Abstract:
Power supply noise affects the performance of many amplifier circuits. Power supply noise rejection circuits are typically used in conjunction with amplifier circuits to reduce the effects of the noise. Unfortunately, the main issue with a transimpedance amplifier (TIA) is that it has a single input port and a single output port, and the output ports are often required to be of a differential type in order to interface with a differential input post amplifier circuit. As a result, the conversion from single input port operation to a dual input port configuration for differential operation is often the cause of poor power supply noise rejection. A circuit is thus provided that overcomes the limitations in the prior art by providing a differential TIA for use with a filter circuit and differential amplifier that overcomes the limitations of the prior art.

Description:
FIELD OF THE INVENTION  
       [0001]     The invention relates to the field of amplifier circuits and more specifically to the field of power supply noise rejection circuitry for use with a transimpedance amplifier circuit.  
       BACKGROUND OF THE INVENTION  
       [0002]     The ever increasing demands for high capacity communications systems has resulted in a wide spread deployment of optical fiber networks across the world. A fundamental component used in such systems receives pulses of light and converts these into electrical signals. The pulses of light in such systems comprise a bit stream of information. This fundamental component employed in the fiber optic networks is commonly known as an optical receiver module. Within the optical receiver, a photodetector is typically employed to receive the light pulses and an amplifying circuit is employed for amplifying photocurrent generated within the photodetector.  
         [0003]     Transimpedance amplifiers (TIAs) are typically used within optical receiver modules to amplify and transform weak photocurrents received from the photodetector, in the form of a photodiode or a PIN diode. The TIA transforms the photocurrent into an output voltage that is further provided to other stages of the optical receiver module. Since TIAs are used to deal with both strong and weak photocurrents, noise in the resultant amplification and transformation to a voltage signal is typically a problem. Indeed, for those skilled in the art of the design of TIAs, it is well understood and appreciated that the noise introduced by the TIA, in many circumstances, limits the ability of the optical receiver module to faithfully reconstruct the intended stream of information. Furthermore, a relationship between the rate at which errors are produced by the receiver—often called the Bit Error Rate (BER), and the noise generated by the TIA can be shown. Thus, the optical receiver module needs to have low noise amplification performed on the weak photocurrents in order to facilitate optical transmission of information. This is especially true in circumstances where the distance that the optical signal must travel is long and results in weak optical pulses at the receiver. It is known to those skilled in the art that long transmission distances—the distance between a transmitter and a receiver—serves to attenuate the initial transmitted optical signal strength and places a greater burden upon the receiver module to avoid errors. Furthermore, it is also known that cost of an optical communication system is reduced if a signal is transmitted along a longer length of optical fiber or, in the alternative, if less optical power is transmitted. Thus, providing low noise amplification for the TIA is important in order to reduce the bit error rate (BER) of the received and amplified signal.  
         [0004]     However, if the power supply noise is present, this can appear at the output port of the TIA in the form of an interference signal along with output noise of the TIA. The resulting effect is to cause an optical penalty of the TIA with the effect of reducing optical sensitivity. Therefore, to preserve the low noise properties of the TIA, a high power supply rejection (PSR) is typically required at frequencies below 100 KHz. For frequencies above 100 KHz, off-chip power supply filters are typically used.  
         [0005]     Most amplifiers circuits offer a differential input signal and output signal mode of operation and as such the differential amplifier circuits are designed to have high gain and high common mode rejection. Unfortunately, the main issue with a transimpedance amplifier (TIA) is that it has a single input port and a dual output port, and the output ports are often required to be of a differential type in order to interface with a differential input post amplifier circuit. As a result, the conversion from single input port operation to a dual output port configuration for differential operation is often the cause of poor power supply rejection.  
         [0006]     A need therefore exists to provide a power supply noise rejection circuit for use with a TIA that allows for a reduction in amplifier output signal noise. It is therefore an object of the invention to provide a power supply noise rejection circuit for use with a TIA that provides power supply noise rejection using a reduced amount of circuitry, thus reducing power dissipation and semiconductor circuit area.  
       SUMMARY OF THE INVENTION  
       [0007]     In accordance with the invention there is provided a circuit for connecting to a power supply having power supply noise, the circuit for amplifying an input signal from a photodetector comprising: a transimpedance amplifier (TIA) circuit for operating in a differential manner and having a gain and a first input port for receiving the input signal from the photodetector, a second input port and an output port, the TIA for providing a TIA output signal comprising a voltage ripple signal dependent on the power supply noise and an amplified signal dependent on the input signal and the gain of the TIA; a first biasing circuit coupled to the second input port for providing a first bias signal thereto, the first bias signal including the voltage ripple signal dependent on the power supply noise; and, a filter circuit coupled to the output port of the TIA for filtering the TIA output signal to form a filtered signal, the filter circuit for filtering the TIA output signal in dependence upon an AC component of the first bias signal and for providing the filtered signal to a filter output port thereof.  
         [0008]     In accordance witrh the invention there is provided a method of performing power supply noise rejection for providing an output signal comprising the steps of: providing a TIA having two input ports; providing an input signal for amplification to a negative input port of the TIA; providing a first bias signal having a voltage ripple to a positive input port of the TIA; amplifying the input signal to form a portion of an amplified signal, the amplified signal comprising a noise signal representative of power supply noise and an amplified version of the input signal; filtering the amplified signal to form a filtered signal comprised of power supply noise; differentially amplifying the filtered signal and the amplified signal to provide an output signal from the amplifier circuit having an amplified version of the input signal and an attenuated version of the power supply noise.  
