Abstract:
A trans-impedance amplifier receives an input current and is operable to generate an output voltage responsive to the input current. The amplifier is responsive to an increased range of input currents and has a wide bandwidth. The amplifier includes an input stage having a first and a second transistor and is configured to receive the input current. The amplifier includes an output stage coupled to the input stage and having a third and a fourth transistor. A variable resistor is coupled to the output stage to adjust the amount of current in the output stage. A variable current source is coupled to the output stage and is operable to adjust the amount of current in the output stage. A output driver, which is coupled to the output stage, includes at least another transistor. The output driver is operable to provide the output voltage and is operable to reduce the output impedance of the amplifier.

Description:
FIELD OF THE INVENTION 
   The invention relates generally to communication systems. In particular, the invention relates to a trans-impedance amplifier for current to voltage conversion in communication systems. 
   BACKGROUND OF THE INVENTION 
   Trans-Impedance Amplifiers (TIAs) are used for current to voltage conversion in communication systems. For example, in optical communication systems, TIAs are used for current to voltage conversion in a power control loop to control a LASER or a Light Emitting Device (LED). The terms “LED” and “LASER” are used interchangeably hereinafter. The power contained in the light emitted by the LED is controlled by adjusting the current supplied to the LED. The TIAs are used in many other applications besides power control in a communication system. 
     FIG. 1  illustrates a Power Control Loop  100  used in a communication system. Light emitted by a LED  104  is detected by a monitor photo detector  108 . The photo detector  108  converts the light into a current  112 , the amount of the current  112  being proportional to the power contained in the light. The current  112  is received by a Trans-Impedence Amplifier (TIA)  116 , which generates a voltage  120  responsive to the current  112 . A comparator circuit  124  compares the voltage  120  to a reference voltage  128 . If the voltage  120  is higher than the reference voltage  128 , the comparator circuit  124  instructs a laser driver  132  to increase the current supplied to the LED  104 . If the voltage  120  is lower than the reference voltage  128 , the comparator circuit  124  instructs the Laser Driver  132  to decrease the current supplied to the LED  104 . Accordingly, the laser driver  132  adjusts the current supplied to the LED  104 . 
   As will be understood by those skilled in the art, in order to precisely control the LED power, the TIA  116  must maintain its linearity error below a desired percentage. Also, the TIA  116  must have the necessary bandwidth to be responsive to the high-speed output of the LED  104 . If the output of the LED  104  is 2.5 Gb/s, but the TIA  116  has a low bandwidth (e.g., 100 MHz bandwidth), the TIA  116 &#39;s output will contain error. Also, the output current from different photo detectors often vary even when the photo detectors are paired with the same LED because of variations in process and manufacture. Thus, the TIA  116  must be able to operate with the same speed and accuracy for a wide range of input currents from the photo detector  108 . Also, as will be understood by those skilled in the art, the parasitic capacitance of the photo detector  108  can vary significantly and can often be as high as 15 pF. The variation of the parasitic capacitance of the photo detector  108  is caused by the reverse bias variation of the photo detector  108  and also caused by the fact that system manufacturers frequently change photo detectors. Thus, the TIA  116  must be tolerant of the variations of the parasitic capacitance. 
   Several variations of TIAs are currently being used.  FIG. 2  illustrates a TIA  200  with a high gain differential amplifier and a negative feedback. A photo detector  204  detects light emitted by a LED (not shown in  FIG. 2 ) and generates a current Iin  208 . An operational amplifier  212  is coupled to the photo detector  204 . The operational amplifier  212  has a non-inverting input terminal  216 , an inverting input terminal  220 , and an output terminal  224 . A feedback resistor  228  is connected between the output terminal  224  and the inverting input terminal  220  of the operational amplifier  212 . A bias voltage V bias    232  is connected to the non-inverting input terminal  216 . The output voltage signal is −I in *R. 
   In order for the TIA  200  to have high speed capability (i.e., large bandwidth), the operational amplifier  212  must have a large bandwidth. For example, if a several hundred MHz TIA bandwidth is desired, the operational amplifier  212  must have a bandwidth of several GHz, which is difficult to achieve. Also, when different photo detectors  204  are used, different currents may be generated by the photo detectors even when paired with the same LED. For example, if the output current of the photo detector  204  increases by a factor of 3, the feedback resistor  228  value must be decreased accordingly so that the output voltage of the operational amplifier  212  is not distorted. However, if the feedback resistor  228  becomes too small, the amplifier bandwidth may not be large enough to ensure stability. Thus, adjusting the feedback resistor value to achieve linearity across a desired input current range affects the stability of the circuit. Furthermore, if the photo detector  204  is replaced with another that has a different parasitic capacitance, the operational amplifier  212  may not have enough bandwidth to ensure stability. 
