Abstract:
An integrated circuit transimpedance amplifier for an optical receiver is formed by a bipolar process comprising fixed value resistive elements in a network to provide AGC. The resistive elements are arranged in a parallel configuration and a transistor switching network is provided to enable at least one of said elements to be isolated from a gain stage of the amplifier thereby providing a means of varying amplifier gain.

Description:
FIELD OF THE INVENTION 
     The present invention relates to a wide dynamic range transimpedance amplifier which finds application in optical receivers. The invention particularly relates to a bipolar IC transimpedance amplifier. 
     BACKGROUND OF THE INVENTION 
     The ever increasing demands for high capacity communications systems has seen the wide spread employment of optical fibre networks across the world. A fundamental component for such systems is a means of converting optical pulses comprising a digital bit stream into electrical signals. This component of such a system is commonly known as an optical receiver. 
     The operational requirements of such a receiver are very demanding. The receiver is required to exhibit a very low noise characteristic, such that it is capable of detecting very low levels of optical input in systems employing maximum optical fibre lengths, thus requiring high gain amplification for maximum sensitivity, but is conversely required to cope with high levels of optical input in systems employing short fibre lengths, thus requiring low gain amplification. As such, the optical receiver is required to have a wide dynamic range which can only be practically achieved with some form of automatic gain control (AGC). A typical integrated circuit (IC) optical receiver  10  is illustrated in block schematic form in FIG.  1 . This comprises an IC (denoted by broken line  12 ) including a transimpedance amplifier stage (denoted by broken line  14 ) with an integrator in a control loop providing AGC. 
     As illustrated by FIG. 1, optical input power OP IN  is converted into an electrical current I IN  by a PIN diode photodetector  16 . This current I IN  is applied as an input to the IC optical receiver  10 . The input current I IN  is amplified by a transimpedance amplifier (Tz Amp)  18  which converts the input current I IN  into an amplified voltage output signal V OUT . To meet the requirement of wide dynamic range the output voltage V OUT  of the Tz Amp  18  which is in the form of a broadband data signal and may be considered as an ac, multi-frequency signal, is rectified or peak detected by a rectifier/peak detector  20  to provide a dc signal level V REC  for comparison with a pre-determined dc reference voltage V REF . The difference between the rectified/peak detected output voltage V REC  and the reference voltage V REF  is considered as an error signal which is amplified and integrated by a Miller Integrator  22  to provide a control signal V CONTROL . A Miller Integrator is a well known form of integrator incorporating an active device such as a transistor amplifier. The Miller Integrator  22  is required to have a high gain, in order to ensure that the error signal approaches zero (ie in order to ensure that the difference between the rectified/peak detected output voltage V REC  and the reference voltage V REF  becomes zero) by means of controlling the gain of the Tz Amp by varying the impedance of a feedback resistor  24 . 
     If the rectified/peak detected output voltage V REC  is smaller than the dc reference voltage V REF , then the Tz Amp  18  must operate at high gain to provide high sensitivity of the optical receiver. When the rectified/peak detected output voltage V REC  becomes just greater than the dc reference voltage V REF , then the on-set of AGC occurs and continues whilst the input channel I IN  increases. When the feedback resistor  24  is at a minimum the Tz Amp is operating at very low gain and approaches an overload condition. 
     The most effective method of controlling the gain of the Tz Amp  18  is to vary the value Rf of the feedback resistor  24 , as shown by FIG.  1 . This can be achieved when using such technologies as CMOS, BiCMOS or GaAs MESFET were the availability of Field Effect Transistors (FETs) allows them to be configured as variable resistors connected in parallel with the feedback resistor  24  to vary the Tz Amp gain. To achieve the highest performance receiver GaAs MESFET technology is often used but at greatest expense. Alternatively, CMOS technology can offer the lowest cost but offers the poorest performance in terms of sensitivity. A good compromise is often provided by using BiCMOS technology, where a high performance is achieved through use of bipolar transistors for low noise while the gain control is achieved by the FET connected in parallel with the feedback resistor. The use of bipolar technology can achieve sensitivities comparable to GaAs MESFET technology, but poor gain control techniques may result in either lowering of the sensitivity of the receiver or providing an inadequate dynamic range. 
     OBJECTS OF THE INVENTION 
     The invention seeks to provide a bipolar IC transimpedance amplifier including gain control. 
     The invention also seeks to provide a bipolar IC transimpedance amplifier which obviates some of the problems encountered with other IC transimpedance amplifiers. 