         [0009]     In accordance witrh the invention there is provided a single ended to dual conversion circuit comprising: a transimpedance amplifier (TIA) circuit for operating in a differential manner and having a gain and a first input port for receiving the input signal from the photodetector, a second input port and an output port, the TIA for providing a TIA output signal comprising a voltage ripple signal dependent on the power supply noise and an amplified signal dependent on the input signal and the gain of the TIA; a first biasing circuit coupled to the second input port for providing a first bias signal thereto, the first bias signal comprising the voltage ripple signal dependent on the power supply noise; a filter circuit coupled to the output port of the TIA for filtering the TIA output signal to form a filtered signal, the filter circuit for filtering the TIA output signal in dependence upon an AC component of the first bias signal and for providing the filtered signal to a filter output port thereof; and, a differential amplifier having a positive input port for receiving the TIA output signal and having a negative input port for receiving the filtered signal, the differential amplifier having a two output ports for providing a differential output signal therefrom, the differential output signal representative of an amplified difference between these two input signal and an attenuated other than a difference between these two input signals. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0010]     Exemplary embodiments of the invention will now be described in conjunction with the following drawings, in which:  
         [0011]      FIG. 1  illustrates a prior art amplifier circuit arrangement used to achieve a high degree of PSR for use with a TIA; and,  
         [0012]      FIG. 2  illustrates an amplifier circuit in accordance with an embodiment of the invention that utilizes a differential TIA, filter circuit and differential amplifier to overcome the limitations of the prior art.  
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0013]     The most commonly used technique to achieve power supply rejection (PSR) is to design amplifier circuits in a fully differential manner for receiving a differential input signal and for providing a differential output signal. However, transimpedance amplifiers (TIAs) typically provide a single ended input port for connection to a photodetector and a differential voltage output signal, where the single ended to differential conversion is typically performed on chip. The manner of this single ended to differential conversion determines the degree of Power Supply Rejection (PSR) achievable by the TIA.  
         [0014]      FIG. 1  illustrates a prior art amplifier circuit arrangement  100  used to achieve a high degree of PSR for use with a TIA  102 . A photodetector  101 , in the form of a PIN diode  101 , anode terminal is connected to the input port  102   a  of a TIA  102  while its cathode is connected to a first voltage input port  100   c  for receiving Vpos, or some other positive bias voltage, from a power supply (not shown). The output port  102   b  of the TIA  102  is connected using a feedback resistor  103  to the input port  102   a  of the TIA  102  for establishing a gain of the TIA  102 . The output port  102   b  of the TIA provides an output signal that has amplification—gain—dependent upon a value of the feedback resistor  103  as well as some unwanted power supply noise. The output signal of the TIA  102  is provided to a positive input port  104   a  of a differential amplifier circuit  104 .  
         [0015]     A replica of the TIA  105  is reproduced with its output port  105   b  connected to a negative input port  104   b  of the differential amplifier  104 . A feedback resistor  106 , having a same value as feedback resistor  103 , is disposed between the replica TIA  105  input port  105   a  and its output port  105   b . The replica, or dummy TIA, provides a same DC output voltage and the same amount of unwanted power supply noise as the TIA  102  connected to the photodiode. The differential amplifier  104  is therefore designed to have a high differential gain and a high common mode rejection (CMR), thus the output signals of the differential amplifier  104  preferably result in an amplification of a desired signal, which is the photocurrent generated by the photodetector  101  and attenuates the unwanted signal, which is the power supply noise.  
         [0016]     Although the circuit illustrated in  FIG. 1  achieves high PSR, it unfortunately has a number of disadvantages. The use of a dummy TIA stage  105  increases both integrated circuit (IC) die real estate, but also increases power dissipation of the IC. In addition, the noise of the dummy TIA  105  adds to the noise of the TIA  102  in an RMS manner, which in turn reduces the optical sensitivity of the amplifier circuit  100  by approximately 1.5 dB. Of course, this 1.5 dB optical sensitivity penalty is reduceable by decreasing the bandwidth of the dummy TIA  105  to a point where its noise contribution is minimized. However, in order to reduce the bandwidth a large ‘on chip’ capacitor is typically utilized, which further increase IC die area.  
         [0017]     An amplifier circuit  200  that overcomes the limitations of the prior art in accordance with an embodiment of the invention is shown in an embodiment illustrated in  FIG. 2 . Within the circuit  200  there is disposed a TIA  202  with a first input port  202   a  and a second input port  202   b . The first input port  202   a  is preferably a negative input port  202   a  and the second input port  202   b  is preferably the positive input port  202   b . The first input port  202   a  is connected to a photodetector  201 , preferably a photodiode  201  in the form of a PIN diode anode terminal, with the cathode of the photodiode  201  connected to the first voltage input port  200   c  for receiving a preferably positive input voltage (Vpos) from a power supply (not shown). The photodiode  201  provides a current (I signal ) to the TIA  202 . A shunt feedback resistor  213  is disposed between the first input port  202   a  and a TIA output port  202   c , the shunt feedback resistor  213  for propagating a feedback signal from the output port  202   c  affecting a gain of the TIA  202 . The second input port  202   b  is connected to a first biasing circuit for receiving a first bias signal therefrom. The first biasing circuit includes a third node e 3   233  having a resistor R 2   212  connected from this node  233  to the first voltage input port  200   c . A first current source I 1   220  is disposed in series between a second voltage input port  200   d , preferably for receiving a ground potential from the power supply (not shown), and the third node e 3   233 . A capacitor C 2   222  is disposed in parallel with the first current source  220  to limit thermal noise contributions of resistor R 2   212 .  
         [0018]     A voltage at a third node e 3   233  is determined by Equation (1):  
               a   .     
     ⁢   e3     =       Vpos   -       I   1     *   R2         1   +     S   *   C2   *   R2                 (   1   )             
 