     FIG. 3  illustrates a TIA  300  with a current mirror. A photo detector  304  generates a current Iin  308  responsive to light emitted by a LED (not shown in  FIG. 3 ). A current mirror circuit is coupled to the photo detector. The current mirror circuit is formed by two NMOS transistors  312  and  316  and a resistor  324 . The transistor  312  is supplied with a bias current Ibias  320  from a supply voltage V dd    322 . The transistor  316  is connected to the voltage V dd    322  via the resistor  324 . The gate and the drain of the transistor  312  are shorted. The current flowing into the transistor  312  is equal to (I in +I bias ). If the transistors  312  and  316  have the same size, the output voltage V out    328  is equal to V dd −R*(I bias +I in ). The signal portion of the output voltage is equal to −R*I in . However, if the transistor  316 &#39;s size is X times the transistor  312 &#39;s size, then V out    328 =V dd −X*R*(I bias +I in ) and the signal portion of the output voltage is −X*R*I in . 
   The TIA  300  lacks a large bandwidth because the parasitic capacitance (not shown in  FIG. 3 ) of the photo detector  304  is added to the parasitic capacitances of the transistors  312  and  316 . Also, while the TIA  300  provides good linearity because the load resistor  322  can be programmed independently, especially if a cascaded structure is used, the bandwidth of the TIA  300  is dependent on the input impedance. To mitigate the variation of the input impedance, and therefore the bandwidth, the current Ibias  320  is added to the input current. However, the addition of the current I bias    320  causes the current through the load resistor  324  to increase, thereby reducing the available voltage swing for the signal at the output node  328 . 
   Accordingly, there is a need for a TIA that does not suffer from the foregoing disadvantages. There is a need for a high-speed TIA that operates with accuracy in response to a wide range of input currents. 
   SUMMARY OF THE INVENTION 
   A trans-impedance amplifier receives an input current and is operable to generate an output voltage responsive to the input current. The amplifier is responsive to an increased range of input currents and has a wide bandwidth. The amplifier includes an input stage having a first and a second transistor and is configured to receive the input current. The amplifier includes an output stage coupled to the input stage and having a third and a fourth transistor. A feedback loop is coupled to the input stage. The feedback loop is operable to reduce the input impedance of the amplifier. A variable resistor is coupled to the output stage to adjust the amount of current in the output stage. A variable current source is coupled to the output stage and is operable to adjust the amount of current in the output stage. A output driver, which is coupled to the output stage, includes at least a sixth transistor. The output driver is operable to provide the output voltage and is operable to reduce the output impedance of the amplifier. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The following detailed description of the invention will be better understood when read in conjunction with the appended drawings. For the purpose of illustrating the invention, there are shown in the drawings embodiments which are presently preferred. It should be understood, however, that the invention is not limited to the precise arrangements and instrumentalities shown. In the drawings: 
       FIG. 1  is a Power Control Loop. 
       FIG. 2  is a Trans-Impedance Amplifier (TIA) with a high gain differential amplifier and a negative feedback loop. 
       FIG. 3  is a TIA implemented with a current mirror. 
       FIG. 4  is a TIA in accordance with one embodiment of the invention. 
       FIG. 5  shows an example input current and an output voltage signal. 
       FIG. 6  is a TIA in accordance with another embodiment of the invention. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 4  is a TIA  400  in accordance with one embodiment of the invention. As will be explained further, the TIA  400  includes a low impedance input stage decoupled from a low impedance output stage, thereby providing a high speed response. The TIA  400  also exhibits stability under a wide range of input currents. 
   A photo detector  404  generates a current I in    408  responsive to light emitted by a LASER or a LED (not shown in  FIG. 4 ). The terms “LASER” and “LED” are used interchangeably hereinafter. A parasitic capacitance C par    412  is associated with the photo detector  404 . The input current I in    408  is supplied to a current mirror formed by two NMOS transistors  416  and  420  and two bipolar transistors  424  and  428 . The input stage of the current mirror comprising the transistors  416  and  424  also receives a bias current I bias    432  from a supply voltage V dd    434 . The bias current Ibias  432  provides the necessary current to properly bias the input stage comprising transistors  416  and  424 . 
