Abstract:
An optical receiver is disclosed that exhibits an improved phase margin and substantially constant output in response to changes in operating conditions (e.g., temperature, process, etc.). In accordance with the illustrative embodiment, a common-mode feedback comparison is performed prior to conversion of the signal from single-ended to differential voltage. When the common-mode feedback comparison is performed in this way, there are fewer amplifiers in the signal path and the phase margin of the common-mode feedback loop is increased. In addition, as the common-mode feedback is performed at the first stage of the transimpedance amplifier, the gain response of the transimpedance amplifier remains substantially constant in response to changes in temperature, input current range, and for different integrated circuit fabrication processes.

Description:
REFERENCE TO RELATED APPLICATIONS 
   This application claims the benefit of U.S. Provisional 60/613,370, filed Sep. 27, 2004, which is also incorporated by reference. 

   FIELD OF THE INVENTION 
   The present invention relates to electronics in general, and, more particularly, to transimpedance amplifiers. 
   BACKGROUND OF THE INVENTION 
     FIG. 1  depicts the salient components of an optical telecommunications system in the prior art. The system comprises: optical fiber  110  and optical receiver  100 , which itself comprises: photodetector  101 , transimpedance amplifier  102 , and receiver circuit  103 . Optical fiber  110  is used to carry optical signals, and optical receivers are used in optical communications systems to detect and convert optical signals into electrical signals that can be processed by electronic systems or converted into sound. 
   Photodetector  101  is a device that receives light from optical fiber  110  and outputs an electrical current that is based on the intensity of the light. Transimpedance amplifier  102  converts the current signal from photodetector  101  into a voltage signal for receiver circuit  103 . Receiver circuit  206  comprises electronics for processing the input signal in preparation for output to the rest of a processing system. 
   The quality of the output of the optical receiver is measured by a parameter known as the signal-to-noise ratio, which is the ratio of the magnitude of the output signal to the magnitude of the noise output by the optical receiver. Each component in transimpedance amplifier  102  is a source of some of the noise, and the manner in which transimpedance amplifier  102  is designed affects the quantity and quality of the noise. 
     FIG. 2  depicts the salient components of transimpedance amplifier  102  in the prior art. Transimpedance amplifier  102  comprises transimpedance stage  204 , amplification stage  205 , output stage  206 , feedback network  207 , and converter  208 . The input current signal from photodetector  101  is carried on line  109  to one of two inputs of transimpedance stage  204 . Transimpedance stage  204  converts the input current signal into a differential voltage signal (i.e. the signal comprises the difference between the voltages carried individually on each of two lines). Amplification stage  205 , which provides amplification of the voltage signal (i.e. gain), amplifies the differential voltage in a manner well-known to those skilled in the art. Output stage  206  provides an appropriate interface between the output of amplification stage  205  and receiver circuit  103 , in a manner well-known to those skilled in the art. 
   The differential voltage output signal on output lines  110   A  and  110   B  comprises a noise component known as Common-Mode Voltage. This common-mode voltage causes a voltage offset of the signal that degrades the performance of transimpedance amplifier  102 . Feedback network  207  is positioned between the differential output of transimpedance amplifier  102  (i.e., the output of output stage  206 ) and the two inputs of transimpedance stage  204  for the purpose of canceling the voltage offset associated with the common-mode voltage. Converter  208  converts the voltage signals on each of the two output lines of feedback network  207  into current signal inputs for transimpedance stage  204 . In some cases, converter  208  is included as part of transimpedance stage  204 , and in other cases it is included as part of feedback network  207 . 
   Since feedback network  207  spans the entire topology of the signal path of transimpedance amplifier  102 , the total phase shift in the feedback signal is equal to the sum of the phase shift contributed by each of the components in the signal path. The signal-to-noise ratio of transimpedance amplifier  102  is, therefore, degraded by the phase shift associated with the feedback signal. 
