Amplifiers – With periodic switching input-output
Reexamination Certificate
2002-05-08
2004-06-29
Van, Nguyen Khanh (Department: 2817)
Amplifiers
With periodic switching input-output
C330S051000, C330S258000
Reexamination Certificate
active
06756842
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to an a.c. coupled high gain multistage operational amplifier.
BACKGROUND OF THE INVENTION
Switched capacitance operational transconductance amplifiers commonly use two stages: a telescopic amplifier with a preamplifier to increase gain without degrading stability and to increase bandwidth. T. B. Cho & P. Gray, “A 10b, 20M sample/s, 35 mW Pipline A/D converter”, IEEE JSSC, Mar. 1995, page 166. K. Vleugels, S. Rabii, & B. Woday, “A 2.5 MHz Broadband Multi-Bit &Sgr;&Dgr; Modulator with 95 dB Dynamic Range”, Proc. ISSCC, 2001, page 50. However, this places tight voltage headroom restrictions on the preamplifier circuit which is constrained by the bias of the input circuits of the telescopic amplifier. This constraint can be reduced somewhat if NMOS and PMOS devices are used but PMOS devices tend to reduce bandwidth and gain which is undesirable.
BRIEF SUMMARY OF THE INVENTION
It is therefore an object of this invention to provide an improved a.c. coupled high gain multistage operational amplifier.
It is a further object of this invention to provide an improved a.c. coupled high gain multistage operational amplifier which decouples the stages so that the bias voltages in each stage can be independent of each other.
It is a further object of this invention to provide an improved a.c. coupled high gain multistage operational amplifier which decouples the stages so that the device areas at the input of each stage can be independent of each other.
It is a further object of this invention to provide an improved a.c. coupled high gain, high bandwidth, multistage operational amplifier which is advantageous for low voltage designs.
It is a further object of this invention to provide an improved a.c. coupled high gain multistage operational amplifier which cancels the offset of the first stage.
The invention results from the realization that an improved a.c. coupled high gain, high bandwidth, multistage operational amplifier in which the bias voltages of the input devices are independent of each other can be achieved by employing an a.c. coupling capacitance between the stages and charging that capacitance during the track phase when the operational amplifier is not operating and using the charge on the coupling capacitance to bias the next stage during the hold phase without constraining the bias on the input of the previous stage.
This invention features an a.c. coupled multistage high gain operational amplifier including at least two gain stages, each having an input and an output. There is an a.c. coupling level shifting capacitance interconnecting the output of the first stage to the input of the second stage and a charging circuit interconnecting with the a.c. coupling level shifting capacitance and the input of the second stage to charge the a.c. coupling level shifting capacitance in a track phase and to connect the a.c. coupling capacitance to the input of the second stage during a hold phase for dissociating the bias voltages of the stages.
In a preferred embodiment the charging circuit may include a common mode switching circuit interconnected with the a.c. coupling level shifting capacitance and the input of the second stage to charge the a.c. coupling level shifting capacitance in the track phase and to connect the a.c. coupling level shifting capacitance to the input of the second stage during the hold phase. There may be a common mode bias circuit for charging the a.c. coupling level shifting capacitance during the track phase. The stages may be implemented in NMOS. There may be a parasitic capacitance associated with the input of the second stage and the a.c. coupling level shifting capacitance may be larger than the parasitic capacitance. The second stage may include a telescopic amplifier. The telescopic amplifier may include a tail current source. The a.c. coupling level shifting capacitance may effect the cancellation of the offset of the first stage.
REFERENCES:
patent: 5600275 (1997-02-01), Garavan
patent: 5745002 (1998-04-01), Baschirotto et al.
patent: 6252454 (2001-06-01), Thompson et al.
patent: 6259316 (2001-07-01), Nagaraj
Vleugefs et al.,A 2.5V Broadband Multi-Bit Sigma-Delta Modulator with 95dB Dynamic Range, 2001 IEEE International Solid State Circuits Conference, Session 3, Oversampling ADCs, 3.4, Feb. 5, 2001.
Cho et al.,A 10 b, 20 Msample/s, 35 mW Pipeline A/D Converter, IEEE Journal of Solid-State Circuits, vol. 30, No. 3, Mar. 1995, pp 166-172.
Mehr Iuri
Singer Lawrence A.
Yang Wenhua
Analog Devices Inc.
Iandiorio & Teska
Van Nguyen Khanh
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