Abstract:
A high gain input stage for a Radio Frequency Identification (RFID) transponder uses an amplifier for increasing a magnitude of an input signal. A DC bias circuit is used for controlling the operation of the amplifier. A resonant circuit is coupled between the amplifier and the DC bias circuit. The resonant circuit is used for receiving a signal generated by an electromagnetic field and for generating the input signal which is sent to the amplifier. The resonant circuit has an inductive portion which is used to bias the amplifier thereby removing the requirement of using a decoupling capacitor.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates generally to a Radio Frequency Identification (RFID) transponder and, more specifically, to a high gain low current input stage for an RFID transponder. 
     2. Description of the Prior Art 
     It is desirable to have a sensitive input into a transponder. In order to do this, one must amplify the input signal. Presently, most input stages require the use of a decoupling capacitor. The decoupling capacitor is required to isolate the DC bias component generated by the amplifying circuit from the external L-C circuit. It is desirable to isolate the DC bias component since one does not want the DC component to be short-circuited to ground via the inductor element of the external L-C circuit. Furthermore, the problem with using a decoupling capacitor is that the decoupling capacitor that is required is very large and consumes valuable silicon real estate. 
     Therefore, a need existed to provide an improved high gain input stage for a transponder. The improved high gain input stage must require fewer components to implement than prior art input stages. The improved high gain input stage must not require a decoupling capacitor. The improved high gain input stage must allow an automatic gain control circuit to be easily integrated therein. The improved high gain input stage must further have a low current consumption. 
     SUMMARY OF THE INVENTION 
     In accordance with one embodiment of the present invention, it is an object of this invention to provide an improved high gain input stage for a transponder. 
     It is another object of the present invention to provide an improved high gain input stage for a transponder which requires fewer components than prior art input stages. 
     It is still another object of the present invention to provide an improved high gain input stage biasing circuit for a transponder that does not require a decoupling capacitor. 
     It is yet another object of the present invention to provide an improved high gain input stage for a transponder wherein an automatic gain control circuit may be easily integrated therein. 
     It is still a further object of the present invention to provide an improved high gain input stage for a transponder that has a low current consumption. 
     BRIEF DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     In accordance with one embodiment of the present invention, a high gain input stage for a Radio Frequency Identification (RFID) transponder is disclosed. The high gain input stage uses an amplifier for increasing a magnitude of an input signal. A DC bias circuit is used for controlling the operation of the amplifier. A resonant circuit is coupled between the amplifier and the DC bias circuit. The resonant circuit is used for receiving a signal generated by an electromagnetic field and for generating the input signal which is sent to the amplifier. The resonant circuit has an inductive portion which is used to DC bias the amplifier. 
     In accordance with another embodiment of the present invention, a method of providing a high gain input stage for a Radio Frequency Identification (RFID) transponder is disclosed. The method comprises the steps of: providing an amplifier for increasing a magnitude of an input signal; providing a DC bias circuit for controlling operation of the amplifier; and providing a resonant circuit coupled between the amplifier and the DC bias circuit for receiving a signal generated by an electromagnetic field and for generating the input signal sent to the amplifier wherein an inductive portion of the resonant circuit is used to DC bias the amplifier. 
     The foregoing and other objects, features, and advantages of the invention will be apparent from the following, more particular, description of the preferred embodiments of the invention, as illustrated in the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a simplified electrical schematic of a prior art high gain input stage. 
     FIG. 2 is a simplified electrical schematic of another prior art high gain input stage. 
     FIG. 3 is a simplified electrical schematic of one embodiment of the present invention. 
     FIG. 4 is a simplified electrical schematic of a second embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring to FIG. 1, a prior art high gain input stage for a transponder  10  (hereinafter input stage  10 ) is shown. The input stage  10  has an external inductor-capacitor (L-C) circuit  12 . The external L-C circuit  12  is comprised of an inductive element  14  coupled in parallel with a capacitive element  16 . The external L-C circuit  12  will pick up a signal generated by an electromagnetic field. The L-C circuit  12  will generate a voltage after receiving the signal that was generated by the electromagnetic field. The voltage signal generated by the L-C circuit  12  is then fed into a first input of an amplifier  18 . A second input of the amplifier  18  is coupled to a reference voltage V REF  source. A feedback resistor  22  is coupled to an output and to the first input of the amplifier  18 . A second resistor  24  is also coupled to the first input of the amplifier  18 . The feedback resistor  22  and the second resistor  24  are used to set the voltage gain of the amplifier  18 . 
     A decoupling capacitor  20  is also coupled to the first input of the amplifier  18 . The decoupling capacitor  20  is required because the amplifier  18  needs to be biased at a certain voltage level to achieve optimal gain. Furthermore, the decoupling capacitor  20  is required to isolate the DC bias component generated by the amplifier  18  from the external L-C circuit  12  since one does not want the DC component to be short-circuited to ground via the inductive element  14  of the external L-C circuit  12 . As stated above, the problem with using the decoupling capacitor  20  is that the decoupling capacitor  20  that is required is very large as is the amplifier  18 . These two components consume valuable silicon real estate. 
