Circuit arrangement for an optical receiver

Optical receivers with a photodiode (PD) and a transimpedance amplifier are optimized for signals with a particular transmission rate. A circuit is provided whereby the optical receiver can be adapted to different transmission rates. To attain this object, the feedback resistor of the transimpedance amplifier is divided into at least two resistors (R.sub.F1, R.sub.F2) connected in series. According to the transmission rate, the respective optimum resistance value is switched into the negative-feedback path with the aid of a control circuit. This makes it possible to operate such a universal optical receiver in a broadband subscriber interface both at a transmission rate of 150 Mb/s and at a transmission rate of 600 Mb/s, for example.

BACKGROUND OF THE INVENTION 
1. Technical Field 
The present invention relates to a circuit arrangement for an optical 
receiver used as an optical-to-electrical transducer in optical 
communication systems. 
2. Background of the Invention 
If great band-width and high dynamic range are recruited, a transimpedance 
amplifier is frequently used as the preamplifier, see Dieter Lutzke, 
"Lichtwellenleiter-Technik", Pflaum Verlag M/e,uml/u/ chen, 1986, pp. 282 
to 289. FIG. 1 shows a generally known equivalent circuit of an optical 
receiver. The photodiode PD is shown as a current source I.sub.Ph shunted 
by a capacitance C.sub.D, and the succeeding transimpedance amplifier TIV 
consists of an operational amplifier V with a feedback resistance R.sub.F, 
an input resistance R.sub.V, and an input capacitance C.sub.V. The 
photodiode PD delivers a photocurrent I.sub.Ph which, to a first 
approximation, is proportional to the received light power. The 
operational amplifier, whose output is fed back through the feedback 
resistance R.sub.F, converts the photocurrent I.sub.Ph into an output 
voltage U.sub.a. Assuming a high gain v, the corresponding current-voltage 
transfer function of the transimpedance amplifier is 
##EQU1## 
follows that the 3-dB cutoff frequency is 
##EQU2## 
The feedback resistance R.sub.F is thus fixed by specifying a desired 
bandwidth. However, the resistance value influences the noise 
characteristic and, hence, the receiver sensitivity. If the receiver 
sensitivity is to be high, the feedback resistance R.sub.F must be made as 
large as possible. This reduces the noise contribution of the feedback 
resistance R.sub.F, but also the bandwidth of the optical receiver. 
In prior art solutions, the feedback resistance value is chosen in 
accordance with the maximum required transmission rate in an optical 
communication system, see R. G. Meyer, R. A. Blauschild, "A Wide-Band 
Low-Noise Monolithic Transimpedance Amplifier", IEEE Journal of 
Solid-State Circuits, Vol. SC-21, No. 4, Aug. 1986. If a receiver is used 
at a lower transmission rate, the mismatched feedback resistance will 
cause a loss of receiver sensitivity. 
SUMMARY OF THE INVENTION 
It is the object of the invention to provide a circuit arrangement for an 
optical receiver which is universally applicable at different transmission 
rates and suitable for monolithic integration. 
This object is attained by providing the circuit arrangement wherein the 
feedback resistor is connected as a series combination of two resistors 
between the input and output of a transimpedance amplifier, and that 
between the junction of the first and second amplifier stages and the 
junction of the two feedback resistors, an amplifier is inserted which has 
essentially the same transfer function as the second amplifier stare and 
includes means for switching the amplifier to an active state or a blocked 
state. By dividing the feedback resistor into two resistors connected in 
series, it becomes possible to match the value of the effective feedback 
resistance to the respective transmission rate. A large feedback 
resistance causes a low cutoff frequency or small bandwidth, and a small 
feedback resistance causes a high cutoff frequency or wide bandwidth. It 
is also possible to divide the feedback resistor into more than two 
resistors and thus optimize the transimpedance amplifier for the 
transmission of different bit rates. 
The effect of the circuit arrangement is based on the fact that the first 
amplifier stage is followed by two equally acting amplifier stages having 
respective inputs together having a common input mode, one of them being 
controllable. The outputs of the two amplifier stages are coupled to the 
terminals of one of the two feedback resistors, so that if the amplifiers 
have the same response, no voltage difference will appear across this 
resistor, which thus has no effect on the negative feedback. If the 
amplifier stage having its output coupled to the junction of the two 
resistors is blocked by the control voltage, the whole feedback resistance 
will become effective. 
Thus, an optical receiver suitable for monolithic integration is provided 
which can be operated, for example, is a broadband subscriber interface 
both at a transmission rate of 150 Mb/s and at a transmission rate of 600 
Mb/s.

