Amplifiers – Signal feedback – Nonlinear impedance element in loop path
Reexamination Certificate
2000-02-15
2001-06-12
Pascal, Robert (Department: 2817)
Amplifiers
Signal feedback
Nonlinear impedance element in loop path
C330S086000, C330S308000, C250S2140AG
Reexamination Certificate
active
06246284
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a negative feedback amplifier being suitably used for a high-speed optical communication system and more particularly to a negative feedback amplifier with automatic gain control function that allows automatic control of a gain.
2. Description of the Related Art
An optical communication system employs an optical-electrical converter used to convert an optical signal to an electrical signal. The electrical signal obtained from the optical-electrical converter such as a photodiode is amplified by a negative feedback amplifier. Such negative feedback amplifiers are disclosed in Japanese Laid-open Patent Application No.
10-284953.
The disclosed negative feedback amplifier is comprised of an inverting amplifier circuit and a first feedback resistor connected in parallel to the same. In the above disclosed negative feedback amplifier, a second feedback resistor which can be connected in parallel to the first feedback resistor through a switching device. When a gain of the feedback amplifier is not more than a predetermined value, the switching device is in an open state, while, when the gain exceeds the predetermined value, the switching device is in a closed and connected state. If the gain of the feedback amplifier exceeds the predetermined value much and changes greatly, the switching device is so operated that its opening and closing operation serves to change impedance of the feedback amplifier, thus enabling the gain of the feedback amplifier to be maintained within a specified range.
When the second feedback resistor is connected to the feedback circuit to reduce the gain, due to a decrease in the impedance of the feedback circuit, frequency characteristics of the impedance extends. At this point, in the high frequency range, a peak may appear in the frequency characteristics of the impedance in some cases. In order to remove the peak in the high frequency range, in the disclosed negative feedback amplifier as shown in FIG. 3 of the publication, it is proposed that a by-pass condenser is additionally connected in parallel to the second resistor. By inserting such a by-pass condenser, the frequency characteristics are flattened while the impedance of the feedback circuit is in a lowered state, thus providing better amplifying characteristics.
However, in the above-mentioned conventional negative feedback amplifier, though one end of the by-pass condenser is interrupted from the inverting amplifier circuit due to interrupting actions of the switching device while the by-pass condenser is not in an active state, the other end of the by-pass condenser is connected to an output terminal of the inverting amplifier circuit. Because of this, parasitic capacity of the by-pass condenser is added to the inverting amplifier circuit, thus causing bad influence on the frequency characteristics of the negative feedback amplifier.
SUMMARY OF THE INVENTION
In view of the above, it is an object of the present invention to provide a negative feedback amplifier comprised of a by-pass condenser which can smoothly make impedance of its feedback circuit variable, thus improving frequency characteristics of the amplifier and eliminating a problem of parasitic capacity caused by the by-pass condenser mounted thereon.
According to a first aspect of the present invention, there is provided a negative feedback amplifier for use with integrated circuits comprising an inverting amplifier circuit provided with an input terminal and an output terminal used to amplify an input signal fed to the input terminal and to output the amplified signal in reverse phase and a feedback circuit mounted between the input terminal and the output terminal in parallel to the inverting amplifier circuit;
whereby the feedback circuit has a first feedback resistor connected between the input terminal and the output terminal, a first switching device one end of which is connected to the input terminal and which is put in an open state while a gain of the output signal to the input signal is not more than a predetermined value and is put in a closed and connected state while the gain exceeds the predetermined value, a second switching device one end of which is connected to the output terminal and which is put in an open state while the gain is not more than the predetermined value and is in a closed and connected state while the gain exceeds the predetermined value, a second feedback resistor connected between both switching devices and connected in parallel to the first feedback resistor used to decrease impedance of the feedback circuit by closing and connecting operations of the switching devices, and a by-pass condenser connected in parallel to the second feedback resistor used to improve flatness of frequency characteristics for high frequency components or region on an output signal from the output terminal while the both switching devices are in a closed and connected state.
In the foregoing, a preferable mode is one wherein the first and second switching devices are variable impedance devices the impedance of which is changed depending on a voltage applied between both terminals of the switching devices.
Also, a preferable mode is one wherein the variable impedance devices are comprised of diodes which allow a current to flow when a voltage applied to both ends of the diodes exceeds the predetermined value.
Also, a preferable mode is one wherein the variable impedance devices are field effect transistors (FET) having a pair of electrodes composed of a source and a drain, and a gate.
Furthermore, a preferable mode is one wherein one electrode of a first FET is connected to the input terminal, the other electrode of the first FET is connected to one end of the second feedback resistor, the gate is connected to either electrode of the pair of electrodes of the first FET, one electrode of a second FET is connected to the other end of the feedback resistor and the other electrode of the second FET is connected to the output terminal, and the gate is connected to either electrode of the pair of electrodes of the second FET.
It is also preferable that the first FET is an enhancement mode n-channel FET a drain and a gate of which are connected to the input terminal and a source of which is connected to the one end of the second feedback resistor, and the second FET is an enhancement mode n-channel FET a drain and a gate of which are connected to the other end of the second feedback resistor and a source of which is connected to the output terminal.
Also, it is preferable that a voltage difference between the source and the gate of each of the FETs becomes equal to a threshold value of each of the FETs when the gain reaches the predetermined value.
Also, it is preferable that each of the FETs reduces impedance between the source and the drain when a voltage difference between the source and the gate of each of the FETs exceeds a threshold value of each of the FETs, causing the second feedback resistor connected between both FETs to be connected substantially to the feedback circuit.
Also, it is preferable that each of the FETs is an enhancement mode transistor having a high impedance which substantially interrupts the second feedback resistor from the feedback circuit when a voltage difference between the source and the drain is 0 (zero) volts.
Furthermore, it is preferable that the by-pass condenser is substantially interrupted from the feedback circuit when a voltage difference between the source and the gate of both FETs is not more than a threshold value of each of both FETs, preventing the occurrence of parasitic capacity caused by the insertion of the by-pass condenser in the inverting amplifier circuit.
REFERENCES:
patent: 3588729 (1971-06-01), Satterfield
patent: 5210503 (1993-05-01), Sawamura
patent: 5302911 (1994-04-01), Miyashita
patent: 6140878 (2000-10-01), Masuta
patent: 10-284953 (1998-10-01), None
Choe Henry
Frank Robert J.
OKI Electric Industry Co., Ltd.
Pascal Robert
Venable
LandOfFree
Negative feedback amplifier with automatic gain control... does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Negative feedback amplifier with automatic gain control..., we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Negative feedback amplifier with automatic gain control... will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2479311