middle node to the second node, the method further comprising: coupling the bulk node of at least one of the first and second switch transistors to a first bulk voltage during the on state; and coupling the bulk node of the at least one of the first and second switch transistors to a second bulk voltage during the off state. 
     Yet another aspect of the present disclosure provides an analog switch for coupling a first node to a second node during an on state, and for isolating the first node from the second node during an off state, the switch comprising first and second switch transistors, the first switch transistor coupling the first node to a middle node, the second switch transistor coupling the middle node to the second node, the switch further comprising: bulk voltage generation means for varying the threshold voltage of at least one of the first and second switch transistors to reduce the threshold voltage during the on state. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  depicts an implementation of a prior art switch  100 . 
         FIG. 2  depicts an embodiment of a switch  200  according to the present disclosure. 
         FIG. 3  depicts an exemplary method according to the present disclosure. 
         FIG. 4  depicts an alternative embodiment of a switch  200  according to the present disclosure. 
     
    
    
     DETAILED DESCRIPTION 
     The detailed description set forth below in connection with the appended drawings is intended as a description of exemplary embodiments of the present invention and is not intended to represent the only exemplary embodiments in which the present invention can be practiced. The term “exemplary” used throughout this description means “serving as an example, instance, or illustration,” and should not necessarily be construed as preferred or advantageous over other exemplary embodiments. The detailed description includes specific details for the purpose of providing a thorough understanding of the exemplary embodiments of the invention. It will be apparent to those skilled in the art that the exemplary embodiments of the invention may be practiced without these specific details. In some instances, well known structures and devices are shown in block diagram form in order to avoid obscuring the novelty of the exemplary embodiments presented herein. 
       FIG. 1  depicts an implementation of a prior art switch  100 . Note the prior art switch  100  in  FIG. 1  is shown for illustrative purposes only, and is not meant to restrict the scope of application of the techniques described in the present disclosure. 
     In  FIG. 1 , the switch  100  includes NMOS transistors M 1  and M 2 . An ENABLE voltage is directly coupled to the gates of M 1  and M 2 , turning M 1  and M 2  on when ENABLE is high, and turning M 1  and M 2  off when ENABLE is low. In the present disclosure, the “on state” corresponds to the state wherein ENABLE is high, while the “off state” corresponds to the state wherein ENABLE is low. 
     In  FIG. 1 , the ENABLE voltage further controls an NMOS transistor M 3  via inverting buffer B 1 . M 3  pulls the node MID between M 1  and M 2  to ground when ENABLE is low, and does not drive the node MID when ENABLE is high. A voltage Vx is provided through resistor R 1  to DC bias the output of the switch  100 . 
     One of ordinary skill in the art will appreciate that the switch  100  is designed to couple the output voltage OUTPUT to the input voltage INPUT when ENABLE is high, and to isolate OUTPUT from INPUT when ENABLE is low. As earlier described, it is desirable to provide a low series on-resistance between OUTPUT and INPUT when ENABLE is high, and conversely, to provide high series impedance between OUTPUT and INPUT when ENABLE is low. It is further desirable to present low parasitic capacitance to the OUTPUT and INPUT voltages when the switch is on. 
     Note in the prior art switch  100 , the bulk (or substrate) nodes B of transistors M 1  and M 2  are both coupled to ground. When ENABLE is high, the source nodes S of the transistors M 1  and M 2  may approach the bias voltage Vx, resulting in negative bulk-to-source voltages Vbs for M 1  and M 2 . One of ordinary skill in the art will appreciate that a negative bulk-to-source voltage generally increases the corresponding transistor threshold voltage, which decreases the overall gate overdrive voltage available to M 1  and M 2 . This undesirably results in higher series on-resistance for M 1  and M 2  during the on state of the switch. 
       FIG. 2  depicts an embodiment of a switch  200  according to the present disclosure. In  FIG. 2 , the switch  200  includes NMOS transistors M 1  and M 2 , whose gates are coupled to the ENABLE voltage. The ENABLE voltage further controls a PMOS transistor M 4  coupled to the node MID between M 1  and M 2 . M 4  pulls MID high to the supply voltage VDD when ENABLE is low, and does not drive MID when ENABLE is high. 
