The present invention relates to an optical receiver comprising a photodiode and a transimpedance amplifier, with the photodiode contained in a direct-current circuit between a supply-voltage source and the input of the transimpedance amplifier, and further comprising an element limiting the direct-current in the direct-current circuit and an alternating-current circuit fed from the photodiode, which acts as an alternating-current voltage source, and containing an impedance which converts alternating voltage of the photodiode into an alternating current flow into the transimpedance amplifier. Such an optical receiver is disclosed in DE-OS 36 33 984. In this prior-art optical receiver, the AC circuit, which serves to convert the alternating voltage developed across the photodiode in the event of a great reduction of direct current into an alternating current flowing into the transimpedance amplifier, runs from the photodiode to ground via parts of the DC circuit of the photodiode.

It is the object of the invention to provide an improved optical receiver based on this prior-art optical receiver.

This object is attained as set forth in claim 1. Further advantageous aspects of the invention are claimed in the subclaims. The invention will now be explained in more detail, by way of example, with reference to the accompanying drawings, in which:

FIG. 1 is a basic circuit diagram of the optical receiver in accordance with the invention;

FIG. 2 shows the family of characteristics of a typical photodiode to explain the invention, and

FIG. 3 is a basic circuit diagram of an embodiment of the invention incorporating a transistor circuit instead of the variable resistor R_v of FIG. 1.

The optical receiver of FIG. 1, like the prior-art optical receiver, contains essentially a photodiode PD, a supply-voltage source U_O for the photodiode PD, and a transimpedance amplifier TIV (bounded by a dashed line). The anode 1 of the photodiode PD is directly connected to the input of the transimpedance amplifier TIV.

The transimpedance amplifier can be constructed in any of the conventional forms, which are characterized by an amplifier circuit V with a gain of -V_O which is shunted by a feedback resistor R_T. It converts the photocurrent of the photodiode PD into an output voltage U_a, which appears at the output designated by the same reference characters.

Another feature which the optical receiver of FIG. 1 has in common with the above-mentioned prior-art receiver is that the DC circuit of the photodiode includes a variable resistor R_V between the cathode terminal 2 of the photodiode and the supply-voltage source U_O. This resistor, as is described in detail in the printed publication mentioned above, serves to limit the DC circuit of the photodiode at high levels of the light impinging on the photodiode and, thus, to change the DC operating point of the photodiode.

This explanation will now be briefly repeated with the aid of FIG. 2. The latter shows schematically the characteristics of the photodiode for three different levels of the received optical signal, P₁ <P₂ <P₃, with I_d and U_d representing, respectively, the current and the voltage of the photodiode. I_d is linearly dependent on U_d, with the slope of the straight line depending on the value of the resistor in the DC circuit of the photodiode. The intersections of this straight line and the characteristics are the DC operating points of the photodiodes.

In FIG. 2 it can be seen that a gradually sloping straight line which intersects the photodiode characteristics in their steeply sloping portions, e.g., the straight line through the operating points AP4, AP5, and AP6, ensures even at high light levels that the maximum permissible current I_max for the transimpedance amplifier is not exceeded, whereas in the case of an I_d /U_d function passing through operating points AP1, AP2, and AP3, which is the case if the resistor in the DC circuit has a small value, a nonpermissible operating point AP3 would be obtained at the high light level P3.

As is known from the printed publication mentioned at the beginning, the changeover from the operating points AP1 through AP3 to the operating points AP4 through AP6 is effected by changing the resistance of the resistor from a low to a high value. At a high resistance, i.e., at the operating points AP4 through AP6, there is hardly any change in the current through the photodiode in case of light-level variations, but only the voltage U_d changes, so that in this mode, the photodiode is an AC voltage source.

To convert this alternating voltage into an alternating current serving as the input current for the transpedance amplifier, the prior art uses an AC circuit which is partly included in the DC circuit of the photodiode, from which it is grounded. In this respect, the invention differs considerably from the prior art.