     Referring to FIG. 2, another prior art high gain input stage for a transponder  30  (hereinafter input stage  30 ) is shown. The input stage  30  uses a very simple amplifier  32 . The amplifier  32  is a single transistor amplifier. The amplifier  32  is comprised of a current source  34 . The current source  34  has a first terminal coupled to a voltage source V DD . A second terminal of the current source  34  is coupled to a transistor  36 . The transistor  36  has three terminals. The first terminal of transistor  36  is coupled to the current source  34 . The second terminal of the transistor  36  is coupled to a bias circuit  38 . The third terminal of the transistor  36  is coupled to ground. 
     As stated above, the second terminal of the transistor  36  is coupled to a bias circuit  38 . The bias circuit  38  is used to control the operation of the amplifier  32  by biasing the amplifier  32  to the amplifier&#39;s threshold voltage. The bias circuit  38  is comprised of a current source  40 . The current source  40  has a first terminal coupled to a voltage source V DD . A second terminal of the current source  40  is coupled to a transistor  42 . The transistor  42  has three terminals. The first terminal of transistor  42  is coupled to the current source  40 . The second terminal of the transistor  42  is coupled to the first terminal of the transistor  42 . The third terminal of the transistor  42  is coupled to ground. 
     Like the prior art input stage  10  shown in FIG. 1, the input stage  30  has an external inductor-capacitor (L-C) circuit  44 . The external L-C circuit  44  is comprised of an inductive element  46  coupled in parallel with a capacitive element  48 . The external L-C circuit  44  will pick up a signal generated by an electromagnetic field. The L-C circuit  44  will generate a voltage after receiving the signal that was generated by the electromagnetic field. The voltage signal generated by the L-C circuit  44  is then fed into the bias circuit  38  and the amplifier  32 . 
     A decoupling capacitor  50  is coupled to the bias circuit  38  and the amplifier  32 . The decoupling capacitor  50  is required to isolate the DC bias component generated by the bias circuit  38  from the L-C circuit  44  since one does not want the DC component to be short-circuited to ground via the inductive element  46  of the LC circuit  44 . As stated above, the problem with using the decoupling capacitor  50  is that the decoupling capacitor  50  that is required is very large and consumes valuable silicon real estate. 
     Referring to FIG. 3, a high gain input stage for a transponder  60  (hereinafter input stage  60 ) is shown. The input stage  60  is unique in that the decoupling capacitor of the prior art is no longer required. This is accomplished by including the external L-C circuit  68  as part of the amplifier/DC bias circuit. Furthermore, the input stage  60  requires fewer components to implement, thereby saving valuable silicon real estate. The input stage  60  also may be biased via the inductive element  76  of the L-C circuit  68  which, as stated above, form part of the input stage  60 . 
     The input stage  60  uses a very simple amplifier  62 . The amplifier  62  is a single transistor amplifier. The amplifier  62  is comprised of a current source  64 . The current source  64  has a first terminal coupled to a voltage source V DD . A second terminal of the current source  64  is coupled to a transistor  66 . The transistor  66  has three terminals. The first terminal of transistor  66  is coupled to the current source  64 . The second terminal of the transistor  66  is coupled to the L-C circuit  68 . The third terminal of the transistor  66  is coupled to ground. 
     A DC bias circuit  70  is coupled to the L-C circuit  68 . The DC bias circuit  70  is used to control the operation of the amplifier  62  by DC biasing the amplifier  62  to the amplifier&#39;s threshold voltage via the L-C circuit  68 . The DC bias circuit  70  is comprised of a current source  72 . The current source  72  has a first terminal coupled to a voltage source V DD . A second terminal of the current source  72  is coupled to a transistor  74 . The transistor  74  has three terminals. The first terminal of transistor  74  is coupled to the current source  72 . The second terminal of the transistor  74  is coupled to the first terminal of the transistor  74 . The third terminal of the transistor  74  is coupled to ground. 
     The L-C circuit  68  is coupled in between the amplifier  62  and the DC bias circuit  70 . The L-C circuit  68  is comprised of an inductive element  76  coupled in parallel with a capacitive element  78 . The L-C circuit  68  now forms part of the amplifier-DC bias circuit (i.e., input stage  60 ). The biasing of the amplifier  62  now flows through the inductive element  76  so that the amplifier  62  is biased at a DC operating voltage level. Therefor, the prior art decoupling capacitor is no longer required. 
     Referring now to FIG. 4 wherein like numerals represent like elements, another embodiment of the input stage  60  of the present invention is shown. The embodiment depicted in FIG. 4 is similar to that shown in FIG.  3 . One difference between the embodiments is that the input stage  60  shown in FIG. 4 has an automatic gain control circuit  80  coupled to the amplifier  62 . The automatic gain control circuit  80  is used to adjust the gain of the amplifier  62 . The automatic gain control circuit  80  accomplishes this by adjusting the resistance level of the resistor  82  which is coupled to the transistor  66  of the amplifier  62 . The automatic gain control circuit  80  may use any adjustable gain element such as a current controlled resistor (ICR) or a voltage controlled resistor (VCR). Another difference in the embodiments is that the input stage  60  shown in FIG. 4 has a resistor  84  coupled between the L-C circuit  68  and the DC bias circuit  70 . The resistor  84  improves the dynamic range of the input stage  60  and does not influence the DC bias of the amplifier  62 . 
     While the invention has been particularly shown and described with reference to preferred embodiments thereof, it will be understood by those skilled in the art that the foregoing and other changes in form and details may be made therein without departing from the spirit and scope of the invention.