Abstract:
Method and system for reducing parasitic feedback and resonances in high-gain transimpedance amplifiers. In a first embodiment of the present invention, a resistive layer is implemented in the gaps of a high-gain transimpedance amplifier&#39;s metallic planes. In a second embodiment of the present invention, a resistive layer is implemented underneath a high-gain transimpedance amplifier&#39;s ground plane, vias are implemented to create contact between the resistive layer and the ground plane.

Description:
BACKGROUND INFORMATION 
   1. Field of Invention 
   The present invention relates to transimpedance amplifiers, and more particularly, to high-gain transimpedance amplifiers with reduced parasitic feedback and resonances. 
   2. Description of Related Art 
   A transimpedance amplifier is used to convert an input current to a proportional output voltage. A typical transimpedance amplifier comprises: an input current that is supplied through a transistor input stage, the input current is typically produced by a photodiode; an output carrying an output voltage; and a coupling member connecting the input to the output. Furthermore, the input current may be small, such as 1 μA, or comparatively large, such as 1 mA. 
   Typical uses of transimpedance amplifiers include summing currents as part of a frequency impulse response filter or processing reverse current produced by a photodiode as function of infrared signal energy received by the photodiode. 
     FIG. 1A  illustrates a layout diagram  100  of a conventional high-gain transimpedance amplifier. Layout diagram  100  comprises: a power plane denoted  1 ; a group of one or more capacitors such as the block illustrated in diagram  100  denoted as  3 ; a group of one or more transistors such as the circuitry illustrated in diagram  100  denoted as  4 ; and a group of one or more pads such as the block illustrated in diagram  100  denoted as  5 , wherein the group of one or more pads connecting power plane  1  to components adjacent to the amplifier. 
   However, conventional transimpedance amplifiers are composed almost entirely of metallic structures comprising material such as gold, aluminum, or copper. Therefore, use of resistive materials is restricted to resistors only. Such metallic geometries in layout and packaging may form parasitic feedback paths and resonances that may turn a high-gain transimpedance amplifier into an oscillator. 
   According, there is a need to include resistive layers in transimpedance amplifiers in order to reduce parasitic feedback and resonances. 
   SUMMARY OF THE INVENTION 
   The present invention provides a method and system for reducing parasitic feedback and resonances in high-gain transimpedance amplifiers. 
   In a first embodiment of the present invention, a layer of resistive (e.g. Tantalum-Nitride, Nickel-Chrome, etc.) material is added in the gaps of a high-gain transimpedance amplifier&#39;s metal ground plane in order to reduce parasitic feedback and resonances. 
   In a second embodiment of the present invention, a layer of resistive (e.g. Tantalum-Nitride, Nickel-Chrome, etc.) material is added below a high-gain transimpedance amplifier&#39;s metal ground plane, along with vias for contacting the resistive layer to the metal ground plane. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The accompanying drawings that are incorporated in and form a part of this specification illustrate embodiments of the invention and together with the description, serve to explain the principles of the invention: 
       FIG. 1A  is a layout diagram illustrating a conventional transimpedance amplifier. 
       FIG. 1B  is a layout diagram illustrating a transimpedance amplifier according to one embodiment of the present invention. 
       FIG. 2  is a diagram illustrating a sectional view of a transimpedance amplifier in accordance to one embodiment of the present invention, wherein a layer of resistive material is implemented in a gap on the metal ground plane of the amplifier. 
       FIG. 3  is a diagram illustrating a section view for a transimpedance amplifier in accordance to one embodiment of the present invention, wherein a layer of resistive material is implemented below the metal ground plane of the amplifier. 
       FIG. 4A  is a graph illustrating the frequency response of an oscillating transimpedance amplifier without reduced parasitic feedback and resonances. 
       FIG. 4B  is a graph illustrating the frequency response of a transimpedance amplifier in accordance to one embodiment of the present invention. 
   

   DETAILED DESCRIPTION 
   The following description is presented to enable one of ordinary skill in the art to make and use the invention and is provided in the context of a patent application and its requirements. In the following description, specific nomenclature is set forth to provide a thorough understanding of the present invention. It will be apparent to one skilled in the art that the specific details may not be necessary to practice the present invention. Furthermore, various modifications to the embodiments will be readily apparent to those skilled in the art and the generic principles herein may be applied to other embodiments. Thus, the present invention is not intended to be limited to the embodiments shown but is to be accorded the widest scope consistent with the principles and features described herein. 
