Abstract:
An Improved Signal Receiver Having Wide Band Amplification Capability is disclosed. Also disclosed is a receiver that is able to receive and reliably amplify infrared and/or other wireless signals having frequency bandwidths in excess of 40 MHz. The receiver of the present invention reduces the signal-to-noise ratio of the received signal to ⅕th of the prior systems. The preferred receiver eliminates both the shunting resistor and the feedback resistor on the input end by amplifying the signal in current form. Furthermore, the receiver includes transconductance amplification means for amplifying the current signal without the need for Cascode stages. Finally, the receiver includes staged amplification to amplify the current signal in stages prior to converting the signal into a voltage output.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates generally to wireless signal transmission systems and, more specifically, to an Improved Signal Receiver Having Wide Band Amplification Capability. 
     2. Description of Related Art 
     In a conventional infrared transceiver system  10  depicted by the diagram of  FIG. 1 , infrared signals  14  are received by an infrared diode  12 . These incident infrared signals  14  generate a current within the infrared diode  12 , which is conventionally converted to a voltage signal by shunting the system with resistor R S , as shown. This relatively low-voltage signal is then passed through a voltage amplifier  16 . The signal then passes through various stages of staged amplification  18  before being carried on out of the system as the output signal V IRRX . What should be appreciated is at node V OUT  the signal is essentially the incident IR signal  14 , plus any noise created by the IR diode  12  or the resistor R S . It should be apparent that the better the signal-to-noise ratio at V OUT , the better and cleaner the amplification through the voltage amplifier  16  and the subsequent staged amplification  18 . 
     Now turning to  FIG. 2 , we can discuss the operation of the conventional system in more depth.  FIG. 2  is a schematic of a single-ended version of a conventional infrared transceiver system of FIG.  1 . As can be seen in  FIG. 2 , the IR diode  12  is simulated by current source I 1  and capacitance C 1 . R S  of  FIG. 1  is here R 7 , shunted with the current source. Essentially, what we have in this diagram is a current mirror  20  and a voltage amplifier  22 . What should be appreciated from this circuit is that in normal operation the typical input level for fast infrared (FIR) frequency bandwidth will result in approximately 0.5 micro amps of current at current source I 1 , which results in 106 micro volts across a “real” 212 ohm resistor R 7 . Under such conditions, the resistor R 7  will have a thermal noise of 17.8 micro volts (at 40 MHz frequency bandwidth), which results in a noise ratio of 15.5 decibels without even having entered the amplification stages. If we now look at the operation of the amplifier  22 , we can see that typically, it is a high impedance voltage amplifier. The problem with this type of voltage amplifier is that R 7 , which is required for the specified system bandwidth, also provides additional noise that is added to the incident infrared signal  14  (at V OUT ) before the signal is amplified—this further decreases the signal-to-noise ratio. It should also be understood that since the “Miller Effect” will apply to the input stage, the value of the intrinsic gate-to-drain capacitance of such a stage is multiplied by the voltage gain. For example, a voltage gain of 10 will result in a “Miller Effect” drain-to-gate capacity of 11 times. In order to achieve the desired bandwidth, a Cascode stage becomes a necessity. The addition of this Cascode stage results in a corresponding addition of another transistor-based noise contribution discussed above (i.e. a total of two equal noise-contributing stages). Consequently, this phenomena further degrades the signal to noise ratio and harms the amplifier performance. Another type of amplifier has been conventionally used, in which R 7  is replaced by a feedback resistor. This amplifier has not been discussed herein, since its design is limited to a lower bandwidth, in particular, because of its poor noise performance. 
     Now turning to  FIG. 3 , we can see a preferred model for the prior art circuit of FIG.  2 .  FIG. 3  is a simulation of the circuit of  FIG. 2  provided for the purposes of modeling the performance of the circuit; the pertinent results of this modeling are shown in  FIGS. 4 and 5 .  FIG. 4  is a plot of noise vs. frequency bandwidth for the conventional circuit of  FIGS. 1 through 3 . As can be seen, at a frequency of approximately 40 MHz (which is in the FIR bandwidth), the spot noise is 1.6×10 −21 /√{square root over (Hz)} approximately. This number will become more significant once we discuss the improvements of the present invention. 
     Now turning to  FIG. 5  we can see the effect of these noises and capacitance&#39;s created in the prior art voltage feedback type amplification circuit.  FIG. 5  a response plot of output voltage (V IRRX ) for the prior system of FIG.  2 . As can be seen, the peaks and valleys are extremely erratic and choppy, which creates an unstable signal and ultimately inferior data processing. What is needed is an improved amplifier system to reliably handle in excess of 40 MHz frequency bandwidth. 
     SUMMARY OF THE INVENTION 
     In light of the aforementioned problems associated with the prior systems and devices, it is an object of the present invention to provide an Improved Signal Receiver Having Wide Band Amplification Capability. The preferred receiver should be able to receive and reliably amplify infrared and/or other wireless signals having frequency bandwidths in excess of 40 MHz. It is an object of the present invention to reduce the signal-to-noise ratio of the received signal to ⅕ th  of the prior systems. In its preferred form, the receiver will eliminate both shunting and feedback resistors on the input end by amplifying the signal in current form. Furthermore, the receiver will include transconductance amplification means for amplifying the current signal without the need for Cascode stages. It is a further object that the receiver include staged amplification to amplify the current signal in stages prior to converting the signal into a voltage output. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The objects and features of the present invention, which are believed to be novel, are set forth with particularity in the appended claims. The present invention, both as to its organization and manner of operation, together with further objects and advantages, may best be understood by reference to the following description, taken in connection with the accompanying drawings, of which: 
