Abstract:
An optical receiver circuit is constructed to be immune to interference from external interference signals. The optical receiver circuit includes a differential amplifier having an optical reception device connected to one input of the differential amplifier. The optical receiver circuit also includes an electrical element for simulating the electrical behavior of the reception device in the illumination-free state. The electrical element is connected to the other input of the differential amplifier.

Description:
BACKGROUND OF THE INVENTION 
   Field of the Invention 
   The invention relates to an optical receiver circuit having an optical reception device and an amplifier device connected downstream. Light incident on the reception device (e.g. photodiode)—for example light from an optical waveguide of an optical data transmission system—is detected by the reception device with the formation of an electrical signal (e.g. photocurrent); the electrical signal is subsequently amplified by the amplifier device connected downstream. 
   Optical receiver circuits of this type require a very high sensitivity since the optical light signals to be received and thus the electrical signals formed by the reception device are generally very small. For this reason, the susceptibility to interference is also very high; this means that high-frequency interference—for example on the supply voltage of the optical receiver circuit and/or received radiated electromagnetic interference (EMI: electromagnetic interference)—can considerably impair the functionality of the optical receiver circuit. 
   In order to avoid interference from the outside, a considerable outlay on electromagnetic shielding is expended in the case of the previously known optical receiver circuits. In particular, considerable efforts are undertaken to suppress high-frequency interference, for example on the supply voltage. 
   It is evident that these shielding measures lead to considerable additional costs in the production of the previously known receiver circuits. 
   A previously known optical receiver circuit having a reception device and an amplifier connected downstream is described for example in the article “High Gain Transimpedance Amplifier in InP-Based HBT Technology for the Receiver in 40 Gb/s Optical-Fiber TDM Links” (Jens Mallrich, Herbert Thurner, Ernst Mullner, Joseph F. Jensen, Senior Member, IEEE, William E. Stanchina, Member, IEEE, M. Kardos, and Hans-Martin Rein, Senior Member, IEEE—IEEE Journal of Solid State Circuits, vol. 35, No. 9, September 2000, pages 1260 to 1265). In this receiver circuit, a differentially operated transimpedance amplifier—that is to say a differential amplifier—is present at the input end, which amplifier is connected by one input to a photodiode as reception device. The other input of the differentially operated transimpedance amplifier is connected to a DC amplifier which feeds a “correction current” into the differential amplifier for the offset correction of the photocurrent of the photodiode. The magnitude of this “correction current” that is fed in amounts to half the current swing of the photodiode during operation. 
   SUMMARY OF THE INVENTION 
   The invention is based on the object of specifying an optical receiver circuit which is immune to interference with respect to external interference signals. 
   Accordingly, the invention provides an optical receiver circuit having an optical reception device and a differential amplifier connected downstream. In this case, the reception device is connected to one of the two inputs of the differential amplifier. An electrical element, which simulates the electrical behavior of the reception device in the “illumination-free case”, is connected to the other of the two inputs of the differential amplifier. An “illumination-free case” is understood here to mean that the electrical element behaves electrically to the greatest possible extent like the reception device if no light to be detected impinges on the reception device. 
   One essential advantage of the optical receiver circuit according to the invention is to be seen in the fact that this receiver circuit is particularly immune to interference. In the receiver circuit according to the invention, this is achieved by means of the “fully differential” design of the circuit or the quasi-symmetrical input-end circuitry of the differential amplifier. In this case, the fully differential design is based on the electrical element according to the invention, which simulates the electrical behavior of the reception device in the illumination-free case. On account of the electrical element, the differential amplifier is connected up symmetrically, so that high-frequency interference is effectively suppressed. This is because high-frequency interference will occur simultaneously on account of the symmetrical input-end circuitry of the differential amplifier at the two inputs of the differential amplifier, so that the interference is suppressed to the greatest possible extent by virtue of the common-mode rejection that is always high in the case of differential amplifiers. 
   The optical receiver circuit according to the invention thus differs significantly from the previously known receiver circuits mentioned in the introduction which, although they have a differential amplifier at the input end, are connected up asymmetrically at the input end. In the prior art, potential interference elements such as, for example, a bonding wire of the reception device, the capacitance of the reception device and further capacitive construction elements—for example capacitances and inductances in the region of the reception device—thus occur exclusively at a terminal of the differential amplifier and are thus amplified directly. In the previously known “asymmetrical” circuitry of the differential amplifier, the interference signals thus pass to the “most sensitive location” of the optical receiver circuit and are amplified directly. In contrast to this, in the receiver circuit according to the invention, the interference signals are “virtually” filtered out and effectively suppressed, to be precise on account of the symmetrical circuitry of the differential amplifier with the aid of the element which simulates the behavior of the reception device in the non-illuminated case and on account of the common-mode rejection of the differential amplifier. 
   In summary, it can thus be established that, in the case of the optical receiver circuit according to the invention, a particularly high degree of immunity to interference is achieved as a result of the symmetrical input-end circuitry of the differential amplifier of the receiver circuit. 
   A further essential advantage of the optical receiver circuit according to the invention is that this receiver circuit can be produced more cost-effectively than the previously known receiver circuits since it is possible largely to dispense with complicated shielding measures for suppressing interference influences on account of the “inner” interference suppression by the differential amplifier. 
   In order to provide for the optical receiver circuit to achieve a particularly high gain, an advantageous development of the receiver circuit provides for the reception device and the electrical element to be connected to the differential amplifier in each case by means of a preamplifier. Interference possibly caused by the preamplifiers is suppressed, and thus becomes ineffective, on account of the common-mode rejection of the differential amplifier—in the same way as the rest of the interference signals. Consequently, a high gain is achieved with the preamplifiers, without interference being amplified. 
   The two preamplifiers are preferably identical preamplifiers in order to achieve a maximum interference suppression of the interference signals that are possibly generated by the preamplifiers themselves or the interference signals that are coupled into the preamplifier. 
   The electrical element which simulates the electrical behavior of the reception device in the illumination-free case may be, for example, a reception device (“dummy reception device”) which is identical to the reception device (“useful” reception device) of the receiver circuit and is darkened in such a way that no light can fall on it. The “useful” reception device and the “dummy” reception device are preferably monolithically integrated together on a semiconductor chip in order to ensure that both reception devices have an approximately identical electrical behavior. 
   As an alternative, the electrical element which simulates the electrical behavior of the useful reception device in the illumination-free case may also be formed by a capacitor which simulates the capacitive behavior of the useful reception device. 
   Transimpedance amplifiers are preferably used as preamplifiers. 
   Furthermore, it is regarded as advantageous if the optical receiver circuit has an integrated control circuit which makes it possible to control the feedback impedances of the two transimpedance amplifiers. By changing over the impedance magnitude of the feedback impedances, the gain of the transimpedance amplifiers and thus indirectly also the bandwidth of the receiver circuit can be set externally at the user end. 
   In order to ensure that the optical receiver circuit is constructed as symmetrically as possible, it is regarded as advantageous if the integrated control circuit is in each case connected to the feedback impedances of the two transimpedance amplifiers. 
   For symmetrical operation of the optical receiver circuit and thus for maximum interference suppression, it is advantageous if the reception device and the simulating electrical element are in each case connected to the same supply voltage. A low-pass filter is preferably connected to the supply voltage, which filter filters out high-frequency interference from the supply voltage. 
   The reception devices are preferably photodiodes. 
   The receiver circuit is preferably packaged in a TO-46 package or in a corresponding plastic package (e.g. TSSOP10 or VQFN20). 

