JPH0575355A - Bias circuit for avalanche photodiode - Google Patents

Abstract

PURPOSE:To expand the maximum light receiving range of an optical reception circuit employing an avalanche photodiode. CONSTITUTION:An avalanche photodiode 1 and a bias voltage circuit VH are connected by using a parallel circuit comprising a resistor 2 and a constant voltage element 3 and a voltage of the constant voltage element is selected in a range that the avalanche photodiode 1 is operated even at a voltage resulting from subtracting a voltage generated across the constant voltage element from the voltage generated at the bias voltage circuit. Thus, the required frequency band of the avalanche photodiode is kept and the maximum light receiving range is expanded.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a bias circuit in the field of optoelectronics centering on optical communication, in which the maximum light receiving range of an optical receiving circuit using an avalanche photodiode is expanded.

[0002]

2. Description of the Related Art Conventionally, in an optical receiving circuit using an avalanche photodiode (hereinafter abbreviated as APD), one end of an APD1 is connected to a high voltage bias circuit VH via a resistor 2 as shown in FIG. And the other end is connected to the preamplifier 3. At this time, the resistor 2 is the APD
When the light input level is high as well as when it is inserted as a protection resistance of 1, the APD multiplication factor is lowered by the voltage drop due to the load current flowing through the resistor 2 and the light input dynamic range is expanded. The generality of this circuit and a similar circuit are disclosed in Japanese Patent Laid-Open No. 2-212707.

[0003]

Generally, the frequency band and multiplication factor of the bias voltage applied to the APD change as shown in FIG. The multiplication factor of APD can be controlled by the APD bias voltage, but the frequency band also changes at that time. The reason that the frequency band decreases at a high APD multiplication factor is that the gain band product of the APD is constant, and that the frequency band decreases at a low APD multiplication factor is that the light absorption region and avalanche of the APD. This is because the electric field strength in the region decreases, the transit time of carriers becomes long, and the time constant is limited due to the increase of the junction capacitance. Therefore, in the optical receiver circuit that handles high-speed signals, it is necessary to maintain the minimum value of the APD bias voltage so as to secure the necessary frequency band.

However, in the circuit shown in FIG. 2, the bias voltage applied to both ends of the APD decreases as the optical input increases. Therefore, when the optical input level is large, the frequency characteristic of the APD itself deteriorates, and the optical receiving circuit is affected. The required band may not be secured and reception may be impossible. Further, when the value of the resistor is reduced to reduce the amount of voltage drop, the preamplifier is saturated and the electric output waveform is distorted and unreceivable due to the large multiplication factor of the APD. Therefore, it is necessary to set the value of the resistor 2 in consideration of both the frequency characteristic of the APD and the maximum input level of the preamplifier, and the adjustment range is narrow, resulting in the sacrifice of the maximum light receiving level. there were.

An object of the present invention is to provide an optical receiving circuit in which the maximum light receiving level is expanded by ensuring the frequency band required for APD.

[0006]

In order to achieve the above object, in the optical receiving circuit according to the present invention, a light receiving element and a bias voltage circuit are connected by a parallel circuit of a resistor and a constant voltage element. Even with a bias obtained by subtracting the voltage generated across the constant voltage element from the circuit voltage, the AP
The voltage of the constant voltage element is selected within the range in which D operates.

[0007]

The operation of the present invention will be described with reference to FIG. Resistor 2
The constant-voltage element 3 connected in parallel with is conductive when the voltage generated in the resistor 2 by the current photoelectrically converted by the APD1 becomes equal to the voltage of the constant-voltage element 3, and for a current value higher than that, , Is always fixed to a constant voltage and the amount of voltage drop is blocked. At this time, the bias voltage applied to the APD is from the voltage VH of the high voltage generation circuit to the constant voltage element voltage V
Given by the difference in Z. Therefore, the APD required for the receiving circuit
If the constant voltage element is selected so as to be equal to or higher than the APD voltage VL that gives the lower limit fL of the frequency band of, even if the optical input increases, the frequency band of the APD does not become insufficient and reception becomes impossible. The optical input level of the receiving circuit can be extended to the saturation point of the connected preamplifier.

[0008]

FIG. 1 shows an embodiment of the present invention. The APD 1 and the bias voltage circuit VH are connected to the resistor 2 and the Zener diode 3.
Are connected in parallel with each other, and the output of the APD 1 is connected to the preamplifier circuit 4. APD is InGaA
s-APD was used. The bias voltage VH can be fixed at a level at which the minimum receiving sensitivity becomes the best. FIG. 4 shows the relationship between the bias voltage of the APD and the optical input level in the example. Here, the voltage that gives the frequency band fL (= 1 GHz) of the APD necessary for the receiving circuit is VL (= 60 V).
And the multiplication factor at this time is ML (= 2). When the minimum light receiving level is PL (= -40 dBm), the multiplication factor of APD1 is set to MH (= 15) and the bias voltage VH (= 85 V). The saturation input current of the preamplifier circuit 4 is 400 μA. The bias voltage VAPD of the APD1 with respect to the optical level P input to the APD1 is
Let S (= 1 A / W) be the light receiving sensitivity of APD1, M be the multiplication factor, and R be the constant of resistor 2 (= 300 kΩ).

[0009]

## EQU00001 ## VAPD = VH-R.times.M.times.S.times.P
… (1). Here, the Zener voltage VZ is VZ = VH-VL

[0010]

[Expression 2] = 25V
... (2) is set. As a result, even if the light input is increased, the APD voltage is maintained at VL and biased as shown by the solid line in the figure. When Zener diode is not added, it is biased as shown by the broken line in the figure.

[0011]

## EQU00003 ## VAPD = VL
(3) The optical input PH1 (= -13.8 dBm) or more and fL
However, according to the present embodiment, the saturation level of the preamplifier can be increased to PH2 (= -7).
dBm).

[0012]

According to the present invention, even when the optical input is large, the voltage drop amount of the APD is limited by the constant voltage element connected in parallel with the resistor, and the frequency band required for the APD is secured. The maximum light receiving level of the receiver can be increased.

[Brief description of drawings]

FIG. 1 is a circuit diagram of an embodiment according to the present invention,

FIG. 2 is a circuit diagram of a conventional example,

FIG. 3 is a general characteristic diagram of APD,

FIG. 4 is a characteristic diagram illustrating the operation of an embodiment according to the present invention.

[Explanation of symbols]

1 ... Avalanche photo diode, 2 ... Resistor, 3 ... Constant voltage element, 4 ... Preamplifier.

Claims (1)

[Claims] 1. An avalanche photo diode for self-multiplying an optical signal and converting it into an electric signal, a bias circuit for supplying a voltage necessary for the operation of the avalanche photo diode, and an amplifier for amplifying the electric signal. In an optical receiver circuit including a preamplifier circuit, the avalanche photodiode and a bias voltage circuit are connected by a parallel circuit of a resistor and a constant voltage element, and the voltage of the bias circuit changes the constant voltage element from A bias circuit for an avalanche photodiode, characterized in that the voltage of the constant voltage element is selected within a range in which the avalanche photodiode operates even with a bias voltage obtained by subtracting a voltage generated at both ends.