     SUMMARY OF THE INVENTION 
     The present invention provides a transimpedance amplifier for an optical receiver comprising an input for receiving an input current signal from a photodetector and a gain stage for amplifying the input current signal. It also comprises a resistor network coupling the output and input of the gain stage, the resistor network comprising at least first and second resistive elements in a parallel arrangement and being arranged such that one of said resistive elements can be isolated from the output of the gain stage wherein the one of said resistive elements can be isolated from the output of the gain stage by means of a transistor switching network. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     For a better understanding of the invention and to show how the same may be carried into effect, there will now be described by way of example only preferred embodiments with reference to the accompanying drawings of which: 
     FIG. 1 is a block schematic diagram of a typical IC optical receiver with automatic gain control (AGC); 
     FIG. 2 is a block schematic diagram of a practical implementation of an optical receiver in accordance with the invention; 
     FIG. 3 is a circuit diagram for a transimpedance amplifier in accordance with the invention; 
     FIG. 4 is a circuit diagram illustrating a first configuration of a current control for the amplifier of FIG. 3; 
     FIG. 5 is a circuit diagram of a second configuration of a current control for the amplifier of FIG. 3; and 
     FIG. 6 is a circuit diagram of a second embodiment of a transimpedance amplifier in accordance with the invention. 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     In a practical implementation of an optical receiver, FIG. 1 is an oversimplification used to illustrate the general principles of operation. A more detailed design of a practical realisation of an optical receiver with AGC in accordance with the invention is shown in FIG.  2 . 
     The integrated circuit optical receiver  30  includes a Tz Amp  32  to amplify an input current I IN  received from a photodetector  34 . As in the typical receiver illustrated in FIG. 1, the receiver  30  includes a rectifier/peak detector  36  and integrator  38  in a control loop for providing AGC. However, the receiver  30  also includes a dummy Tz Amp  40  and a 50Ω line driver  42  which provides a differential output V DIFF . 
     As indicated in FIG. 2, the Tz Amp  32  and Dummy Tz Amp  40  are of the same type differing only in that the Dummy Tz Amp  40  is not connected to any input. Its purpose is to provide the correct DC voltage to bias the 50Ω line driver  42 . The 50Ω line driver  42  may be a long tail pair configuration with 50Ω collector resistors to interface to the outside world. In addition, the long tail pair configuration converts the single ended signal from the Tz Amp  32  into a differential signal at the output. This output differential signal is rectified/peak detected and compared to a reference DC voltage to determine the correct output signal level required for AGC. The output of the rectifier/peak detector  36  is applied to the integrator  38  to provide the required control voltages to both the Tz Amp  32  and Dummy Tz Amp  40 , thus maintaining a DC match at the input of the 50Ω line driver. An additional requirement of the integrator  38  is to provide a long time constant to prevent pattern dependent jitter of the digital data. 
     Referring to FIG. 3, a first embodiment of a bipolar IC transimpedance amplifier  50  comprises an input  52  for receiving an input current I IN  signal from an optical receiver photodetector (FIG. 2) such as a PIN diode and a gain stage comprising first and second transistors Q 1 , Q 2  for amplifying said input current signal I IN  to provide an output voltage signal V OUT . The gain stage includes a feedback loop coupling its output and input comprising a parallel network of feedback resistors Rfh and Rfl. Rfh has a value of resistance considerably larger than Rfl, in the order of 100:1. The feedback resistive network establishes the gain of the gain stage. As such, the network includes a transistor switching stage which enables the low value resistive element Rfl to be isolated from the gain stage input leaving only the high valve resistive element Rfh in circuit thus establishing high gain operation. The circuit also includes a second input  54  which receives an input voltage control signal V CONTROL  to control the transistor switching network. The voltage control signal V CONTROL  is taken from the optical receiver integrator stage (FIG.  2 ). The input control voltage V CONTROL  determines by its action on the transistor switching network whether the low value resistive element Rfl is isolated from the gain stage input or switched in circuit. 