   
     BRIEF DESCRIPTION OF THE DRAWING 
     The sole FIGURE of the drawing is a block diagram of an exemplary embodiment of an optical receiver circuit. 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENT 
   The FIGURE shows a photodiode  10  as a reception device (“useful” reception device) , which is connected to one terminal E 30   a  of a differential amplifier  30  via a transimpedance amplifier  20 . The other input E 30   b  of the differential amplifier  30  is connected via a further preamplifier  40 , embodied as a transimpedance amplifier, to a “dummy” photodiode  50  provided as a “dummy” reception device which electrically simulates the electrical behavior of the reception device  10  in the illumination-free case. 
   The transimpedance amplifier  20  is formed by a voltage amplifier  60 , for example an operational amplifier, which is connected up to a feedback impedance RF 1 . In a corresponding manner, the further transimpedance amplifier  40  is formed by a voltage amplifier, for example an operational amplifier  70  identical to the operational amplifier  60 , which is connected up to a further feedback impedance RF 2  (RF 1 =RF 2 ). 
   The output of the differential amplifier  30  is connected to a second differential amplifier  80 , which further amplifies the output signal of the first differential amplifier  30 . The output of the second differential amplifier generates an output signal S res ′ corresponding to the optical signal of the photodiode  10  and the inverted signal −S res ′, which is inverted with respect to the output signal S res ′. 
   The output of the differential amplifier  80  is connected to an AGC (amplitude gain control) control circuit  90 . The output of the control circuit  90  connected to the two feedback impedances RF 1  and RF 2 . The control circuit  90  sets the impedance RF 1  and RF 2  in a manner dependent on a control signal S 3  present at a control input S 90  of the control circuit  90 . Via the control input S 90 , the gain of the two transimpedance amplifiers  20  and  40  can be set externally at the user end. Since the achievable gain V and the bandwidth B of the circuit are to an approximation related to one another (V*B=constant) , by altering the gain it is also possible to set the achievable bandwidth at the user end. As an alternative or in addition, the control circuit  90  can also connect additional capacitances (or inductances) in parallel or in series with the two feedback impedances RF 1  and RF 2  in order to modify the feedback behavior and in order to avoid the occurrence of electrical oscillations, for example. Furthermore, the output signals S res ′ and −S res ′ of the second amplifier  80  are applied to the control circuit  90 , so that the control circuit can prevent overdriving of the amplifier, for example. 
   Furthermore, the optical receiver circuit is equipped with a DCC circuit  100  (DCC: Duty Cycle Control), which effects a control of the optical receiver circuit. The DCC circuit  100  or the duty cycle control (offset control) formed by it controls the sampling threshold for the downstream differential amplifiers, so that the signal is sampled at the 50% value of the amplitude and, as a result, no signal pulse distortions (duty cycle) are produced. This can be effected by feeding a current into a respective one of the preamplifiers (transimpedance amplifiers) or else by feeding in a voltage at the inputs of the differential amplifiers directly. 
   As can furthermore be gathered from the FIGURE, the two photodiodes  10  and  50  are both connected in each case to a supply voltage VCC 1 , which is connected to a low-pass filter  110 —comprising a capacitor C PD  and a resistor R PD . 
   The optical receiver circuit is operated as follows: 
   In the event of light being incident on the photodiode  10 , an electrical signal S 1  is fed into the transimpedance amplifier  20  and amplified by the latter. The amplified signal S 1 ′ is thus formed at the output of the transimpedance amplifier  20  and passes to the input E 30   a  of the differential amplifier  30 . 
   Interference signals St 1 , which are coupled or fed into the photodiode  10  or into the leads of the photodiode  10 , are likewise amplified by the transimpedance amplifier  20  and transmitted as amplified interference signals St 1 ′ to the differential amplifier  30 . 
   The dummy photodiode  50  is darkened—as indicated by the vertical bar in the FIGURE—in such a way that no light can fall onto the dummy photodiode  50 . The photodiode  50  is thus optically inactive and merely has the function of a “dummy”. 
   Despite all this, interference signals ST 2  can be coupled into the “dummy” photodiode  50 , for example via the leads of the photodiode  50 . The interference signals ST 2  are amplified by the further transimpedance amplifier  40  and pass as amplified interference signals ST 2 ′ to the further input E 30   b  of the differential amplifier  30 . Consequently, at the output of the differential amplifier  30 , output signals S res —and the inverted signals −S res  with respect thereto—are formed in accordance with
 
