Abstract:
Method and system for a diode shunting configuration wherein the configuration prevents a transimpedance amplifier from saturation while maintaining high transimpedance gain and bandwidth. In one embodiment of the present invention, a diode is coupled to the input of a transimpedance amplifier in order to prevent the transimpedance amplifier from saturation. Moreover, the diode serves to divert current such that in cases where the input current is low the diode never turns on and only represents a minimal, mostly capacitive load on the input; in cases where the input current is high, the diode conducts and diverts any excess input current from the transimpedance amplifier.

Description:
BACKGROUND INFORMATION 
     1. Field of Invention 
     The present invention relates to amplifiers in general, and more particularly, to a method and system for implementing transimpedance amplifiers with high current input while maintaining high transimpedance gain and bandwidth. 
     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 a function of infrared signal energy received by the photodiode. 
     In circumstances where a transimpedance amplifier receives high input current from a photodiode, the current may drive the transimpedance amplifier into a state of deep saturation where a large input signal spoils the sensitivity of the amplifier to a following smaller signal. 
     In order to prevent the transimpedance amplifier from saturation, several conventional methods have been established. FIG. 1 illustrates one embodiment of a conventional method for preventing saturation wherein a transimpedance optical receiver denoted  100  comprises: an input modulated light denoted  1 , a photodiode denoted  3 , a transimpedance amplifier denoted  5 , a load resistor denoted  7 , a non-linear diode denoted  9 , and an output denoted  11 . 
     Optical receiver  100  receives modulated light  1  from an optical fiber. Subsequently, the light falls onto photodiode  3 . Moreover, photodiode  3  is connected to the inverting input of amplifier  5 , resistor  7  is connected across amplifier  5 , and diode  9  such as a Schottky diode is connected in parallel with resistor  7 . 
     As shown in FIG. 1, diode  9  is added to receiver  100  in order to prevent amplifier  5  from saturation. Conventionally, a Schottky diode is chosen as diode  9  due to the fact that a Schottky diode has negligent effect on the receiver at low current input. At large current inputs, a Schottky diode conducts to limit the receiver&#39;s output and prevents saturation. 
     However, traditional methods that prevent saturation such as shown in FIG. 1 are limited and cause an increase in determninistic jitter at the output. Furthermore, the bit error rate (BER) increases in response to the increased jitter and eventually reaches a point where the transferred data can no longer be recovered. 
     Accordingly, there is a need to prevent transimpedance amplifiers with high current input from saturation while maintaining high transimpedance gain and bandwidth. 
     SUMMARY OF THE INVENTION 
     The present invention provides a method and system for preventing a transimpedance amplifier from saturation while maintaining high transimpedance gain and bandwidth. 
     In a first embodiment of the present invention, a diode is coupled to the input of a transimpedance amplifier in order to prevent the transimpedance amplifier from saturation. Moreover, the diode serves to divert current such that in cases where the input current is low the diode is not turned on and represents only a minimal, mostly capacitive load on the input; in cases where the input current is high, the diode conducts and diverts any excess input current from the transimpedance amplifier. 
     In a second embodiment of the present invention, a diode is coupled to the input of a transimpedance amplifier in order to prevent the transimpedance amplifier from saturation. The diode serves to divert excess current from the transimpedance amplifier in cases where the input current is high. Additionally, a resistive divider is coupled to the diode in order to further optimize the overall performance of the transimpedance amplifier. 
     In a third embodiment of the present invention, a diode is coupled to the input of a transimpedance amplifier in order to prevent the transimpedance amplifier from saturation. The diode serves to divert excess current from the transimpedance amplifier in cases where the input current is high. Furthermore, in order to alleviate switching noise generated by the excess current through the diode, an extra buffer is added in the diode shunt configuration. 
     In a fourth embodiment of the present invention, a diode is coupled to the input of a transimpedance amplifier in order to prevent the transimpedance amplifier from saturation. The diode serves to divert excess current from the transimpedance amplifier in cases where the input current is high. Furthermore, the configuration of the embodiment is such that the input stage may be independent of the output. 
     The diode shunt configuration of the present invention diverts excess current from a transimpedance amplifier in order to prevent the amplifier from saturation while maintaining high gain and bandwidth. 
    
    
     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. 1 is a prior art diagram illustrating a conventional solution to transimpedance amplifier saturation. 
     FIG. 2 is a circuit diagram illustrating a diode coupled to the input of a transimpedance amplifier in order to prevent saturation in accordance to a first embodiment of the present invention. 
     FIG. 3 is a circuit diagram illustrating a second embodiment of the present invention that employs an additional resistive divider. 
     FIG. 4 is a circuit diagram illustrating a third embodiment of the present invention that employs an additional buffer. 
     FIG. 5 is a circuit diagram illustrating a fourth embodiment of the present invention where the input stage is independent of the output of the circuit. 