According to the invention, the AC circuit runs from the photodiode terminal not connected to the input of the transimpedance amplifier TIV, i.e., the terminal 2, to the output of the transimpedance amplifier. As shown in FIG. 1, it contains an impedance consisting of a resistor R_S and a capacitor C_S connected in series with this resistor and serving to isolate the DC path from the AC path. Under suitable current and voltage conditions, the capacitor C_S can be dispensed with. Instead of the resistor R_S, an impedance consisting of a resistor and a parallel capacitor or a complex active or passive RLC four-terminal network may be present if equalization of the output voltage U_a is desired.

In any case, the connection from the terminal 2 to the output of the transimpedance amplifier TIV completes an AC circuit from the ground terminal of the transimpedance amplifier TIV through the photodiode, the impedance R_S, C_S and the output voltage U_a back to

In the following, the significance of the resistors R_V and R_S for the dynamic range of the novel circuit will be explained, assuming that the capacitor C_S has a very high capacitance, i.e., that it is virtually a short circuit for alternating current.

With the aid of equivalent circuit diagrams, it can be shown that for the novel circuit, ##EQU1## and I_d =strength of the current flowing through the photodiode

R_d =differential internal resistance of the photodiode

R_T =value of the feedback resistor R_T

V_O =gain of the amplifier circuit V

R_V =value of the resistor R_V

R_S =value of the resistor R_S, and

U_a =voltage (with respect to ground) at the output U_a of the transimpedance amplifier.

The foregoing expression for U_a is based on the assumption that R_S is very small compared with R_T, so that R_S /R_T is nearly zero. However, this is not a prerequisite for the functioning of the novel optical receiver but only serves to facilitate the explanation of the receiver.

The output voltage U_a thus depends on the variables R_V and R_D. At low light levels (e.g., P1, P2),the novel circuit, just as the prior art circuit, will be operated to advantage at the operating points AP1 and AP2. These operating points, as mentioned above, are predetermined by a very small value R_V. On the other hand, the differential resistance R_d of the photodiode, which is given by ##EQU2## is very high at these operating points, because the photodiode characteristics have very gradual slopes. Under these conditions (R_V very small, R_d very large), Eq. (1) becomes

U.sub. ≈RT ·Id                  (3)

This means that the novel circuit will act approximately like the prior-art optical receiver if R_V is made very small, as is done in all "normal" transimpedance receivers.

If, however, in order to avoid overloading the transimpedance receiver at high light levels, R_V is chosen --again in accordance with the principle of the abovementioned prior-art optical receiver --to be so large as to obtain operating points in the steeply rising portions of the diode characteristics, e.g., AP4 through AP6, conditions are as follows: The differential internal resistance R_d is very low at these operating points, so that under these conditions (R_V very large, and R_d very small), Eq. (1) becomes

Ua ≈Rd ·Id (4)

since α≈1 and ##EQU3## and, in most cases, ##EQU4## In Eq. (4), R_d is a function of I_d. From the equation of the characteristic of the photodiode, one can derive that ##EQU5## Thus, the product R_d ·I_d is constant, namely equal to U_T /m, where U_T is the so-called temperature voltage of the photodiode, and m is a correction factor which allows for the deviation from Shockley's simple diode theory, cf. textbook by U. Tietze and Ch. Schenk, "Halbleiter-Schaltungstechnik", eighth, revised edition, Springer-Verlag, Berlin, Heidelberg, New York, London, Paris, Tokio, 1986, p. 24.

From the preceding description it follows that through such a change in the value of the resistor R_V which places the operating points in the steeply rising portions of the characteristics, the output voltage of the transimpedance amplifier can be limited to a constant value, so that neither the transimpedance amplifier TIV nor the subsequent circuits will be overloaded. It can be shown that with the prior-art circuit, limitation of the output voltage to such a low value is not attainable without disadvantages resulting therefrom.

Now the question is how, in a practical circuit, the value of the resistor R_V can be changed in such a manner that the above-described shifts of the operating points are achieved at high light levels.