         FIG. 1  is a functional diagram of a conventional eared transceiver system; 
         FIG. 2  is a schematic of the conventional infrared transceiver system of  FIG. 1 ; 
         FIG. 3  is a simulation of the circuit of  FIG. 2  provided for the purposes of modeling the performance of the circuit; 
         FIG. 4  is a plot of frequency bandwidth of noise to frequency bandwidth for the conventional circuit of  FIGS. 1 through 3 ; 
         FIG. 5  a plot of output voltage keeping the effect of the high system noise characteristics; 
         FIG. 6  is a functional diagram of an improved infrared transceiver system of the present invention using current amplification; 
         FIG. 7  is a preferred circuit design of the circuit of  FIG. 6 ; 
         FIG. 8  is a circuit model of the circuit of  FIGS. 6 and 7 ; 
         FIG. 9  is a plot of noise versus bandwidth of the circuit of  FIGS. 6 ,  7 , and  8 ; and 
         FIG. 10  is a plot of output voltage of the circuit of  FIGS. 6 ,  7 ,  8 , and  9 . 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The following description is provided to enable any person skilled in the art to make and use the invention and sets forth the best modes contemplated by the inventors of carrying out their invention. Various modifications, however, will remain readily apparent to those skilled in the art, since the generic principles of the present invention have been defined herein specifically to provide an Improved Signal Receiver Having Wide Band Amplification Capability. 
     The present invention can best be understood by initial consideration of FIG.  6 .  FIG. 6  is a functional diagram of an improved infrared transceiver system  24  of the present invention, employing current amplification. In this system  24 , the IR signals  14  incident upon the IR diodes remain in the form of a current (I OUT ). The Current (I OUT −I F ) develops voltage across R INEFF  (Effective Input Resistance=R IN /(1+βA OL ). This voltage is multiplied by the Transconductance of the current amplifier  26 , producing a current through R L , giving a voltage input to the buffer  32 . This pre-amplifier output voltage is converted to a feedback current (I F ) by device X 3 . I F  is then combined with I OUT  which results in a reduction in the size of R IN  (noiselessly), which ultimately improves the bandwidth of the system. 
     Now turning to  FIG. 7 , we can see the preferred circuit design for the improved transceiver system  24  of FIG.  6 . As can be seen, in this case, current generator  13  and capacitor C 4  simulate the IR diode  12 . In contrast to the prior voltage-type amplifier depicted in  FIGS. 2 and 3 , the amplifier  30  of this  FIG. 7  is a transimpedence-type amplifier. With the transimpedence amplifier, since there is typically no resistive feedback loop (i.e. there is no feedback resistor), the intrinsic system noise is substantially reduced. Furthermore, the significant benefit of using this topology for the transimpedence amplifier is that it does not result in a Miller effect, and therefore there is only a noise contribution from a single input stage (since the full Cascode stage is rendered unnecessary by the absence of a Miller effect). The result is an amplifier that is capable of extremely high signal-to-noise ratios, in addition to very good bandwidth, since R INEFF  is equal to R IN /(1+βA OL ). 
     In order to potentially achieve further performance improvements, the transistors X 3 , X 6 , X 7  and/or X 4  might include dynamically-adjustable bias voltage control in order to operate these transistors in the “weak inversion” range for certain portions of their operational curves. Since weak inversion operations are well known in the art, the particulars of this operational mode are not discussed herein. For the purposes of this discussion, a 0.7μ CMOS process is employed; it should be understood that additional system capacitance reductions (and therefore performance improvements) might be achievable through the use of smaller geometry. 
       FIG. 8  is a circuit model of the circuit of  FIGS. 6 and 7  constructed in order to provide simulation data on the circuit, as reported below in  FIGS. 9 and 10 .  FIG. 9  is a plot of noise versus frequency bandwidth of the circuit of  FIGS. 6 ,  7 , and  8 . If we look at the 40 MHz line we can see that the spot noise at 0.54×10 −21 /√{square root over (Hz)} this point is This compares to 1.6×10 −21  of the prior circuit, or approximately ⅓ the spot noise at equivalent frequency in the new circuit of  FIG. 7  (as compared to the old circuit of Figure  2 ), which equates to a 13 dB improvement when integrated over the full frequency range. Also, at 3 dB signal-to-noise ratio, the frequency bandwidth exceeds 64 MHz. 
     As can be seen from  FIG. 10 , the improvement in responsiveness of the transimpedence solution is dramatic.  FIG. 10  is a plot of output voltage of the circuit of  FIGS. 6 ,  7 ,  8 , and  9 . In contrast to the sawtooth response curve of  FIG. 5 ,  FIG. 10  shows a smooth output through several signal pulses. It should be understood from  FIGS. 9 and 10  that the device of the present invention will provide extremely high bandwidths with low noise while at the same time giving very, very smooth response. It also should also be understood that while throughout this application the embodiments discussed have been in regard to infrared signal receipt, this method can also be expected to provide the same benefits for other wireless signal receipt, for example radio frequency, and in particular cellular phones and other devices. Through application of this technology it is believed that the noise improvement of 15 to 16 decibels will result in an incredible increase in-range and coverage that heretofore has not been achievable. 
     Theoretical Noise Comparison to the Prior Art 
     The following analysis is provided in order to further explain the significant benefits of the signal receiver of the present invention. A noise comparison between the prior art amplifier and the amplifier of the present invention revolves around the input transistor and the input resistor, since the system signal-to-noise ratio is essentially determined at this point in the respective circuits. In the prior art circuit (see FIG.  2 ), R 7  is the input resistor, X 5  is the input transistor—as discussed above, X 5  is a Cascode connection. In the preferred circuit of the present invention, there is NO input resistor, as well as NO Cascode connection. 
     Input Resistor Contribution 
     In the prior circuit, assume that a Bandwidth of 40 MHz drives R 7  to be 265 Ω (in order to have adequate gain without decreasing the signal-to-noise ratio to an unacceptable level). The formula for RMS noise generated in a resistor is: 
           i   RMS     ⁡     (   resistor   )       =         4   ⁢   xkxT     R           
 