     FIG. 6 is a flow diagram illustrating 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. 2 illustrates a circuit  200  of a diode shunt configuration in accordance to one embodiment of the present invention. Circuit  200  generally comprises: an input current signal denoted  13 , a circuit denoted  15  generally comprising a diode, an amplifier denoted  17  generally comprising a transimpedance amplifier (TIA), a reference point denoted  19 , an amplifier denoted  21  generally comprising a TIA, a circuit denoted  20  generally comprising a buffer, an amplifier denoted  23  generally comprising a limiting amplifier, an amplifier denoted  25  generally comprising a direct current (DC) amplifier, a first output denoted  27 , and a second output denoted  29 . 
     Typically, a photodiode produces the input signal  13  that is directed to circuit  15 . Circuit  15  is coupled to a first input of amplifier  17 , and a first output of amplifier  17  is coupled to a first input of amplifier  23 . Amplifier  23  produces outputs  27  and  29  coupled to a first and a second input of amplifier  25  respectively. Amplifier  25  in turn produces an output coupled to an input to amplifier  21 , and amplifier  21  produces an output coupled to a second input of amplifier  23 . 
     As shown in FIG. 2, circuit  15  functions to shunt excessive current from amplifier  17 . In cases where input current  13  is low (i.e. below a preset threshold value), circuit  15  does not turn on and represents only a minimal, mostly capacitive load on the input. However, in cases where input current  13  is high (i.e. above a preset threshold value), the voltage at reference point  19  falls in response to the high current and circuit  15  conducts and diverts the excess input current towards reference point  19  as indicated by the dashed arrow denoted  22 , effectively preventing amplifier  17  from saturation. 
     Furthermore, because a fixed DC voltage at reference point  19  can optimally accommodate only a particular input current level, a control voltage at reference point  19  is adjusted adaptively in order to optimize the overall transimpedance amplifier performance. The connection of DC voltage at reference point  19  as shown in FIG. 2 decreases with increasing average input current which results in improved diode shunting at high currents while improving the diode cut-off state at low currents where shunting is not necessary. 
     FIG. 3 illustrates a circuit  300  of a diode shunt configuration in accordance to a second embodiment of the present invention. Circuit  300  generally comprises: an input current signal denoted  35 , a circuit denoted  37  generally comprising a diode, an amplifier denoted  39  generally comprising a TIA, a reference point denoted  40 , an amplifier denoted  41  generally comprising a TIA, an amplifier denoted  43  generally comprising a limiting amplifier, an amplifier denoted  45  generally comprising a DC amplifier, a first output denoted  47 , a second output denoted  49 , and a circuit denoted  31  generally comprising a resistive divider. 
     Typically, a photodiode produces the input signal  35  that is directed to circuit  37 . Circuit  37  is coupled to a first input of amplifier  39 , and a first output of amplifier  39  is coupled to a first input of amplifier  43 . Amplifier  43  produces outputs  47  and  49  coupled to a first and a second input of amplifier  45  respectively. Amplifier  45  in turn produces an output coupled to an input to amplifier  41 . Amplifier  41  produces an output coupled to a second input of amplifier  43 . 
     As shown in FIG. 3, circuit  37  shunts excessive current from amplifier  39 . In cases where input current  35  is low (i.e. below a preset threshold value), circuit  37  does not turn on and only represents a minimal, mostly capacitive load on the input. However, in cases where input current  35  is high (i.e. above a preset threshold value), the voltage at the output of circuit  41  and at reference point  40  falls in response to the high current and circuit  37  conducts and diverts the excess input current towards reference point  40  as indicated by the dashed arrow denoted  24 , effectively preventing amplifier  39  from saturation. 
     Furthermore, because a fixed DC voltage at reference point  40  can optimally accommodate only a particular input current level, a control voltage at reference point  40  is adjusted adaptively in order to optimize the overall transimpedance amplifier performance. The connection of DC voltage at reference point  40  as shown in FIG. 3 decreases with increasing average input current which results in improved diode shunting at high currents while improving the diode cut-off state at low currents where shunting is not necessary. 
     Circuit  31  is added to the embodiment shown in FIG. 3 in order to further optimize the adaptability of the control voltage at reference point  40 . 
     FIG. 4 illustrates a circuit  400  of a diode shunt configuration in accordance to a third embodiment of the present invention. Circuit  400  generally comprises: an input current signal denoted  51 , a circuit denoted  53  generally comprising a diode, an amplifier denoted  55  generally comprising a TIA, a reference point denoted  54 , an amplifier denoted  59  generally comprising a TIA, a circuit denoted  57  generally comprising a buffer, a circuit denoted  61  generally comprising a buffer, an amplifier denoted  65  generally comprising a limiting amplifier, an amplifier denoted  63  generally comprising a DC amplifier, a first output denoted  67 , a second output denoted  69 , and a reference voltage input denoted  62 . 