     The bulk nodes B of M 1  and M 2  are both coupled to a bulk control voltage BULK_C generated by a bulk voltage generation module  210 . The module  210  includes transistors M 5  and M 6 . The gate of M 5  is directly coupled to ENABLE, while the gate of M 6  is coupled to ENABLE via an inverting buffer B 2 . When ENABLE is high, M 6  is turned off, and M 5  is turned on, thus coupling the voltage BULK_C to the voltage Vb at the drain of M 5 . When ENABLE is low, M 5  is turned off, and M 6  is turned on, thus coupling the voltage BULK_C to ground. One of ordinary skill in the art will appreciate that the configuration of M 5  and M 6  effectively raises the bulk voltage BULK_C of the switch transistors M 1  and M 2  to a level Vb when ENABLE is high. This acts to decrease the threshold voltage of M 1  and M 2  when the switch is on. In an exemplary embodiment, the level Vb may be set at half the supply voltage. Furthermore, the configuration of M 4  effectively drives the node MID to a high voltage when ENABLE is off. 
     One of ordinary skill in the art having the benefit of the present disclosure will appreciate that the exemplary switch  200  depicted in  FIG. 2  offers at least two advantages over the prior art switch  100  shown in  FIG. 1 . 
     First, the controlled voltage BULK_C results in a bulk-to-source voltage during the on state of switch  200  that is less negative than the corresponding bulk-to-source voltage of the prior art switch  100 . This increases the gate overdrive voltage of M 1  and M 2  during the on state of switch  200 , thereby decreasing the series resistance of the transistors. In an exemplary embodiment, small device aspect ratios for M 1  and M 2  may further be used to decrease the parasitic capacitances associated with M 1  and M 2  during the on state. 
     Second, the coupling of the node MID to a high voltage during the off state of switch  200  results in negative gate-to-source voltages for M 1  and M 2 . This increases the series resistance of M 1  and M 2  during the off state of switch  200 , and also reduces the associated parasitic capacitances of M 1  and M 2 . 
     In an alternative exemplary embodiment illustrated in  FIG. 4 , PMOS devices M 1 ′ and M 2 ′ may be used for switch transistors, and an NMOS device M 4 ′ used to pull the corresponding node MID in such an alternative embodiment to ground. One of ordinary skill in the art may readily derive the appropriate modifications to the circuitry depicted in  FIG. 2  to realize such an alternative exemplary embodiment. 
     In alternative exemplary embodiments (not shown), the functionality of bulk generation voltage module  210  may be implemented using alternative transistor configurations known to one of ordinary skill in the art. For example, PMOS transistors, or a combination of NMOS and PMOS transistors, may be used. Such alternative exemplary embodiments are contemplated to be within the scope of the present disclosure. 
       FIG. 3  depicts an exemplary method according to the present disclosure. Note the method of  FIG. 3  is provided for illustration only, and is not meant to restrict the scope of the present disclosure to any exemplary embodiment explicitly disclosed. 
     In  FIG. 3 , at step  300 , the method switches the switch transistors (e.g., M 1  and M 2  in  FIG. 2 ) between an on state and an off state. 
     At step  310 , the method couples the bulk node of a switch transistor to a first bulk voltage during the on state of the switch. 
     At step  320 , the method couples the bulk node of a switch transistor to a second bulk voltage during the off state of the switch. In an exemplary embodiment wherein the switch transistors are NMOS devices, the second bulk voltage may be lower than the first bulk voltage. 
     At step  330 , the method couples a middle node of the switch to a high voltage during the off state. 
     In this specification and in the claims, it will be understood that when an element is referred to as being “connected to” or “coupled to” another element, it can be directly connected or coupled to the other element or intervening elements may be present. In contrast, when an element is referred to as being “directly connected to” or “directly coupled to” another element, there are no intervening elements present. 
     A number of aspects and examples have been described. However, various modifications to these examples are possible, and the principles presented herein may be applied to other aspects as well. These and other aspects are within the scope of the following claims.