   Transimpedance amplifier  102  is required to provide enough amplification to provide an input signal to receiver circuit  103  with a suitable voltage level. However, too much noise is introduced into the output signal of transimpedance amplifier  102  if transimpedance stage  204  has a transimpedance gain that is too high. These competing requirements lead to a need to include amplification stage  205 , and they also determine the magnitude of signal amplification required of amplification stage  205 . Similarly, the gain of any one amplifier included in amplification stage  205  should be kept relatively low so as to reduce generated noise on the signal. As a result, in order to obtain sufficient amplification from amplification stage  205  to ensure maximum signal strength at the input of output stage  206 , amplification stage  205  comprises a cascade of many limiting amplifiers—each of which introduces some amount of phase shift which further degrades the signal-to-noise ratio of transimpedance amplifier  102 . 
   It is often difficult build a transimpedance amplifier with a lot of gain, but low phase shift, and, therefore, a trade-off exists between gain and phase shift. Furthermore, it is challenging to suppress the voltage offset caused by the common-mode voltage without inducing phase shift. 
   Therefore, the need exists for an optical receiver that avoids or mitigates some or all of these problems. 
   SUMMARY OF THE INVENTION 
   The present invention enables an optical receiver that avoids some of the costs and disadvantages found in the prior art. In particular, the illustrative embodiment comprises a transimpedance that exhibits an improved phase margin and substantially constant output in response to changes in operating conditions (e.g., temperature, process, etc.). 
   In accordance with the illustrative embodiment, a common-mode feedback comparison is performed prior to conversion of the signal from single-ended to differential voltage. When the common-mode feedback comparison is performed in this way, there are fewer amplifiers in the signal path and the phase margin of the common-mode feedback loop is increased. In addition, as the common-mode feedback is performed at the first stage of the transimpedance amplifier, the gain response of the transimpedance amplifier remains substantially constant in response to changes in temperature, input current range, and for different integrated circuit fabrication processes. 
   The illustrative embodiment comprises an optical telecommunications system comprising: (1) an optical fiber; (2) a photodetector for generating an electric current based on an optical signal from said optical fiber; (3) a transimpedance amplifier for generating an output voltage based on said electric current; and (4) a receiver circuit for processing said output voltage; wherein said transimpedance amplifier comprises: (a) an amplification stage comprising a first input, a second input, a first output, and a second output; and (b) a feedback network comprising an input, a first output, and a second output; wherein said first input of said amplification stage is also said input of said feedback network; and wherein said second input of said amplification stage is said second output of said feedback network. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  depicts the salient components of an optical telecommunications system that utilizes an optical receiver as is known in the prior art. 
       FIG. 2  depicts the salient components of a transimpedance amplifier, as is known in the prior art. 
       FIG. 3  depicts a schematic diagram of the salient components of a transimpedance amplifier in accordance with the illustrative embodiment of the present invention. 
       FIG. 4  depicts a schematic diagram of the salient components of an amplification stage in accordance with the illustrative embodiment of the present invention. 
       FIG. 5  depicts a schematic diagram of the salient components of a feedback network in accordance with the illustrative embodiment of the present invention. 
       FIG. 6  depicts a circuit schematic comprising the salient components of the illustrative embodiment of a transimpedance stage in accordance with the present invention. 
   

   DETAILED DESCRIPTION 
     FIG. 3  depicts a schematic diagram of the salient components of transimpedance amplifier  302  in accordance with the illustrative embodiment of the present invention. In accordance with the illustrative embodiment, transimpedance amplifier  302  exhibits a gain of 104 dBΩ. It will be clear to those skilled in the art, however, after reading this specification, how to make and use embodiments of the present invention that have a different gain. 
   Transimpedance amplifier  302  comprises: transimpedance stage  304  (comprising signal input  109 , bias input  312 , and output  310 , amplification stage  305 , output stage  206 , and feedback network  307 , interconnected as shown. 
   Transimpedance stage  304  converts a current at its signal input into a single-ended voltage signal at its output. In accordance with the illustrative embodiment, transimpedance stage  304  has a transimpedance gain of 60 dBΩ. The details of transimpedance stage  304  are described in detail below and with respect to  FIG. 6 . 