     As shown in FIG. 3, if high gain, high sensitivity is required from the transimpedance amplifier, the control voltage (Vcontrol provided by the rectifier and integrator shown in FIG. 2) would be biased to turn on fully transistors Q 4  and Q 5  while turning off transistors Q 3  and Q 6 . This would result in current flow through transistor Q 1  determined by the collector resistor R 2 . Conversely, since Q 5  is turned on the current  12  would flow through resistors R 1  and R 3  provided just the required voltage drop to turn off transistors Q 8 , Q 10  and in turn Q 7 . In addition a control current connected to the emitter of Q 7  would also be turned off, isolating the feedback resistor Rfl. While current flows through R 2 , transistor Q 11  is biased on and connected to the feedback resistor Rfh via the emitter follower formed by transistor Q 9  and resistor R 6 . Thus the transimpedance gain would be determined by the value of Rfh. The current source  13  and the resistor R 4  provides the DC bias for the input of the transimpedance amplifier so that the PIN diode detector may be reversed biased. Under these conditions the current flowing in Q 1  would be set to provide the lowest noise for maximum sensitivity and the majority of the current of  11  would flow through transistor Q 2  to provide a virtual ac earth. The transistor Q 12  and resistor R 8  form an emitter follower output drive for the next stage. 
     In the above circuit diagram, it is important to carefully choose the values of resistors R 1 , R 2  and R 3  such that R 2 &gt;R 1  and R 1 +R 3 ≡R 2 . 
     When the input current has reached a signal level requiring gain control, the dc control voltage ‘Vcontrol’, would begin to increase the current in Q 1  by allowing current to flow in R 1  and R 2  while lowering the voltage drop across R 1  and R 3  by allowing a proportion of current  12  to flow through Q 6 . This would in turn begin the turn on the transistors Q 8  and Q 7  provided that the control current, ‘Icontrol’ also begins to allow current flow through transistor Q 7 . Since transistors Q 10  and Q 11  are effectively an analogue OR function, feedback is always applied to the base of Q 1  via Rfh, thus the emitter of Q 7  would approximately remain at a constant DC bias point. This has the effect of controlling the impedance seen by Q 7  determined by re=VT/Icontrol, where VT is the thermal voltage (KT/e). By controlling re which appears in series with Rfl, the high gain feedback resistance is shunted by the combination of Rfl+ re. 
     Under very high input current levels, Vcontrol would be such that transistors Q 3  and Q 6  would be fully turned on while transistors Q 4  and Q 5  would be turned off. This would result in current flow through transistor Q 1  determined by the collector resistor R 1 . Conversely, since Q 6  is turned on the current I 2  would flow through resistor R 2  only, which allows transistors Q 8 , Q 10  and in turn Q 7  to be fully on since Icontrol would also be fully on. Under these conditions, re of transistor Q 7  would be small compared to Rfl and Rfl would be small compared to Rfh, thus the transimpedance gain would be determined by the value of Rfl. It is important to note that for correct gain control to be achieved, the ratio of change in current from transistors Q 3  to Q 4  must be represented by the correct transistor ratio of Q 3  and Q 4 . 
     There are various ways that Icontrol in FIG. 3 may be achieved as illustrated by FIGS. 4 and 5. 
     In FIG. 4, Icontrol is achieved by the use of a long tail pair driven by a control signal Vcontrol 2  which would be derived by Vcontrol (in FIG. 2) differing by a common mode voltage of only two diode drops (ie 2vbe). When Vcontrol was steering the current through transistors Q 3 , Q 4  and Q 5 , Q 6 , the derived Vcontrol 2  would be steering the current I 4  from transistor Q 14  into transistor Q 13 , thus increasing the current through the control transistor Q 7 . Alternatively, FIG. 5 uses the action of change of current through transistor Q 2  during gain control. When high gain is required the transistor Q 2  delivers a maximum collector current which in turn provides a voltage drop across R 9 . This in turn would be designed to turn off the transistor Q 15  via diodes D 1  to Dn thus preventing current flow by the current mirror formed by transistors Q 16  and Q 17 . As gain control is required the current at the collector of transistor Q 2  reduces, thus reducing the voltage drop across resistor R 9 . This in turn allows the transistor Q 15  via diodes D 1  to Dn to turn on and provide the required current flow to start via the current mirror formed by transistors Q 16  and Q 17 , providing Icontrol. The regulated voltage supply ‘Vregulated’ in FIG. 5 is required to prevent the activation of the gain control by the supply voltage variation. 
     The transimpedance amplifier with gain control illustrated in FIG. 3 may be re-configured to operate with reduced voltage supply as shown by FIG.  6 . 
     The basic principles of operation of the gain control technique are the same as FIG. 3 with the exception that the control transistor Q 8  in FIG. 3 which provided an additional vbe drop to transistor Q 7  is not required. In addition, a parallel analogue OR function (Q 10  and Q 11  in FIG. 6) is required to buffer the effects of the next stage from feedback resistors formed by resistors Rfl and Rfh. This function was performed by the emitter follower of Q 12  and R 8  in FIG.  3 .