This can be done in the same manner as in the prior-art circuit, namely by selectively connecting a suitable resistor from a major supply of resistors into circuit and by changing from one resistor to another if high light levels are to be expected. The necessary current limiting can also be accomplished with a continuously variable resistor R_V and a control circuit which controls the resistance of this resistor in accordance with the light level as explained above.

The ideal solution, however, is an element which limits the direct current in the DC circuit of the photodiode by itself when the light level rises, thus causing the above-described shifts of the operating points automatically. According to an essential aspect of the invention, this is accomplished by replacing the resistor R_V of FIG. 1 with a circuit whose current-voltage characteristic resembles the output characteristic of a bipolar transistor at a constant base current.

As is well known, the output characteristic of a bipolar transistor at a constant base current shows that the current first rises steeply to a given value with increasing voltage and then remains nearly constant. In this region of constant current, the transistor circuit is a nearly ideal current source. Such transistor circuits and their output characteristics are described in the relevant literature, e.g., in the above-cited textbook on pages 28 and 29.

FIG. 3 shows the circuit according to the invention, which differs from the circuit of FIG. 1 in that the resistor R_V has been replaced by a transistor circuit. The latter consists of a bipolar pnp transistor T whose emitter is connected via an emitter resistor R_E to the supply-voltage source U_O, and whose collector is coupled to the terminal 2 of the photodiode. By means not shown, it is ensured that the base current I_b is constant.

Simple considerations taking into account the current and Voltage direction show that, as a result of the action of this transistor circuit, the current I_d first increases rapidly in the negative direction with increasing diode voltage U_d, similarly to the steep straight line through points AP1 to AP3, but soon slopes more gradually and becomes more and more monotonous. Such a curve is shown in FIG. 2 and gives the operating points AP7 to AP9.

The transistor circuit thus limits the current I_d in such a way that operating points at which I_max would be exceeded do not form. Without any control or switching operations, it is ensured that at high light levels the operating points lie in the steeply rising portions of the characteristics (AP8, AP9). In other words, the diode current I_d does not increase with increasing light level as would be the case with the straight line through the operating points AP1 to AP3, but the current is limited to an acceptable value, and the voltage across the photodiode is inverted from the negative range to the positive range. Any circuit capable of doing this is suitable for implementing the invention. The transistor circuit of FIG. 3, which is only a basic circuit, is just an example.

For example, all variants of the constant-current sources given in the above-mentioned textbook on pages 59 to 63, including the current mirror, and the current sources shown on pages 356 to 364 are suitable, it being possible to reduce the resistance in the AC circuit at low light levels by an additional circuit element, e.g., in the circuit of FIG. 13.11 on p. 357 by a resistor in parallel with R₁.

Finally, it should be pointed out that the shift of operating points described with the aid of FIG. 2 can also be accomplished by changing another parameter, namely the bias U_O. The latter determines the point at which a straight line whose slope is determined by a resistor intersects the abscissa, independently of the slope of the straight line. This point of intersection is denoted by U' in FIG. 2. From FIG. 2 it is readily apparent that by a control which shifts the point U' of the straight line through AP1 to AP3 and, thus, this straight line itself to the right with increasing light levels, it is possible to avoid operating points such as AP₃, at which I_max is exceeded.

Thus, it is also possible to extend the dynamic range with a fixed resistor R_V of a very low value by changing the bias, which is easier to implement than the control of a resistor. In the simplest case, analogously to the above-indicated change in the value of the resistor R_V, a suitable voltage from a supply of voltages serves as the bias voltage.

It is also possible, of course, to combine the change in the value of the resistor with the bias change, continuously or discontinuously, or to change the bias U_O applied to the transistor circuit used in FIG. 3 as a current-limiting element.

In principle, the current-limiting element in the DC circuit of the photodiode, i.e., the resistor R_V or the transistor circuit, could also be located on the other side of the photodiode PD.

In the arrangement of FIG. 1, an inductor can be placed betWeen R_V and the terminal 2 of the photodiode to reduce the effect of stray capacitances of R_V on the overall circuit.