   As will be understood by those skilled in the art, the current flowing through the transistor  416  is mirrored in the transistor  420 . The transistor  424  is used as a cascode transistor, which reduces the TIA  400 &#39;s input impedance. 
   A CMOS transistor  444  creates a feedback loop around the input stage. The transistor  444  (implemented either as a CMOS or a bipolar transistor) also acts as a level shifting transistor. In one embodiment of the TIA wherein all transistors are bipolar transistors, the level shifting transistor  444  ensures that the transistor  424  remains in the linear region and does not saturate. In another embodiment of the invention wherein all transistors are CMOS transistors, the level shifting transistor  444  allows the transistor  424  to operate at a higher V DSAT , thereby providing higher speed. A bias current I bias2    446  is connected between the source of the transistor  444  and ground. The bias current I bias2    446  provides the necessary current to bias the transistor  444 . 
   The output stage of the current mirror comprises a variable resistor  436  and the transistors  420  and  428 . The variable resistor  436  is coupled between the transistor  428  and the supply voltage V dd    434 . By adjusting the value of the variable resistor  436 , the voltage V res  at the node  438  is controlled. In one embodiment, the variable resistor  436  is a programmable resistor that allows the resistance value to be adjusted and the trans-impedance gain of the TIA changed according to the input current I in    408 . 
   A bias voltage V ref    466  is connected to the base of the transistors  424  and  428 . The bias voltage V ref    466  allows the input and the output stages of the TIA  400  to operate in the saturation or the linear regions. 
   The current I in +I bias  conducts through the transistor  416 . If the transistors  416  and  420  have equal size, the current conducting through the transistor  420  will also be equal to I bias +I in . A variable offset current I offset    440  is supplied to the transistor  420 . The variable offset current I offset    440  provides a programmable current used to tune the current in the variable resistor  436 . The current in the variable resistor  436  is I out =I bias +I in −I offset . 
   Ideally, I offset    440  would be equal to I bias    432  so that only I in  (i.e., the signal current) flows in the resistor  436 , which significantly improves the dynamic range of the TIA  400  especially in a situation where I in    408  is much smaller than I bias    432 . In another embodiment of the TIA, I offset  can be used to correct mismatches between the transistors in the TIA and thereby provide improved accuracy in the current to voltage conversion. 
   As will be appreciated, when I in    408  increases, the current through the transistor  420  also increases. In existing current mirrors, the current through the resistor  436  will also increase, causing the voltage V res    438  to become too low. In the TIA  400 , however, the variable current I offset    440  and the variable resistor  436  value can both be adjusted, thereby increasing the voltage V res    438  yet at the same time maintaining the required current level through the transistor  420 . As will be appreciated, an increase in I offset    440  causes the current through the resistor to decrease, thereby reducing the voltage drop across the resistor  436  and increasing Vres  438 . Thus, when the input current Iin  408  increases, the offset current I offset    440  can also be increased, thereby preventing v res    438  from becoming too low. When the value of the resistor  436  is increased, V res    438  decreases and vice versa. Thus, the resistor value  436  can also be adjusted accordingly for proper operation of the TIA. 
   When I in    408  decreases, the current through the transistor  420  also correspondingly decreases. In existing current mirrors, the current through resistor  436  will also decrease, causing the voltage V res    438  to become too high. In the TIA  400 , the variable current I offset    440  and the variable resistor  436  can both be adjusted, thereby reducing the voltage V res    438  yet at the same time maintaining the required current level through the transistor  420 . Thus, the variable resistor  436  and the variable current I offset    440  allows the TIA  400  to operate within a wider range of input current I in    408 . 
   In one embodiment of the invention that is useful especially for very low input current I in    408 , the offset current I offset    440  can be tied to the node  438  allowing higher current through the transistor and improved bandwidth given that the current in the transistor  428  is I in +I bias . 
   A NMOS transistor  444  and the bias current I bias2    446  form a feedback loop in the input stage of the current mirror. Due to this feedback loop, the input impedance at the node  410  as seen by the photo detector  404  is low. Accordingly, the bandwidth of the input stage is large and thus high speed signal can pass through the input stage without being filtered. 