 where S is the Laplace operator. From Equation (1) it is seen that the noise in the power supply (Vpos) is propagated to the positive input port  202   b  of the TIA  202  and the thermal noise from R 2  is preferablyattenuated by a pole formed by C 2 *R 2 . The output signal of the TIA  202  at a first node e 1   231  is determined by Equation (2):  
               a   .     
     ⁢     e   1       =           V   pos     -     I   *     R   2           1   +     S   *     C   2     *     R   2           -       I   signal     *     R   f                 (   2   )             
 
         [0020]     Equation (2) is based on the assumption that the TIA  202  has infinite open loop gain.  
         [0021]     The output signal from the TIA  202  is in the form of an amplified signal derived from amplification of the photocurrent (I signal ) received by the photodiode  201  as well as unwanted power supply noise, arising from the power supply connected to the first and second voltage input ports  200   c  and  200   d.    
         [0022]     The output port  202   c  of the TIA  202  is connected to a positive input port  204   a  of the differential amplifier  204  for providing the TIA output signal thereto. In addition, the TIA output signal is provided to a filter circuit  206 . The filter circuit  206  is formed from resistor R 1   211  and capacitor C 1   221 . A second node e 2   232  forms an output port of the filter circuit  206 , where this filter output port  232  is for providing a filtered signal and is coupled to the negative input port  204   b  of the differential amplifier  204  via a unity gain buffer  205 . The unity gain buffer  205  is used to prevent a DC offset between the input ports of the differential amplifier  204 . Capacitor C 1   221  is disposed between the second node e 2   232  and the positive input port  202   b  of the TIA  202 . With respect to the TIA output signal from output port  202   c , the filter circuit  206  acts as a low high pass filter, for attenuating high frequency components of filter output signal provided at the second node e 2   232 . With respect to the first bias signal provided to the positive input port  202   b  of the TIA  202 , the filter circuit  206  acts as a high pass filter, attenuates low frequency components of this signal at the output port of the second node e 2   232 .  
         [0023]     The voltage at the second node e 2   232 , or at the output port of the filter circuit  206 , is determined by Equation (3):  
               e   2     =             V   pos     -       I   1     *     R   2           1   +     S   *     C   1     *     R   1           ⁢     (     1   +     S   *     C   2     *     R   2         )       -         I   signal     *     R   f         1   +     S   *     C   1     *     R   1           +         S   *     C   1     *       R   1     ⁡     (       V   pos     -       I   1     *     R   2         )           1   +     S   *     C   1     *     R   1           ⁢     (     1   +     S   *     C   2     *     R   2         )                 (   3   )             
 
