Abstract:
The invention relates to a feedback circuit for a transimpedance amplifier, which is typically used for converting an input current from a photodiode into an output voltage. The feedback circuit of the present invention linearizes the transconductance feedback, as the input current signal varies, by providing a constant current source for supplementing the DC feedback current through a bypass transistor, thereby reducing a variation in the low frequency cut off.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]     The present invention claims priority from U.S. Patent Application No. 60/529,663 filed Dec. 15, 2003, which is incorporated herein by reference. 
     
    
     TECHNICAL FIELD  
       [0002]     The present invention relates to a feedback circuit for a transimpedance amplifier (TIA), and in particular to a feedback circuit for linearizing the transconductance feedback in a transimpedance amplifier used in combination with a photodiode.  
       BACKGROUND OF THE INVENTION  
       [0003]     A level restoration circuit in a transimpedance amplifier removes the DC component, i.e. the average value which carries no information, of an optical signal exiting an optical fiber onto a photodiode, while at the same time keeping the low frequency −3 dB frequency low enough to meet requirements for both Telecom and Datacom applications.  
         [0004]     With reference to  FIG. 1 , a conventional TIA circuit, generally indicated at  1 , converts the current I PD  exiting a photodiode  2 , into an output voltage V OUT . The photodiode current I PD , which enters the TIA circuit  1  at an input terminal  3 , includes both a DC component and an AC component. The AC component, which carries the information, must be maintained and sent down an amplification chain  4  to final receiving equipment (not shown), while the DC component should be ignored and if possible eliminated. A feedback circuit, generally indicated at  5 , removes the DC component by means of negative feedback, implemented by a feedback amplifier  6 /low pass filter (i.e. Capacitor  7 ) and a bypass transistor  8  combination. The feedback amplifier  6 /low pass filter  7  has gain, and removes the AC component of a voltage feedback signal V FB , leaving only a DC component V FBDC . The capacitor  7  is used to set the low-frequency cutoff that the TIA circuit  1  requires. The bypass transistor  8  takes that DC component V FBDC  of the voltage feedback signal V FB  and generates a DC current I FBDC  in the collector  9 , which by the action of negative feedback equals the incoming DC current I PDDC  from the photodiode  2 . Accordingly, the DC component I PDDC  is removed from the incoming signal I PD  and passed to the ground GRND through the emitter  11  of the bypass transistor  8 .  
         [0005]     Unfortunately, the low frequency −3 dB cut off frequency, i.e. the low frequency cut off, of the TIA  1  can vary dramatically depending on the input current I PD , which makes meeting performance requirements difficult. The low frequency cut off of the entire wideband TIA  1  is proportional to the gain of the feedback circuit  5 , as well as the size of the filtering capacitor  7 . The gain of the feedback circuit  5  is a transconductance because the feedback circuit  5  samples the differential output voltage, V out =Outp−Outm, and produces a current I FBDC  at the collector  9  of the bypass transistor  8 . The transconductance gain of the bypass transistor  9 , according to basic small signal transistor theory is the collector current divided by the thermal voltage (G T =I c /V t =I FBDC /V t ), i.e. the gain varies with the DC current I FBDC  flowing in the device. Accordingly, since the transconductance gain of the whole feedback circuit  5  is proportional to the transconductance gain of the bypass transistor  8 , the low frequency cut off is proportional to the transconductance of the bypass transistor  8 . This can cause a large variation in the low frequency cut off, since the photodiode current I PD  can vary over a large range of values. For example: the photodiode DC current I PD  can vary from about 10 uA, up to about 1 mA, which makes a 40 dB of difference between the low and high values of the transconductance gain of the bypass transistor  8 , i.e. 20*log(1 mA/10 uA). Accordingly, the variation in transconductance gain causes the low frequency cut off to also vary significantly, i.e. if the low frequency cut off is set to 50 kHz at a low input current, the low frequency cut off could get as high as 5 MHz at a high input current, which is quite unacceptable in many applications including Ethernet and Sonet Telecom. Moreover, the filter capacitor  7  must be designed to be large enough to maintain the low frequency cut off small, which can result in unreasonably large capacitors affecting the size of the required packaging.  
         [0006]     U.S. Pat. No. 6,404,281, issued Jun. 11, 2002 in the name of Kobayashi et al; U.S. Pat. No. 6,504,429, issued Jan. 7, 2003 to Kobayashi et al; and U.S. Pat. No. 6,771,132 issued Aug. 3, 2004 to Denoyer et al disclose improvements to TIA feedback circuits that include minimizing the upper limit of the low frequency cut off frequency; however, none of these references addresses the problem caused by the variation in transconductance gain.  
         [0007]     An object of the present invention is to overcome the shortcomings of the prior art by providing a feedback circuit with a relatively small transconductance gain variation resulting in relatively small variation in the low frequency cut off frequency over a range of photodiode input currents.  
       SUMMARY OF THE INVENTION  
       [0008]     Accordingly, the present invention relates to a transimpedance amplifier comprising: 
        an amplifier circuit for converting a variable input current signal, which has AC and DC components, into an output voltage; and     a feedback circuit generating a feedback voltage signal with AC and DC components from the output voltage.        
 
