Amplifiers – With semiconductor amplifying device – Including differential amplifier
Reexamination Certificate
1998-11-20
2001-03-13
Mottola, Steven J. (Department: 2817)
Amplifiers
With semiconductor amplifying device
Including differential amplifier
C330S253000
Reexamination Certificate
active
06201443
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to radio frequency communication and more particularly, to variable gain amplifiers used in radio frequency signal transmission systems.
2. Description of Related Art
Variable gain amplifiers (VGAs), devices that attempt to maintain a desired output signal level by adjusting gain, are generally used with radio receivers. As such, the cost of a VGA is significant to the overall cost of the radio receiver function. Currently, most VGAs are fabricated with bipolar processes. Bipolar processes are expensive, costing nearly twice as much as a CMOS fabrication processes. However, little development of VGAs in CMOS has occurred due to noise concerns and due to concerns of integrating VGAs fabricated in CMOS with other radio frequency functions because variation in the input and output impedances of a VGA fabricated with CMOS technology can significantly affect the overall operation of a radio frequency system.
One development of a variable gain CMOS amplifier may be found in U.S. Pat. No. 5,757,230. The variable gain amplifier of the '230 patent uses a linearized transconductor in conjunction with an output circuit. The transconductor has a transconductance that varies in response to a first control signal while the output circuit has a transresistance that varies in response to a second control signal. Both control signals are provided by a gain controller circuit portion. The voltage gain of the amplifier is equal to the product of the transconductance and the transresistance. The amplifier can be configured to provide an exponential gain. The amplifier of the '230 patent is a complicated device requiring as many as 50 transistors to implement.
In view of the above, there is a need for a VGA that can be fabricated with low-cost CMOS technology, provide a desirable signal-to-noise ratio, and maintain predetermined, and constant input and output impedances while providing a straight forward design.
SUMMARY OF THE INVENTION
The needs described above are in large measure met by a variable gain amplifier (VGA) of the present invention. The VGA generally includes a first gain cell, a second gain cell that is operably connected to the first gain cell, and a current control circuit portion that is operably connected to the first and second gain cells. The first gain cell has a transconductance and preferably comprises a pair of MOSFETs. The second gain cell has a transconductance and also preferably comprises a pair of MOSFETs. The first and second gain cells receive an input signal and produce an output signal that is amplified in proportion to their transconductance. The current control circuit portion controls the amount of current sent to the first and second gain cells and, as such, also controls their transconductance. Note that the transconductance of the first gain cell is designed to be larger by a predetermined amount than the transconductance of the second gain cell for a given amount of current.
The present invention further includes a method of amplifying an input signal. The steps for amplifying the input signal are as follows: (1) providing a variable control voltage; (2) providing a current; (3) receiving an input signal; (4) directing an amount of current to a first gain cell and a second gain cell according the level of the variable control voltage (the amount of current may be zero amps); (5) generating a gain from the first and second gain cells (the amount of gain may be zero); (6) amplifying the input signal with the gain from the first and second gain cells; and (7) producing an output reflective of the amplified input signal.
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patent: 5757230 (1998-05-01), Mangelsdorf
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patent: 5818880 (1998-10-01), Kriz et al.
Applied Micro Circuits Corporation
Gray Cary Ware & Freidenrich
Mottola Steven J.
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