   The output stage of the current mirror comprises the transistors  420 ,  428  and the variable resistor  436 . In order to reduce the impedance of the output stage, a bipolar driver stage comprising a bipolar transistor  448  and a bias current I bias3    452  is added to the output stage. Since the bipolar transistor  448  has a lower impedance (looking into the emitter of the bipolar transistor) than a CMOS-type transistor (looking into the source of the CMOS transistor), the output impedance at the output node  456  of the TIA  400  is low. The bipolar driver stage allows the TIA  400  to drive loads with high parasitic capacitance. 
   In one example implementation of the TIA  400 , the values associated with various elements are as follows: 
   (1) R  436  value may be a few hundred Ohms to 1K Ohm. 
   (2) V dd    434  value may be 2.97V to 3.63V 
   (3) I bias    432 =1.2 mA 
   (4) I bias2    446 =500 uA 
   (5) I bias3    452 =500 uA 
   (6) I offset    440 =1 mA +/−50% 
   (7) C par    412  value may be 2 pF to 15 pF 
   It will be understood by those skilled in the art that the above values are provided as examples only, and other values can be used to implement the TIA. The implementations of I bias , I bias2 , I bias3 , Ioffset and variable resistor R are well understood by those skilled in the art, and thus will not be described here. 
   In one embodiment, a second programmable current source can be used in parallel to the variable current I offset    440 . The second programmable current source can be used during power-up of the TIA  400  to eliminate any offset voltage at the output of the TIA  400  due to components mismatch or process variation. 
   Since the characteristics of the photo detector  404  is known, using a programmable variable resistor  436  and a programmable variable current I offset    440 , a user can adjust the values of the variable resistor  436  and the I offset    440  to obtain a desired output voltage swing. 
   As discussed before, due to variations in process and manufacture, the TIA  400  can generate output errors such as its output voltage can be slightly higher or lower than expected. If the error is not corrected, a comparator circuit that compares the TIA output to a reference voltage will also generate an error. The programmability of the TIA  400  discussed before allows the errors to be corrected. 
     FIG. 5A  shows an example input current signal Iin  408  provided to the TIA  400  and an output voltage signal Vout generated by the TIA  400 . I in ( 1 ) is the input current corresponding to a level 1 and I in ( 0 ) is the input current corresponding to a level 0. Note that when Iin is at level 1, Vout is at level 0, and when Iin is at level 0, Vout is at level 1. 
     FIG. 6  shows a TIA  600  in accordance with another embodiment of the invention. As will be appreciated, the transistors in the TIA  600  are CMOS-type transistors. Unlike the TIA  400  shown in  FIG. 4 , the TIA  600  lacks a feedback transistor in the input stage. 
   In operation, a photo detector  604  generates a current I in    608  responsive to light emitted by a LED (not shown in  FIG. 6 ). The current Iin  608  is received by a current mirror formed by 4 CMOS transistors  616 ,  620 ,  624 , and  628 . As will be appreciated, the current mirror includes an input stage formed by the transistors  616  and  624  and an output stage formed by the transistors  620  and  628 . 
   A first bias current I bias    632  is provided to the transistor  624  for proper operation of the current mirror. A variable resistor  638  couples the transistor  628  to a supply voltage V dd    634 . A variable offset current I offset    640  provides a programmable current to tune the current flowing through the variable resistor R  638 . An output driver stage comprising transistor  648  and a second bias current I bias2    652  lowers the output impedance of the TIA  600 . The output of the TIA  600  is provided at the node  656 . An advantage of the TIA  600  is that its input impedance is significantly lower than the simple current mirror-type TIA shown in  FIG. 3 . As will be understood by those skilled in the art, the input impedance of the TIA in  FIG. 3  is 1/gm, wherein gm is the trans-conductance of the transistor  312 . In contrast, the input impedance of the TIA  600  is (1/gm1)/(gm2*Z2), where gm1 is the trans-conductance of the transistor  624 , gm2 is the trans-conductance of transistor  616  and Z2 is the output impedance seen looking at the drain of the transistor  624  when the loop is open. 
   It will be appreciated by those skilled in the art that changes could be made to the embodiments described above without departing from the broad inventive concept thereof. For example, the TIAs can be implemented with only CMOS-type transistors or with only bipolar-type transistors, or using both CMOS and bipolar type transistors. It is understood, therefore, that this invention is not limited to the particular embodiments disclosed, but it is intended to cover modifications within the spirit and scope of the present invention as defined by the appended claims.