, where:
         k=Boltzman&#39;s constant=1.38×10 −23      T=Temperature (deg. Kelvin)=290   R=Resistor value=265
 
, such that: 
           i   RMS     ⁡     (   R7   )       =           4   ⁢   x1   ⁢   .38   ×     10     -   23       ⁢   x290     265       =     49.16   ⁢           ⁢   nanoAmperes           
 
Input Transistor Contribution
       

     The thermal noise of one input MOSFET is calculated by the following formula: 
           i   RMS     ⁡     (   MOSFET   )       =         {       8   ⁢   xkxT     3     }     ⁢   x   ⁢       2   ⁢   x   ⁢           ⁢   β   ⁢           ⁢   xId               
 
, where:
         β=K′×W/L   K′ is a transconductance parameter=30.3×10 −6      W/L are width and length dimensions of the MOSFET=55/1 (therefore β=7.575×10 −4 )   Id is the MOSFET drain current=60×10 −6  (for this case)
 
, such that: 
           i   RMS     ⁡     (   MOSFET   )       =         {       8   ⁢   x1   ⁢   .38   ⁢     x10     -   23       ⁢   x290     3     }     ⁢   x   ⁢       2   ⁢   x7   ⁢   .575   ⁢     x10     -   4       ⁢     x60x10     -   6                   
 i RMS (MOSFET)=11.34 nanoAmperes
 
Comparison between the Circuits:
       

     Assume that the input current source may drop as low as 250 nanoAmperes (fairly common for infrared communications). 
     The prior circuit&#39;s input components&#39; noise:
 
i RMS (input)=i RMS (R7)+i RMS (MOSFET) 
 
, but since X 5  is Cascode-connected, there are essentially two noise contributions, making the combined contribution equal to the square root of their squared contributions, therefore: 
           i   RMS     ⁡     (   input   )       =             i   RMS     ⁡     (   R7   )       2     +     2   ⁢   x   ⁢       {       i   RMS     ⁡     (   MOSFET   )       }     2               
           i   RMS     ⁡     (   input   )       =           49.16   2     +     2   ⁢   x   ⁢       {   11.3   }     2           =     51.6   ⁢           ⁢   nanoAmperes           
 
.
 
     The preferred circuit of the present invention&#39;s input components&#39; noise Since there is no input resistor, the formula for the comparable noise current is simply:
 
i RMS (input)=i RMS (MOSFET) 
 
i RMS (input)=11.34 nanoAmperes 
 
Signal-to-Noise Ratio Comparison:
 
S:N(prior circuit)=250:51.6=4.85:1 
 
S:N(present invention)=250:11.34=22.0:1!
 
This represents over 5 (five) times the signal-to-noise ratio of the prior circuit, which, when coupled with the superior frequency performance described previously, clearly demonstrates the previously-unknown benefits of the present circuit and method over the prior devices and methods.
 
     Those skilled in the art will appreciate that various adaptations and modifications of the just-described preferred embodiment can be configured without departing from the scope and spirit of the invention. Therefore, it is to be understood that, within the scope of the appended claims, the invention may be practiced other than as specifically described herein.