Amplifiers – With semiconductor amplifying device – Including differential amplifier
Patent
1980-06-25
1983-04-05
Mullins, James B.
Amplifiers
With semiconductor amplifying device
Including differential amplifier
330255, 330257, 330258, 330260, H03F 345, H03F 316
Patent
active
043792677
DESCRIPTION:
BRIEF SUMMARY
The invention relates to the field of integrated circuit amplifiers and, more particularly, to low power N channel MOS differential amplifier comprised of two input transconductance MOSFET's coupled to positive and negative inputs and having a single common source line in which is placed a feedback device for the purpose of amplifying the difference between an input signal impressed upon the input terminals, and for rejecting common mode input signals.
The recent developments in switched capacitor filtering techniques have caused a need to arise for a low power differential amplifier which can be used to implement precision filters monolithicaly in MOS technology without the need for trimming and which has a gain and frequency response which is substantially stable despite inadvertent production variations in the processing of individual batches of identically designed integrated circuits.
Low power consumption is extremely important in PCM filters since telephone switching systems must operate from battery back-up power. Since numerous differential amplifiers are required throughout each pulse code modulation filter, low power consumption and insensitivity to processing variations are important. That is to say that the maximum amount of gain must be achieved with a minimum number of transistors without sacrificing stability or common mode rejection.
Two important problems in differential amplifier design are differential-to-single-ended conversion without loss of gain and high common mode rejection. With respect to the differential-to-single-ended conversion, it is necessary to convert a normal differential amplifier having two inputs and two outputs to an amplifier having two inputs and one output without losing half of the gain of the amplifier. With respect to common mode rejection, it is desirable to have zero output voltage when the two inputs change by the same amount in the same polarity and at the same time.
Operational amplifiers are used very frequently as building blocks for other electronic systems. They are used primarily with externally applied feedback to perform functions such as providing a stable gain block, integration and filtering. The ideal operational amplifier is a voltage controlled voltage source having two input terminals and a single output terminal. Ideally, an operational amplifier has: a large gain; zero output voltage when the difference between the input voltages is zero; zero input current; zero output resistance to maximize gain; no frequency dependence, no temperature dependence, no distortion, no processing dependence; and low power consumption.
Typically, an operational amplifier is constructed of an input differential amplifier employing a differential-to-single-ended conversion circuit to allow the output signal to be taken from only one output terminal, and employs a common mode feedback loop to increase common mode rejection. Following the input differential amplifier, a level shift stage is used to shift the D.C. component of the output signal down to near zero relative to analog ground so that the output voltage will be near zero for zero input voltage. Finally, an output stage having little gain is used to lower the output resistance of the operational amplifier and to increase the current drive capability since the input and gain stages seldom can supply the output current requirement and generally have high output impedances.
One way of doing the differential-to-single-ended conversion is by simply taking the output signal from only one of the two output nodes of the differential amplifier. The disadvantages of this method are that the gain is only half that which would be obtained if a differential output were used, and the output signal contains not only a differential component but also a common mode component.
In accordance with the principles of the instant invention, an operational amplifier is described which utilizes simple circuitry compatible with N channel MOSFET processing to achieve temperature and processing variation tolerant gain and frequency
REFERENCES:
patent: 3959733 (1976-05-01), Solomon et al.
patent: 4042839 (1977-08-01), Araki
patent: 4219782 (1980-08-01), Stein
patent: 4284959 (1981-08-01), Heagerty et al.
Love, "In-Phase Weak-Signal Input Circuit", IBM Technical Disclosure Bulletin, vol. 19, No. 12, May 1977, p. 4673.
Mostek Corporation
Mullins James B.
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