     FIG. 1B  illustrates a schematic diagram  200  of a high-gain transimpedance amplifier in accordance to one embodiment of the present invention. Schematic diagram  200  comprises: a ground plane denoted  6 ; a group of one or more capacitors such as the block illustrated in diagram  200  denoted as  8 ; a group of one or more transistors such as the circuitry illustrated in diagram  200  denoted as  10 ; a group of one or more pads such as the block illustrated in diagram  200  denoted as  12 , wherein the group of one or more pads connecting ground plane  6  to components adjacent to the amplifier; a layer comprising resistive (e.g. Tantalum-Nitride, Nickel-Chrome, etc.) material such as the blocks illustrated in diagram  200  denoted as  7 ,  9 ,  11 ,  13 ,  14 ,  18 ,  20 ,  24 ,  26 ,  28 ,  30 ,  32 ,  34  and  36 ; and a group of one or more vias on each resistive pads such as the via illustrated in diagram  200  denoted  15 . 
   The resistive layer illustrated in the transimpedance amplifier of  FIG. 11B  allows the amplifier to possibly reduce parasitic feedback and dampen parasitic resonances. Moreover, the resistive layer is constructed in the ground plane of the amplifier and does not alter the design of the amplifier elsewhere. 
   Furthermore, the resistive layer is typically added with a direct current (DC) contact to the ground plane. The group of one or more vias illustrated in diagram  200  provide such a contact between the resistive layer and the ground plane. 
     FIG. 2  illustrates a sectional view of a high-gain transimpedance amplifier  200  in accordance to one embodiment of the present invention. Transimpedance amplifier  200  comprises: a ground plane comprising a first layer of metallic (e.g. gold, aluminum, copper, etc.) material denoted  15  and a second layer of metallic (e.g. gold, aluminum, copper, etc.) material denoted  16 ; a layer of resistive (e.g. Tantalum-Nitride, Nickel-Chrome, etc.) material denoted  17 ; and a layer of dielectric denoted  19 . 
   As shown in  FIG. 2 , the layer of metallic material  15  lies adjacent to the layer of metallic material  16 , the two layers lie parallel on the same plane, and an electric disconnect or gap lies between the layer of metallic material  15  and the layer of metallic material  16 . Moreover, the layer of resistive material  17  lies in the brake between the first metallic material  15  and the second metallic material  16 . 
   Furthermore, as shown in  FIG. 2 , the layer of resistive materials in  FIG. 2  makes direct contact with the ground plane in order to reduce parasitic feedback and dampen parasitic resonances. 
     FIG. 3  illustrates a sectional view of a high-gain transimpedance amplifier  300  in accordance to a second embodiment of the present invention. Transimpedance amplifier  300  comprises: a ground plane comprising a first layer of metallic (e.g. gold, aluminum, copper, etc.) material denoted  21  and a second layer of metallic (e.g. gold, aluminum, copper, etc.) material denoted  22 ; a first group of one or more vias denoted  23 ; a second group of vias denoted  25 ; a layer of resistive (e.g. Tantalum-Nitride, Nickel-Chrome, etc.) material denoted  27 ; and a layer of dielectric denoted  29 . 
   As shown in  FIG. 3 , the layer of resistive material  27  lies directly atop the layer of dielectric  29 . The first group of one or more vias  23  and the second of one or more vias  25  are implemented on top of the layer of resistive material  27 . Moreover, the first layer of metallic material  21  lies directly atop the first group of one or more vias  23 , and the second layer of metallic materials  23  lies directly atop the second group of one or more vias  25 . 
   Furthermore, as shown in  FIG. 3 , the layer of resistive material makes contact with the ground plane via the first and the second groups of one or more vias, and the layer of resistive material reduces parasitic feedback and dampens parasitic resonances. 
     FIG. 4A  illustrates a transimpedance gain vs. frequency graph  500  for a conventional transimpedance amplifier without a layer comprising resistive material. The step denoted  27  in  FIG. 4A  illustrates a condition caused by an oscillation owing to parasitic feedback and resonance. 
     FIG. 4B  illustrates transimpedance gain vs. frequency graph  600  for a transimpedance amplifier in accordance to one embodiment of the present invention. A layer of resistive (e.g. Tantalum-Nitride, Nickel-Chrome, etc.) material is built into the amplifier that reduces parasitic feedback and resonances, thereby eliminating the oscillation condition illustrated in  FIG. 4A . 
   Although the invention has been described in connection with several embodiments, it is understood that this invention is not limited to the embodiments disclosed, but is capable of various modifications that would be apparent to a person skilled in the art. 
   For example, although the resistive layer is described as typically added with a DC contact to the ground plane, a floating (i.e. no DC contact) resistive layer may be added as well. 
   The foregoing descriptions of specific embodiments of the present invention have been presented for purposes of illustration and description. They are not intended to be exhaustive or to limit the invention to the precise forms disclosed, and obviously many modifications and variations are possible in light of the above teaching. The embodiments were chosen and described in order to best explain the principles of the invention and its practical application, to thereby enable others skilled in the arts to best utilize the invention and various embodiments with various modifications as are suited to the particular use contemplated. It is intended that the scope of the invention be defined by the claims appended hereto and their equivalents.