     Typically, a photodiode produces the input signal  51  that is directed to circuit  53 . Circuit  53  is coupled to a first input of amplifier  55 , and a first output of amplifier  55  is coupled to a first input of amplifier  65 . Amplifier  65  produces outputs  67  and  69  coupled to a first and a second input of amplifier  63  respectively. Amplifier  63  produces an output coupled to an input of amplifier  59  having buffers  57  and  61 . Amplifier  59  produces an output coupled to a second input of amplifier  65 . 
     As shown in FIG. 4, circuit  53  shunts excessive current from amplifier  55 . In cases where input current  51  is low (i.e. below a preset threshold value), circuit  53  does not turn on and only represents a minimal, mostly capacitive load on the input. However, in cases where input current  51  is high (i.e. above a preset threshold value), the voltage at reference point  54  falls in response to the high current and circuit  53  conducts and diverts the excess input current towards reference point  54  as indicated by the dashed arrow denoted  52 , effectively preventing amplifier  55  from saturation. 
     Furthermore, because a fixed DC voltage at reference point  54  can optimally accommodate only a particular input current level, a control voltage at reference point  54  is adjusted adaptively in order to optimize the overall transimpedance amplifier performance. The connection of DC voltage at reference point  54  as shown in FIG. 4 decreases with increasing average input current which results in improved diode shunting at high currents while improving the diode cut-off state at low currents where shunting is not necessary. 
     Moreover, the diode shunt configuration illustrated in FIG.  2  and FIG. 3 may generate undesirable switching noise at reference voltage input  62 . The additional buffer at amplifier  59  shown in FIG. 4 is implemented in order to alleviate said switching noise. 
     FIG. 5 illustrates a circuit  500  of a diode shunt configuration in accordance to a fourth embodiment of the present invention. Circuit  500  generally comprises: an input current signal denoted  71 , a circuit denoted  73  generally comprising a diode, an amplifier denoted  75  generally comprising a TIA, an amplifier denoted  79  generally comprising a DC amplifier, an amplifier denoted  87  generally comprising a limiting amplifier, an amplifier denoted  89  generally comprising a DC amplifier, a reference signal denoted  85 , an external filter capacitor denoted  77 , a first output denoted  91 , and a second output denoted  93 . 
     Typically, a photodiode produces the input signal  71  that is directed to circuit  73 . Circuit  73  is coupled to a first input of amplifier  75 , and a first output of amplifier  75  is coupled to a first input of amplifier  87 . Amplifier  87  produces outputs  91  and  93  coupled to a first and a second input of amplifier  89  respectively. Amplifier  89  in turn produces an output coupled to a second input of amplifier  87 . 
     Moreover, the cross denoted  83  illustrates a possible disconnection between the input to amplifier  79  denoted  80  and the output of amplifier  89 . In cases where input  80  is coupled to the output of amplifier  89 , amplifier  79  receives the output of amplifier  89  as an input; in cases where input  80  is disconnected from the output of amplifier  89 , amplifier  79  receives the output of amplifier  75  as an input, as indicated by the dashed line denoted  81 . The disconnection allows the input stage comprising circuit  73 , amplifier  75 , and amplifier  79  to be independent of the output of circuit  500  (i.e. output  91  and output  93 ) where the output of circuit  500  is to be configured without affecting the input stage. 
     As shown in FIG. 5, circuit  73  shunts excessive current from amplifier  75 . In cases where input current  71  is low (i.e. below a preset threshold value), circuit  73  does not turn on and only represents a minimal, mostly capacitive load on the input. However, in cases where input current  71  is high (i.e. above a preset threshold value), the voltage at the output of amplifier  79  falls in response to the high current and circuit  73  conducts and diverts the excess input current, effectively preventing amplifier  75  from saturation. 
     Reference signal  85  is a threshold that may be set to any value according to desired criteria of a design, signal  85  is set to control the threshold at which the diode is turned on or off in order to divert excessive current from amplifier  75 . Moreover, signal  85  may be generated externally by a device (not shown) such as a microprocessor or by other appropriate devices according to the design of a particular implementation. 
     Furthermore, external filter capacitor  77  may be implemented to further filter and integrate the signal output of amplifier  79 . 
     FIG. 6 is a flow diagram illustrating the steps for one embodiment of the present invention. In step  95 , the diode receives an input signal and subsequently detects whether or not the current level of the input signal is higher than a predetermined threshold value in step  97 . 
     If the current of the input signal is greater than the threshold value, the diode conducts and diverts the excess current in step  99 , otherwise, the diode does not turn on and represents only a minimal, mostly capacitive load on the input in step  101 . 
     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.