JPH05243858A - Pre-amplifier for optical communication - Google Patents

Abstract

PURPOSE:To widen a dynamic range, and also, to enlarge the gain at the time of inputting a small signal, and to enhance S/N by varying the gain in accordance with whether an input current value is large or small. CONSTITUTION:When an input current iin is in a small signal area, Vout -iin.Rf (Rf denotes a value of a feedback resistance 8) because an output voltage Vout is small. When the input current iin is in a large signal area, a forward current id of a diode 7B cannot be disregarded, compared with the current iin because the output current Vout becomes large. Also, a part of the current iin is bypassed as the forward current id of the diode 7B, saturation of a transistor 6A is avoided and amplitude of an output voltage is controlled. Subsequently, when the current id of the diode 7B starts to flow, the gain of the pre-amplifier is also varied, and the current iin is set by a value Re of a resistance 9 and a constant-current value Ie of a constant-current source 10. Accordingly, in accordance with whether the current value iin is large or small, the gain is varied, a dynamic range is widened and the gain at the time of inputting a small signal is large, and S/N is enhanced.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a preamplifier in a light receiving circuit section of a repeater for optical communication.

[0002]

2. Description of the Related Art Generally, in an optical communication device, a preamplifier for amplifying a light receiving element output is (A) low noise, (B)
Characteristics such as wide dynamic range, (C) wide band, and (D) high gain are required.

Conventionally, this type of preamplifier has been known as RGSmith.
and SDPersonic `` Receiver design for optical fiber
r communication system ”, in Topics in Applied Phy
sics, Vol.39 (H.Kressl, Ed.). New York: Springer-Verla
g, 1988. That is, FIG. 4 is a circuit diagram showing a conventional preamplifier for optical communication. In the figure, 1 is an input terminal for inputting a current signal i _in from a light receiving element (not shown), 2 is an output terminal for outputting an amplified signal V _out , 3
Is a power supply terminal, 4 is a feedback resistor having a resistance value R _f , 5A and 5
B is a resistor, 6A is an NPN type input transistor, and 6B is an NPN type output transistor.

In the circuit configuration of this preamplifier for optical communication,
The relationship between the current i _{in at the} input terminal 1 and the voltage V _out at the output terminal 2 is V _out = −i _in · R _f, which is called a transimpedance type preamplifier, and its input current value i _in The gain characteristic is shown in FIG.

[0005]

However, in the preamplifier for optical communication having the above configuration, in order to widen the dynamic range, the feedback resistance is set so that the transistor 6A does not operate in saturation even with a large input current i _in . It is necessary to reduce the resistance value R _f (gain) of No. 4, but with this,
There is a problem in that the influence of noise from the peripheral circuits due to the increase of the noise of the amplifier itself and the decrease of the output amplitude is increased, and the S / N is deteriorated.

In order to eliminate the above-mentioned problem that the S / N is deteriorated, the present invention changes the gain according to the magnitude of the input current value so that the dynamic range is wide and the gain is high. The purpose is to provide a preamplifier.

[0007]

The preamplifier for optical communication according to the present invention has a diode connected in parallel with a feedback resistor in the forward direction to determine an input current value at which the diode starts to conduct. A gain varying means composed of a source or a current mirror circuit is connected in series to the output transistor.

[0008]

The present invention has a wide dynamic range, a large gain when a small signal is input, and can improve S / N.

[0009]

1 is a circuit diagram showing an embodiment of an optical communication preamplifier according to the present invention. In the figure, 7A to 7C are diodes, 8 is a feedback resistor having a resistance value R _f , and 9 is a resistance value R.
_e of the resistor 10 is a constant current source of a current value I _e.

The diode 7B has a feedback resistor 8
The parallel current source 10 and the resistor 9 are connected in parallel to the input terminal 2 and are for determining the input current value at which the diode 7B starts to conduct.

Next, the operation of the preamplifier for optical communication having the above configuration will be described. First, the inter-terminal voltage V _d of the diodes 7B, when the forward current _{i in, V out = - (} i in -i d) · R f i d = I S · [exp (V _d / V _t) −1] V _d = (V _out + R _e · I _e ), where I _S is the saturation current of the diode 7B and V _t is the thermal voltage of the diode 7B.

Now, _in a region where the input current i _in is small (hereinafter,
In a small signal region), since the output voltage V _out is small, the forward current i _d of the diode 7B can be compared with the input current i _in and ignored. Therefore, the output voltage V _out should be approximated as follows. You can

V _out ≈−i _in · R _f Next, _{in a} region where the input current i _in is large (hereinafter referred to as a large signal region), the output current V _out becomes large, and therefore the forward current i _{d of the} diode 7B. However, compared with the input current i _in , it cannot be ignored, and a part of the input current i _in is bypassed as the forward current i _d of the diode 7B, the saturation of the transistor 6A is avoided, and the output voltage amplitude V _out is controlled. To be done. Then, when the forward current i _d of the diode 7B starts to flow, the gain (transimpedance value) of the preamplifier also changes, and the input current i _in is the resistance value R _{e of the} resistor 9.
And the constant current value I _e of the constant current source 10 are set,
As shown in FIG. 2, the gain (transimpedance value) changes according to the magnitude of the input current value i _in .

FIG. 3 is a circuit diagram showing another embodiment of the preamplifier for optical communication according to the present invention. In the figure, 11 is N
A current mirror circuit composed of TN type transistors 12A and 12B. The current mirror circuit 11 has the same functions as the diode 7A and the constant current source 10 in the embodiment shown in FIG. Therefore, it goes without saying that the operation of the preamplifier for optical communication according to this configuration is the same as that of the embodiment of FIG.

In the above embodiments, the diode is used, but the present invention is not limited to this, and it goes without saying that the NPN type transistor may be connected in the diode type.

[0016]

As described in detail above, according to the preamplifier for optical communication according to the present invention, the dynamic range is wide, the gain when a small signal is input is large, and the S / N ratio can be increased. Moreover, there is an effect that it can be easily integrated into a circuit.

[Brief description of drawings]

FIG. 1 is a circuit diagram showing an embodiment of a preamplifier for optical communication according to the present invention.

FIG. 2 is a diagram showing an input current value-gain (transimpedance value) characteristic in FIG.

FIG. 3 is a circuit diagram showing another embodiment of the preamplifier for optical communication according to the present invention.

FIG. 4 is a circuit diagram showing a conventional preamplifier for optical communication.

5 is a diagram showing an input current value-gain (transimpedance value) characteristic in FIG.

[Explanation of symbols]

 7A to 7C Diode 8 Feedback resistor 9 Resistor 10 Constant current source 11 Current mirror circuit 12A, 12B Transistor

Claims (3)

[Claims] 1. A feedback resistor having a resistance value R _f for determining gain and an output transistor are provided, and an input current i _in and an output voltage V _out are provided.
In a transimpedance type preamplifier having a relationship of V _out = −i _in R _f, a diode is connected in parallel with the feedback resistor in the forward direction, and a gain varying means for determining an input current value at which the diode starts to conduct. Is connected in series to the output transistor, and a preamplifier for optical communication is provided. 2. The preamplifier for optical communication according to claim 1, wherein the gain varying means comprises a resistor and a constant current source. 3. The preamplifier for optical communication according to claim 1, wherein the gain varying means is composed of a current mirror circuit.