JPH0685556A - Amplifier - Google Patents

Abstract

PURPOSE:To increase the maximum input level of a transfer impedance amplifier used for an optical receiver or the like. CONSTITUTION:This amplifier is composed of a transfer impedance amplifier 2 provided with a feedback part constituted by parallelly connecting a feedback resistor 9 and a transistor 13 for current branch, and transistor 10 for buffer. The output of a buffer part is set so as to be clamped with an input current smaller than the output of the transfer impedance amplifier, and the difference voltage between a transfer impedance amplifier output voltage 14 and a buffer part output voltage 8 is impressed between the gate sources of the transistor 13 for current division. Since the output of the buffer is early clamped when an input current is increased, the transistor 13 for current division is turned on, and the saturation of a main amplifier transistor 3 is relaxed.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an amplifier requiring a wide dynamic range, such as a preamplifier for an optical receiver.

[0002]

2. Description of the Related Art Conventionally, a preamplifier for an optical receiver receives an optical signal by a light receiving element 1 as shown in FIG. 2, amplifies the signal obtained by this and outputs it at an output terminal OU.
The signal voltage is obtained from T. The amplifier 2 has a main amplifying transistor 3 having a gate as an input and having a source grounded, a load resistor 6 connected to the drain of the main amplifying transistor 3 and a power source V _DD , and a gate for the main amplifying. It is composed of a buffer transistor 4 connected to the drain of the transistor 3 and connected to the power source V _DD , a diode 7, and a constant current transistor 5. A part of the output signal of the amplifier 2 is fed back to the input of the amplifier 2 via the feedback resistor 9. As a result, negative feedback is applied, the stability of the circuit is increased, and at the same time the band is expanded. As a conventionally known example, a technical research report of the Institute of Electronics, Information and Communication Engineers (1988 OCS88-7
p. 37-42) show examples of basic transimpedance type amplifiers.

[0003]

In the conventional optical receiving preamplifier, the allowable value of the optical input level is determined by the saturation level of the main amplifying transistor 3. However, in the above-mentioned conventional circuit, when an excessive input arrives at the light receiving element 1, the main amplifying transistor is saturated, the output waveform is deteriorated, and reception becomes impossible. On the other hand, it is possible to raise the saturation level of the main amplification transistor by lowering the feedback resistance, but there is a drawback that the reception sensitivity deteriorates.

As a method of expanding the maximum input,
As disclosed in Japanese Patent Application Laid-Open No. 2-143731 shown in FIG. 4, a method is known in which the gain value is switched by detecting the peak value of the output of the transimpedance type amplifier circuit. However, a time constant circuit is indispensable for the peak value detection circuit, and it is difficult to follow the high speed fluctuation of the output current.

It is an object of the present invention to provide a transimpedance type amplifier circuit which prevents the main amplifying transistor from being saturated with respect to a rapid fluctuation of the input current and increases the maximum input current.

[0006]

In order to achieve the above object, the present invention provides a feedback resistor and a current shunting circuit in the feedback section of a transimpedance type amplifier, and the output of the buffer section is a transimpedance type. By connecting a buffer unit that is clamped with an input current smaller than the amplifier output, and controlling the current flowing into the current shunt circuit by the difference voltage between the transimpedance type amplifier output voltage and the buffer unit output voltage. To be realized.

[0007]

When the current input to the transimpedance amplifier increases and the current flowing into the feedback portion increases,
The current shunt circuit works to control the current flowing into the feedback resistor, relax the saturation of the main amplification transistor, and promote the normal operation of the amplifier even for a large input signal.

[0008]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the present invention will be described below with reference to FIG. In the light receiving element 1, the optical signal is converted into an electric signal,
The light receiving element output signal is amplified in the amplifier 2.
A part of the output signal of the amplifier 2 is fed back to the input via the feedback resistor 9. Further, the buffer transistor 10 is connected to the output of the amplifier 2. Current shunt transistor 13
Is connected in parallel with the resistor 9 and the drain is connected to the input side of the amplifier 2 and the source is connected to the output side of the amplifier 2. The gate of the transistor 13 is the constant current source transistor 1 of the buffer section.
2 connected to the drain.

Here, when the voltage V _RF across the feedback resistor 9 and the input current Ip, the feedback resistor 9 is R _F , the resistance of the transistor 13 is R _FET , and the transimpedance Z _T ,

[0010]

## EQU1 ## V _RF = Ip × Z _T Z _T = R _F × R _FET / (R _F + R _FET )

The light input level to the light receiving element 1 is Ip≈2.
When it is as small as 3 μA, V _RF ≈0 in the equation (1), the gate-source voltage of the main amplifying transistor 3 hardly changes, and the saturation of the main amplifying transistor 3 does not occur.

The _gate-source voltage V _13gs of the transistor 13 is given by the difference voltage between the output voltage 14 of the output buffer and the _{transimpedance} amplifier output voltage 8. Since the voltage gain of the output buffer is about 1 for the voltage 14,
Same change as voltage 8. Therefore, if the level shift voltage by the diode 7 is appropriately selected and V _{13gs ≤V 13P} (the pinch-off voltage of the transistor 13), the transistor 13 is turned off and the amplifier is a _{transimpedance} type amplifier having only a simple feedback resistor. Works like.

When the light input becomes large and the current source transistor 12 passes the saturation region, the voltage 14 is clamped and V
_13gs increase. When V _13gs > V _13P , the transistor 13 is turned on and a shunt current flows. At that time, the change in the voltage caused by the feedback resistance becomes small, the change in the gate-source voltage of the main amplifying transistor 3 is small, and the saturation is relaxed, so that the maximum input current increases. Since this circuit does not need a time constant circuit, high-speed response is possible.

FIG. 3 shows the voltage change of each part.

[0015]

According to the present invention, the saturation of the main amplification transistor is relaxed as compared with the conventional preamplifier, which is effective for expanding the maximum input level of the optical receiver.

[Brief description of drawings]

FIG. 1 is a circuit diagram showing an embodiment of the present invention.

FIG. 2 is a circuit diagram showing an example of a conventional method.

FIG. 3 shows the light input level and the drain of the transistor 13.
It is a graph which shows the relationship between the voltage between sources, and trans impedance.

FIG. 4 is a circuit diagram showing an example of a conventional method.

[Explanation of symbols]

1 ... Light receiving element, 2 ... Amplifier, 3 ... Main amplification transistor,
4 ... Amplifier buffer transistor, 5 ... Constant current transistor, 6 ... Load resistance, 7 ... Diode, 8 ... Voltage,
9 ... Feedback resistor, 10 ... Transistor for buffer, 11 ...
Diode, 12 ... Constant current transistor, 13 ... Current shunt transistor, V _DD , V _SS ... Power supply, 14 ... Voltage,
11 ... Diode, OUT ... Output terminal.

Claims (1)

[Claims] 1. A transimpedance type amplifier for converting an input current into a voltage by connecting an input and an output through a feedback section composed of a feedback resistor and a current shunt circuit connected in parallel, and the transimpedance type amplifier. An amplifier having a buffer section connected to an output, the buffer section output being clamped at an input current smaller than the transimpedance type amplifier output, wherein the transimpedance type amplifier output voltage and the buffer section output An amplifier characterized in that a current flowing into the feedback resistor is controlled by a voltage difference between the voltages.