 S   res   =S 1+ St 1−( S 2+ St 2).
 
   Since the “dummy” photodiode  50  is darkened and thus cannot generate its own useful signal S 2 , the following holds true:
 
S2=0
 
   Furthermore, it can be assumed that interference signals which are coupled into the photodiode  10  are also simultaneously coupled into the “dummy” photodiode  50 , so that the following assumption is justified:
 
St1=St2.
 
   Consequently, at the output of the differential amplifier  30 , overall an output signal is produced in accordance with
 
S res =S1,
 
namely because St 1 =St 2  and S 2 =0.
 
   In summary, it can thus be established that the use of two input paths of the same type—formed by the photodiode  10  and by the “dummy” photodiode  50 —means that the receiver circuit is highly immune to interference since the interference signals St 1  and St 2  “concomitantly supplied” by the two photodiodes  10  and  50  at the input end are largely eliminated on account of the common-mode rejection of the differential amplifier  30 . 
   The low-pass filter  110  at the power supply voltage VCC 1  serves, moreover, to filter out high-frequency interference of the voltage supply VCC 1 , so that the interference cannot even reach the differential amplifier  30  in the first place. The FIGURE furthermore shows terminal pads  200  and  210 , which can be connected to one another by means of a bonding wire  220 . By means of such a bonding wire  220 , the capacitor C SYM  can be connected to the further transimpedance amplifier  40 . In this case, the capacitor C SYM  may replace the “dummy” photodiode  50  if such a photodiode  50  is not available. The capacitor C SYM  is then preferably dimensioned in such a way that it essentially corresponds to the capacitance of the “absent” dummy photodiode  50  or the capacitance of the useful diode  10 .