Amplifiers – Signal feedback – Phase shift means in loop path
Reexamination Certificate
1999-02-11
2001-03-27
Mottola, Steven J. (Department: 2817)
Amplifiers
Signal feedback
Phase shift means in loop path
C330S100000, C330S109000
Reexamination Certificate
active
06208206
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to novel frequency control compensation techniques for low-voltage low-power multistage amplifiers, and to amplifiers incorporating such techniques.
BACKGROUND OF THE INVENTION
High-gain and high-speed amplifiers are vital in analog circuits and are used in a very wide range of applications. The increasing tendency towards low-voltage designs, especially as many devices and integrated circuits are made to smaller dimensions, causes significant problems in amplifier design.
A particular problem is that as the power supply voltage is scaled down in the design, the threshold voltage does not necessarily scale down in the same way. For an operational amplifier in such a situation conventional vertical gain enhancement techniques (cascoding) are no longer suitable for low-voltage applications and instead horizontal gain enhancement techniques (cascading) must be used. However with cascaded amplifiers the stability of the amplifier and its bandwidth are both limited by existing frequency compensation techniques.
PRIOR ART
Frequency compensation techniques for multistage amplifiers are becoming increasingly important as cascode configurations are no longer applicable in low-voltage low-power designs. One very well known prior frequency compensation technique is nested Miller compensation which is commonly used to ensure the stability of a multistage amplifier.
FIG. 1
shows schematically the structure of a three-stage amplifier using nested Miller compensation (NMC). The amplifier of
FIG. 1
suffers from bandwidth reduction due to the two capacitive feedback loops by C
m1
and C
m2
. As is shown in
FIG. 2
, if the second loop and capacitor C
m2
is not used the bandwidth is increased, but the second and third pole will form a frequency “peak” near the unity-gain frequency due to the small value of the damping factor of the second order function of the second and third pole of the amplifier.
Several advanced topologies such as multipath nested Miller compensation (MNMC) and nested Gm-C compensation (NGCC) have been proposed to overcome the bandwidth reduction problem. MNMC techniques are disclosed for example in R. G. H. Eschauzier, L. P. T. Kerklaan, and J. H. Huijsing, “
A
100
MHZ
100
dB Operational Amplifier with Multipath Nested Miller Compensation Structure”, IEEE Journal of Solid-State Circuits,
vol. 27, pp. 1709-1717, 1992 and in R. G. H. Eschauzier and J. H. Huijsing, “
Frequency Compensation Techniques for Low-Power Operational Amplifiers
” Boston: Kluwer Academic Publishers, 1995. NGCC techniques are disclosed in
F. You.
S. H. K. Embabi and E. Sanchez-Sinencio, “
A Multistage Amplifier Topology with Nested Gm
-
C Compensation for Low-Voltage Application”, IEEE ISSCC
, pp. 348-349, 1997.
When compared to an amplifier using NMC, MNMC can increase the gain-bandwidth product by a factor of approximately two, while NGCC can further improve the stability of the amplifier. However, the bandwidth enhancement by MNMC and NGCC topologies is still not sufficient for analog circuits which require high-gain and high-speed amplifiers to drive large capacitive loads, such as for example the error amplifier in a low-dropout regulator within a portable electronic device.
SUMMARY OF THE INVENTION
According to the present invention there is provided a three stage amplifier comprising: first, second and third gain stages wherein said first gain stage receives an amplifier input signal and said third gain stage outputs an amplifier output signal; a feedback loop having a first compensation capacitance provided from the output of the third gain stage to the output of the first gain stage; and damping factor control means for controlling the damping factor of the second and third complex poles of the said amplifier.
In general, the gain-bandwidth product of a three-stage amplifier is reduced by the presence of the additional compensation capacitor included in the inner feedback loop. It is therefore desirable to remove this feedback loop. Doing so, however, as discussed above causes the amplifier to be unstable. By means of the present invention a damping factor control means is introduced in order to control the damping factor of the second and third poles to stabilize the amplifier.
Preferably the damping factor control means comprises a fourth gain stage. The fourth gain stage may include a feedback loop with a second compensation capacitance, and the fourth gain stage is preferably a negative gain stage. By means of this arrangement the damping factor of the amplifier may be adjusted by appropriately setting the values of the transconductance of the fourth stage and of the second compensation capacitance.
The damping factor control means is preferably located such that it receives an input taken from the output of the second gain stage, but alternatively it may be located such as to receive an input from the output of the first gain stage.
The amplifier may preferably also be provided with a feedforward transconductance stage extending from the output of the first gain stage to the output of the third final gain stage.
REFERENCES:
patent: 4560957 (1985-12-01), Cabot
patent: 5155447 (1992-10-01), Huijsing et al.
Eschauzier, Ruud G. H. et al., “A 100-MHZ 100-dB Operational Amplifier with Multipath Nested Miller Compensation Structure”; IEEE Journal of Solid-State Circuits, vol. 27, No. 12, Dec. 1992, pp. 1709-1717.
Eschauzier, R. G. H. et al, “Frequencey Compensation Techniques for Low-Power Operational Amplifiers”; Boston: Kluwer Academic Publishers, 1995, pp. 105-117.
You, Fan et al, “A Multistage Amplifier Topology with Nested Gm-C Compensation for Low-Voltage Application”; 1997 IEEE International Solid State Circuits Conference, Feb. 8, 1997, pp. 348-349.
Ki Wing Hung
Leung Ka Nang
Mok Kwok Tai Philip
Sin Kin On Johnny
Burns Doane Swecker & Mathis L.L.P.
Mottola Steven J.
The Hong Kong University of Science and Technology
LandOfFree
Frequency compensation techniques for low-power multistage... does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Frequency compensation techniques for low-power multistage..., we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Frequency compensation techniques for low-power multistage... will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2539059