BEST MODE FOR CARRYING OUT THE INVENTION 
As shown in FIG. 2, the circuit arrangement for an optical receiver 
consists of a photodiode PD, an amplifier made up of two amplifier stages 
V1, V12, an additional amplifier V22, whose operation is determined by a 
control voltage U.sub.st, and a feedback resistor implemented as a series 
combination of two resistors R.sub.F1 R.sub.F2. A possible technical 
realization is shown in FIG. 3. The first amplifier stage V1 includes a 
first transistor T1 as an active element, and the second amplifier stage 
V12 contains two transistor stages with second and third transistors T3, 
T4. A second signal path is designed in a similar manner with two 
transistors T5, T6 as the additional amplifier V22. A supplementary 
circuit consisting of two transistor switches with transistors T8, T9 
makes it possible to control this second signal path. Also provided is a 
current source which is controlled by a reference voltage U.sub.ref and 
includes a transistor T7. 
FIG.3 shows a transimpedance amplifier in detail in the upper side thereof, 
and shows the control circuit in detail in the lower side thereof. 
The operation of the circuit arrangement is as follows. If the control 
voltage U.sub.st is greater than the base-emitter voltage U.sub.BE of the 
two transistors T8, T9 of the supplementary circuit, the transistors T8, 
T9 will be turned on until the collector-emitter saturation voltage 
U.sub.CEsat is reached. As a result, the second signal path is shunted to 
ground through a resistor R6 and the transistor T8. At the same time, the 
reference voltage U.sub.ref, which generates a constant current through 
the transistor T6 by means of the circuit consisting of transistor T7 and 
resistor R7, is also shunted to ground through a resistor R8 and the 
transistor T9. The transistor T6 in the output of the second signal path 
is thus currentless, and a reverse voltage of about 2 U.sub.BE is applied 
to its base-emitter diode. As a result, only the relatively low 
depletion-layer capacitance of the base-emitter diode of the transistor T6 
is effective at the junction of the two resistors R.sub.F1, R.sub.F2, but 
this capacitance has practically no influence on the transfer function and 
the noise of the transimpedance amplifier. The transimpedance amplifier 
now operates with an effective feedback resistance equal to the sum of the 
values of the two resistors R.sub.F1, R.sub.F2. 
If the control voltage is very much smaller than the base-emitter voltage 
U.sub.BE of the transistors T8, T9 of the supplementary circuit, the 
transistors T8, T9 are turned off and the transistor T6 in the output of 
the second signal path is traversed by a constant current. The signal 
taken from the collector of the transistor T1 of the first amplifier stage 
is fed through the second signal path, containing the transistors T5, T6 
and the first resistor R.sub.F1 of the negative-feedback loop, back to the 
input of the transimpedance amplifier. Since the same signal is present at 
the emitters of the output transistors T4, T6 in the first and second 
signal paths, and the second resistor R.sub.F2 is connected between these 
two transistors, the negative feedback through the second resistor 
R.sub.F2 is ineffective. The transimpedance amplifier now operates with 
the first resistor R.sub.F1 as a feedback resistor. 
FIG. 4 shows an embodiment of the optical receiver where the 
negative-feedback resistors includes two or more additional resistors 
(R.sub.F3, . . . , R.sub.Fn) connected in series between the input and 
output of the transimpedance amplifier means (TIV) and the controllable 
amplifier means (V22) includes two or more additional controllable 
amplifiers (V23, . . . , V2n). 
For example, the negative-feedback resistor consists of a first resistor 
(R.sub.F1), a second resistor (R.sub.F2), a third resistor (R.sub.F3), and 
an nth resistor (R.sub.Fn). Compared to the circuit arrangement of FIG. 2, 
an amplifier (V23) associated with the third resistor (R.sub.F3) and an 
amplifier (V2n) associated with the nth resistor (R.sub.Fn) have been 
added. It is respectfully submitted that any person skilled in the art 
would appreciate that when all the amplifiers (V22, V23, V2n) are in an 
active state, only the first resistor (R.sub.F1) will act as a 
negative-feedback resistor. If the amplifier (V22) is blocked by means of 
the control voltage (U.sub.st1), the sum R.sub.F1 +R.sub.F2 will act as 
the negative-feedback resistor. By blocking further amplifiers associated 
with the resistors in the negative-feedback loop by means of respective 
control voltages (U.sub.st1, U.sub.st2, . . . , U.sub.stn), the resistors 
can be activated. A resistance range from R.sub.F1 to R.sub.F1 +R.sub.F2 
+R.sub.F3 + . . . +R.sub.Fn can be covered, so that optical receivers can 
be implemented which are optimizable for n different transmission rates. 
FIG. 2(a) shows an alternative embodiment of the invention similar to the 
embodiment in FIG. 2. In FIG. 2(a), the photodiode PD is connected between 
ground and an input of the transimpedance amplifier.