   Amplification stage  305  amplifies the voltage difference between its first and second inputs and provides a differential input signal on its first and second outputs to output stage  206 . The gain of amplification stage  305  is determined by the gain of transimpedance amplifier  302 . The details of amplification stage  305  are described in detail below and with respect to  FIG. 4 . 
   Output stage  206  provides an impedance-matched differential signal path between amplification stage  305  and the input load impedance of receiver circuit  103 . It will be clear to those skilled in the art how to make and use output stage  206 . 
   Feedback network  307  provides a common-mode feedback (CMFB) path that is used to affect the biasing of transimpedance stage  304 . This is desirable because it enables a desired common mode voltage shift at the output of output stage  206 . The details of feedback network  307  are described in detail below and with respect to  FIG. 5 . 
     FIG. 4  depicts a schematic diagram of the salient components of amplification stage  305  in accordance with the illustrative embodiment of the present invention. Amplification stage  305  comprises four differential limiting amplifiers,  411   1  through  411   4 , interconnected as shown. 
   Each of limiting amplifiers  411   1  through  411   4  comprises a differential input and a differential output. The gain of each of limiting amplifiers  411   1  through  411   4  is 11 dB, and, therefore, amplification stage  305  has a total gain of 44 dB. 
   Although the illustrative embodiment uses four differential limiting amplifiers, it will be clear to those skilled in the art, however, after reading this specification, how to make and use embodiments of the present invention which: 
   i. use any number of amplifiers, or 
   ii. use any kind of amplifier (e.g., non-differential, non-limiting, etc.), or 
   iii. any combination of i and ii. 
   Moreover, although limiting amplifiers  411   1  through  411   4  do not comprise individual feedback loops to maintain their individual common mode voltages, it will be clear to those skilled in the art, after reading this specification, how to make and use alternative embodiments of the present invention in which amplification stage  305  comprises one or more limiting amplifiers that each comprise an individual feedback loop to maintain its individual common mode voltage. 
     FIG. 5  depicts a schematic diagram of the salient components of feedback network  307  in accordance with the illustrative embodiment of the present invention. Feedback network  307  provides a common-mode feedback comparison for transimpedance amplifier  302 , wherein the comparison is performed before the signal is converted from single-ended to differential voltage. Since the comparison is performed before the signal passes through amplification stage  305 , the phase margin of the feedback loop is improved. Feedback network  307  comprises: low-pass filter  515 , voltage reference  516 , and differential amplifier  517 , interconnected as shown. 
   Low-pass filter  515  comprises a unity-gain buffer followed by a resistor and capacitor low-pass filter combination as is well-known by those skilled in the art. The output of low-pass filter  515  is electrically connected to the second input of amplification stage  305  and the first input of differential amplifier  517 . Low-pass filter  515  passes the low-frequency components of signal  310  to amplification stage  305  as one of two differential inputs such that the output of amplification stage  305  comprises just those frequency components higher than the cut-off frequency of low-pass filter  515 . 
   The output of low-pass filter  515  is also electrically connected to the first input of differential amplifier  517 , which acts as a comparator that compares the output of low-pass filter  515  to voltage reference  516 , wherein voltage reference  516  is chosen to be substantially equal to 1.32 V. The voltage of voltage reference  516  is chosen so as to be suitable to keep limiting amplifier  411   1  in proper operational region. The voltage choice depends upon the desired gain and bandwidth of transimpedance amplifier  302 , and it will be clear to those skilled in the art, after reading this specification, how to make and use embodiments of the present invention that utilize a voltage reference that has a different voltage. The output of differential amplifier  517  is electrically connected to the second (i.e., bias) input of transimpedance stage  304  as depicted in  FIGS. 3 and 6 . 