         [0024]     Assuming the differential amplifier  204  has a voltage gain G diff , an output signal differential voltage V out  provided as a potential difference on a positive output port (V outP )  200   a  and negative output port (V outN )  200   b  of the differential amplifier  204  is expressed by Equation (4): 
 
 a. V   out =( V   outP   −V   outN )= G   diff *( e   1 − e   2 )   (4) 
 
         [0025]     A small signal transimpedance gain (Tz) of the TIA  202  is determined by Equation (5):  
               a   .     
     ⁢   Tz     =       G   diff     *   Rf   ⁢       S   *   Cl   *     R   1         1   +     S   *     C   1     *     R   1                     (   5   )             
 
 where the small signal transimpedance gain (Tz) is determined by gain from the TIA  202  as a result of the feedback resistor Rf  213  and the differential amplifier  204  gain (G diff ). The power supply noise expressed as Vpos is advantageously attenuated. In addition, the inclusion of the RC network formed by the first resistor R 1   211  and the second capacitor C 1   221  determine a zero pole in the small signal gain, where the position of this zero pole is determined by C 1 *R 1 . 
 
         [0027]     The circuit in accordance with an embodiment of the invention shown in  FIG. 2  is absent the dummy TIA  105  used to DC bias the differential amplifier  104  (as shown in  FIG. 1 ). This advantageously reduces the power dissipation of the circuit. Furthermore, capacitor C 2   222  is used to reduce the thermal noise contribution of resistor R 2   212  and thus C 2   222  does not have a high capacitance value. Since capacitor C 2   222  does not have a high capacitance value it therefore does not occupy a large area when integrated to form the IC amplifier of  FIG. 2 . Thus, the PSR amplifier circuit  200  shown in  FIG. 2  requires less chip area and advantageously provides lower power dissipation without having a 1.5 dB optical penalty resulting from the use of a dummy TIA  105  ( FIG. 1 ).  
         [0028]     The TIA  202 , preferably operates as a unity gain voltage amplifier that provides both power supply noise as well as the desired signal I signal *Rf at the output port thereof. Advantageously, the filter circuit  206  formed from resistor R 1   211  and capacitor C 1   221  results in a significant rejection of power supply noise at the output ports  200   a  and  200   b  of the differential amplifier  204 .  
         [0029]     The TIA  202  preferably operates in a differential manner, where the negative input port  202   a  is used to provide a low impedance input port for amplifying input current (I signal ) from the photodiode  201  and the positive input port  202   b  is a high impedance input port that is used to provide the DC bias to the positive input port  202   b . The positive input port also allows for power supply noise in the form of a power supply voltage ripple signal (V ripple ) to appear at the TIA output port  202   c  with the power supply noise having a substantially unchanged amplitude, as found in Equation (6). 
 
 a. V   out   =V   ripple   −Rf*I   signal    (6) 
 
         [0030]     Advantageously, the first biasing circuit comprising the second resistor R 2   212  and the first current source I 1   220  are used to provide the bias voltage, in the form of the first bias signal, to the third node e 3   233 , as opposed to using a resistor potential divider network. Typically, many techniques in the prior art attempt to suppress the power supply ripple at the TIA stage from appearing at the TIA output port, which results in an addition of power supply noise to the desired output signal RfI signal . By allowing power supply ripple to appear in the TIA output signal with a substantially unchanged amplitude, the use of the filter circuit  206  advantageously allows for substantial canceling of the power supply ripple by the differential amplifier  204  when these signals are differentially amplified and provided to the output ports of the amplifier circuit  200   a  and  200   b . Of course, the differential amplifier preferably has circuitry therein for providing a high differential gain and a high common mode rejection (CMR).  
         [0031]     Further advantageously, the amplifier circuit shown in  FIG. 2  facilitates integration within an integrated circuit with a minimal external component count. Typically, the only external component that is provided with the integrated circuit  200  is a photodiode. This allows for disposing the circuit  200  in a confined space of an optical receiver module.  
         [0032]     Optionally, if the embodiment of the invention is used with lower data rates, for example data rates that have a bit transition frequency lower than in the GHz range, then the first capacitor C 1   221  used within the filter circuit  206  is preferably of a larger value and is thus preferably implemented as an external component. The use of a high valued first capacitor C 1   221  allows the low frequency TIA output signal to be substantially unfiltered by the filter circuit  206 .  
         [0033]     Numerous other embodiments may be envisaged without departing from the spirit or scope of the invention.