News Column

Patent Issued for Variable Gain Amplifier

August 20, 2014

By a News Reporter-Staff News Editor at Journal of Engineering -- A patent by the inventors Kao, Shuo-Chun (Sunnyvale, CA); Nedovic, Nikola (San Jose, CA), filed on May 22, 2012, was published online on August 5, 2014, according to news reporting originating from Alexandria, Virginia, by VerticalNews correspondents.

Patent number 8797098 is assigned to Fujitsu Limited (Kawasaki-shi, JP).

The following quote was obtained by the news editors from the background information supplied by the inventors: "An electronic amplifier is a device for increasing the power of a signal. In basic operation, an electronic amplifier uses energy from a power supply and controls its output to match an input signal shape but with a larger amplitude. Amplifiers are used in many electronics applications, including in communications, audio systems, servo motor controllers, and many other applications.

"A key operational parameter of any amplifier is its gain. A gain is typically a multiplicative factor relating the magnitude of the output signal to the input signal. The gain may be specified as the ratio of output voltage to input voltage (voltage gain), output power to input power (power gain), or some combination of current, voltage and power. In some applications, it is beneficial that an amplifier have a variable gain, wherein the gain of the amplifier may be modified or changed. However, existing approaches to adapting conventional amplifiers to provide for variable gain have disadvantages.

"To illustrate, reference is made to FIG. 1, which depicts a conventional amplifier 100, as is known in the art. A conventional amplifier may comprise transistors 108a, 108b, drain resistors 112a, 112b, and current source 106. Each drain resistor 112a, 112b may be coupled between the drain terminal of a corresponding transistor 108a, 108b, respectively, and a high potential voltage source (e.g., V.sub.DD). Each transistor 108a, 108b may in turn be coupled at its gate terminal to a differential input (e.g., or and at its source terminal to current source 106.

"In operation, transistors 108a, 108b form a differential transistor pair to sense and track the input data represented by a differential input voltage, and producing a differential output voltage V.sub.out at the drain terminals of transistors 108a, 108b. Those of skill in the art will appreciate that the gain A of amplifier 100 may be given by the equation A=g.sub.m1R.sub.D, where g.sub.m1 is the transconductance of each transistor 108a, 108b and R.sub.D is the resistance of each resistor 112a, 112b. Thus, to make the gain A variable, one or both of g.sub.m1 and R.sub.D must be made variable. For example, R.sub.D may be variable if resistors 112a, 112b are replaced with variable resistors. As another example, those of skill in the art will appreciate that g.sub.m1 is a function of I.sub.SS, the tail current generated by current source 106. Accordingly, g.sub.m1 may be varied by varying I.sub.SS, which it turn varies the gain A. However, as those of skill in the art will appreciate, the common mode output voltage, out of amplifier 100 may be given by the equation, out=V.sub.DD-I.sub.SSR.sub.D/2. Thus, adjusting the gain by changing either of g.sub.m1 or R.sub.D will lead to a change in the output common mode voltage. Changing the output common mode of amplifier 100 may be undesirable, as it disturbs the bias condition of the circuit."

In addition to the background information obtained for this patent, VerticalNews journalists also obtained the inventors' summary information for this patent: "In accordance with some embodiments of the present disclosure, a method may include applying an input differential voltage to input terminals of an amplifier, the input terminals comprising a first input terminal and a second input terminal, the first input terminal coupled to a gate terminal of a first transistor and the second input terminal coupled to a gate terminal of a second transistor. The method may also include varying a gain of the amplifier by varying at least one of: a resistance of a first resistor, the first resistor coupled between a source terminal of the first transistor and a source terminal of the second transistor; and a resistance of a second resistor, the second resistor coupled between a source terminal of a third transistor and a source terminal of a fourth transistor; wherein: the third transistor is coupled at its drain terminal to the drain terminal of the first transistor; and the fourth transistor is coupled at its drain terminal to the drain terminal of the second transistor and a gate terminal of the third transistor and coupled at its gate terminal to the drain terminal of the third transistor.

"One or more technical advantages of the present disclosure may be readily apparent to one skilled in the art from the figures, descriptions, and claims included herein."

URL and more information on this patent, see: Kao, Shuo-Chun; Nedovic, Nikola. Variable Gain Amplifier. U.S. Patent Number 8797098, filed May 22, 2012, and published online on August 5, 2014. Patent URL:

Keywords for this news article include: Fujitsu Limited.

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Source: Journal of Engineering

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