         [0011]     The feedback circuit including: 
        a feedback amplifier/low pass filter, defining a low frequency cut off, for removing the AC component of the feedback voltage signal;     a bypass transistor, which receives the DC component of the feedback voltage signal and generates a DC feedback current substantially equal to the DC component of the input current signal for removing the DC component of the input current signal, the feedback circuit having a feedback gain; and     a linearizing circuit for linearizing the feedback gain as the input current signal varies, including a constant current source for supplementing the DC feedback current through the bypass transistor, thereby reducing a variation in the low frequency cut off caused by the variation in the input current.       
 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0015]     The invention will be described in greater detail with reference to the accompanying drawings which represent preferred embodiments thereof, wherein:  
         [0016]      FIG. 1  illustrates a conventional TIA amplifier circuit with feedback circuit;  
         [0017]      FIG. 2  illustrates a TIA amplifier circuit with feedback circuit according to the present invention; 
     
    
     DETAILED DESCRIPTION  
       [0018]     The present invention solves the aforementioned problem by linearizing the feedback network gain, which removes the variability of the feedback gain, and hence the variability in the low frequency cut off of the TIA circuit. As illustrated in  FIG. 2 , a TIA circuit  11  receives photodiode current I PD  from photodiode  12  via input terminal  13 . Amplification chain  14  amplifies and converts the input current I PD  producing differential output voltage V OUT =Outp−Outn.  
         [0019]     The linearization is accomplished by a feedback network  15  including a feedback amplifier  16 /low pass capacitor filter  17  combination, and a bypass circuit  18 . The bypass circuit  18  includes an NMOS transistor N 1 , which provides a voltage level shift to the gate of a PFET transistor P 1 . The PFET transistor P 1  and a gain resistor R gm  convert that voltage into a current I FB , which is mirrored by a first NPN transistor Q 3  to a second NPN transistor Q 4  for removal of the DC component I PDDC  from the incoming photodiode current I PD . However, to reduce the variability of the transconductance gain of the PFET transistor P 1  as the input current I PD  from the photodiode  12  varies, a constant current source  21  supplements the feedback current I FB  with a constant current I hot , which is always maintained in the PFET transistor P 1 . I hot  causes P 1  to be always “on” and to amplify at a more consistent gain level. I hot  may be produced by a bandgap current source or by a voltage reference. Ideally, I hot  is equal to or greater than the typical high value of the photodiode current, and preferably two to three times greater. The small signal transconductance of the PFET transistor P 1  is given by gm(p1)=sqrt(2*K′ *W*Id/L), from basic transistor theory, where K′ is a process parameter, W and L are the width and length of the PFET, and I d  is the drain current flowing in the PFET.  
         [0020]     Using the example cited above, the variation in the drain current Id, assuming a constant current I hot  of 2 mA, is from 2.01 mA for the low DC optical input signal (2 mA I hot +10 uA I FB ) to 3 mA for the high DC optical input signal (2 mA I hot +1 mA I FB ). Accordingly, the variation in the return feedback transconductance is only 1.7 dB instead of 40 dB, i.e. 20*log(sqrt(3 mA)/sqrt(2.01 mA)). The reduction in the variability, enables the low frequency cut off frequency to be kept low over all input current ranges, which means that applications for wideband transimpedance amplifiers like Sonet and Ethernet are more easily met with the same TIA. Furthermore, the value of the capacitor  17  can be much lower than previously, because the gain variation of 100:1 has been eliminated. In the prior art, the capacitor  7  would be required to be 100× larger than with the present invention, i.e. the size of the capacitor  17  can be kept so small that it could be integrated on a chip, eliminating the need for a separate external capacitor.  
         [0021]     Since the DC input current can be very low, the excess current (above I hot ) through Rgm must also go very low. This is accomplished easily over process, temperature, and supply voltage variations by making P 1  smaller than N 1 , e.g. W p L p &lt;W n /L n , as well as by making the current through the P 1  larger than that in N 1  by a factor of n as implemented by the current mirror pair I hot  and I ls . This allows the current through Rgm to be brought as low as the input signal requires.  
         [0022]     Note also, for extra flexibility, the relative sizes of Q 3  and Q 4  can be mismatched to produce a current mirror that is not necessarily 1:1; however, Q 3  and Q 4  should be matched, if possible. The current multiplication can also be accomplished by taking linear combinations of Q 3  and Q 4  transistors to produce the desired output current. Multiplication Factor=(# of Q 4  transistors)/(# of Q 3  transistors) assuming equal geometries in Q 4  and Q 3 . The specific transistors can be other equivalent devices, e.g. the PFET transistor P 1  could be a PNP transistor, the NPNs Q 3  and Q 4  could be NFET transistors, and N 1  could be an NPN emitter follower.