   Although low-pass filter  515  comprises an operational amplifier-based unity-gain buffer followed by a resistor and capacitor low-pass filter combination, it will be clear to those skilled in the art, after reading this specification, how to make and use embodiments of the present invention wherein low-pass filter  515  comprises a Butterworth filter, a Chebyshev Filter, or other suitable low-pass filter. 
   Feedback network  307  and transimpedance stage  304  form a negative feedback loop for transimpedance amplifier  302 , wherein the feedback loop has a phase margin of 60 dB. An important consideration for the negative feedback loop is that it has a positive phase margin for amplifier operation to be stable. The gain of transimpedance stage  304  is 60 dB, the gain of the operational amplifier of the unity-gain buffer in low-pass filter  515  is 50 dB, and the gain of differential amplifier  517  is 40 dB, therefore the feedback loop is stable. It will be clear to those skilled in the art, after reading this specification, how to make and use alternative embodiments of the present invention comprising stable feedback loops that have different individual component gain values. 
     FIG. 6  depicts a schematic of the salient components of transimpedance stage  304  in accordance with the illustrative embodiment of the present invention. Transimpedance stage  304  comprises: transistor  620 , transistor  621 , and transistor  622 , resistor R 1 , and resistor R 2 , interconnected as shown. 
   Transimpedance stage  304  converts and amplifies input current signal  109  from photodetector  101  into single-ended voltage signal  310 , which is applied to the first input of amplification stage  205  and the input of feedback network  307 . 
   Transistors  620 ,  621 , and  622  are fabricated in a 0.18 μm-CMOS process. Transistor  620  comprises a gate that has a width and a length of 16.56 μm and 0.18 μm, respectively. Transistor  621  comprises a gate that has a width and a length of 6.98 μm and 0.18 μm, respectively. Transistor  622  comprises a gate that has a width and a length of 17.72 μm and 0.18 μm, respectively. R 1  has a resistance of 1000Ω, and R 2  has a resistance of 700Ω. It will be clear to those skilled in the art how to make and use transistors  620 ,  621 , and  622 , and resistors R 1  and R 2 . 
   The input impedance of transimpedance stage  304  is a function of the gain of transistor  620  and the value of resistor R 1 . It is desirable to have an input impedance of transimpedance stage  304  that matches the impedance of photodetector  101 , which is 30 to 50 Ohms. Therefore, resistor R 1  has a value substantially equal to 1000Ω and resistor R 2  has a value substantially equal to 700Ω. It will be clear to those skilled in the art, after reading this specification, how to make and use alternative embodiments of the present invention that utilize resistors that have other values of resistance. 
   Although transistors  620 ,  621 , and  622  are CMOS transistors fabricated in a 0.18 μm-CMOS process, it will be clear to those skilled in the art, after reading this specification, how to make and use embodiments of the present invention that comprise transistors made in any CMOS technology, or any other suitable IC fabrication technology. 
   It is to be understood that the above-described embodiments are merely illustrative of the present invention and that many variations of the above-described embodiments can be devised by those skilled in the art without departing from the scope of the invention. For example, in this Specification, numerous specific details are provided in order to provide a thorough description and understanding of the illustrative embodiments of the present invention. Those skilled in the art will recognize, however, that the invention can be practiced without one or more of those details, or with other methods, materials, components, etc. 
   Furthermore, in some instances, well-known structures, materials, or operations are not shown or described in detail to avoid obscuring aspects of the illustrative embodiments. It is understood that the various embodiments shown in the Figures are illustrative, and are not necessarily drawn to scale. Reference throughout the specification to “one embodiment” or “an embodiment” or “some embodiments” means that a particular feature, structure, material, or characteristic described in connection with the embodiment(s) is included in at least one embodiment of the present invention, but not necessarily all embodiments. Consequently, the appearances of the phrase “in one embodiment,” “in an embodiment,” or “in some embodiments” in various places throughout the Specification are not necessarily all referring to the same embodiment. Furthermore, the particular features, structures, materials, or characteristics can be combined in any suitable manner in one or more embodiments. It is therefore intended that such variations be included within the scope of the following claims and their equivalents.