Miscellaneous active electrical nonlinear devices – circuits – and – Specific identifiable device – circuit – or system – Nonlinear amplifying circuit
Reexamination Certificate
2000-08-23
2002-01-01
Cunningham, Terry D. (Department: 2816)
Miscellaneous active electrical nonlinear devices, circuits, and
Specific identifiable device, circuit, or system
Nonlinear amplifying circuit
C327S562000, C327S427000, C327S541000, C327S543000, C330S277000, C330S296000
Reexamination Certificate
active
06335657
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an amplifier circuit employing MOSFETs, and in particular, to a circuit for use with a high frequency amplifier circuit employing a MOSFET for compensating for a secular change in a drain current of a MOSFET.
2. Description of the Related Art
A drain current I
DQ
of a MOSFET at the time of no signal input thereto is determined by the magnitude of a gate-source voltage V
GS
thereof.
FIG. 4
shows a V
GS
versus I
DQ
characteristic of such a MOSFET. In general, the electric characteristics such as an operational point of a MOSFET vary depending upon the magnitude of a drain current thereof, so with an amplifier employing a MOSFET, the drain current I
DQ
of the MOSFET is set so as to provide an optimal electrical operational point.
However, a MOSFET has a so-called “hot carrier” phenomenon that its drain current I
DQ
changes over time.
FIG. 5
illustrates an example of such a secular change in the drain current of a MOSFET with a constant gate-source voltage V
GS
thereof. As clearly seen from
FIG. 5
, as the drain current I
DQ
of the MOSFET changes, the electrical operational point of the MOSFET varies, thus posing a problem that the characteristics of the amplifier having an optimal electrical operational point at the time of the manufacture thereof change over time.
SUMMARY OF THE INVENTION
In view of this, the present invention is intended to obviate the above-mentioned problem of the prior art of a change in the operation point of an amplifier due to a change in a drain current of a MOSFET at the time of no signal input, and has its object to provide a MOSFET amplifier circuit which includes a compensation circuit comprising an inexpensive MOSFET and a constant current circuit for making the drain current of the MOSFET constant to thereby maintain the operational point of the amplifier at an optimal state at all the times.
Bearing the above object in mind, according to the present invention, there is provided a MOSFET amplifier circuit comprising: a first MOSFET; a constant current circuit for generating a current of a constant magnitude; a second MOSFET having a constant drain current controlled by said constant current circuit; and a voltage control circuit for applying a gate-source voltage of said second MOSFET to a gate-source voltage of said first MOSFET.
With the above arrangement, the voltage control circuit detects, for example, a gate-source voltage of the second MOSFET, amplifies and supplies it to the first MOSFET as a gate-source voltage thereof, whereby a voltage based on a secular change in the gate-source voltage of the second MOSFET, which provides a constant drain current for the second MOSFET, is given to the first MOSFET as its gate-source voltage, thus reducing a change in the drain current of the first MOSFET or making it at a constant value.
In this regard, it is to be noted that the voltage control circuit has a voltage amplification factor set in such a manner that the detected gate-source voltage of the second MOSFET is amplified in accordance with a difference in magnitude between the secular changes in the first and second MOSFETs, and supplied to the first MOSFET as its gate-source voltage. As a result, according to this invention, employing an inexpensive MOSFET for the second MOSFET, a MOSFET amplifier circuit capable of compensating for the drain current of the first MOSFET can be provided at a low cost.
In a preferred embodiment of the invention, the voltage control circuit comprises operational amplifiers and a plurality of resistors, and the constant current circuit comprises an operational amplifier, a variable resistor, resistors and a power supply.
Preferably, the second MOSFET is formed in a chip into which the first MOSFET is built.
With this arrangement, the secular change characteristics of the first and second MOSFETs can be made substantially the same, thus rendering circuit design easy and simple.
The above and other objects, features and advantages of the present invention will become apparent to those skilled in the art from the following detailed description of preferred embodiments of the invention taken in conjunction with the accompanying drawings.
REFERENCES:
patent: 4051392 (1977-09-01), Rosenthal et al.
patent: 4340867 (1982-07-01), Sano
patent: 5235218 (1993-08-01), Matsuo et al.
patent: 5373226 (1994-12-01), Kimura
patent: 5796278 (1998-08-01), Osborn et al.
patent: 5920232 (1999-07-01), Hariton
patent: 5977832 (1999-11-01), Satyanarayana et al.
Hayase Hiromichi
Ishigami Takeshi
Takenaga Kotaro
Cunningham Terry D.
Kokusai Electric Co. Ltd.
McGinn & Gibb PLLC
Nguyen Long
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