Amplifiers – With periodic switching input-output
Reexamination Certificate
2001-06-27
2003-01-21
Lam, Tuan T. (Department: 2816)
Amplifiers
With periodic switching input-output
C330S282000, C330S284000, C327S337000, C327S345000
Reexamination Certificate
active
06509792
ABSTRACT:
BACKGROUND OF THE INVENTION
Field of the Invention
The invention relates to a compensation circuit for attenuating or eliminating undesired properties of an operational amplifier and to a corresponding method for attenuating or eliminating undesired properties of an operational amplifier.
A variety of embodiments of operational amplifiers have been known for many years. An ideal operational amplifier has an infinitely high input resistance over the entire frequency axis, no output resistance, an infinitely high amplification and a number of further ideal properties.
However, in reality, operational amplifiers are not ideal. The aforementioned properties deviate from the ideal case to a greater or lesser extent, and these deviations are frequently also non-linear.
It is particularly unfortunate here that a phase shift which depends on the frequency of the input signals occurs between the input signals and the output signal of the operational amplifier. This phase shift can have the effect that negative feedback from the output terminal of the operational amplifier to its inverting input terminal becomes positive feedback as a result of which the operational amplifier becomes unstable. In order to avoid this, the frequency response must be corrected. During this correction, it is ensured that the gain factor of the operational amplifier is reduced to values which are less than 1 for frequencies at which the phase shift assumes values an which can be critical for the stability of the operational amplifier.
For this purpose, the configuration and/or the external wiring of the operational amplifier must be changed, the change usually including the addition of a capacitor. One of the best known possible ways of performing a frequency response correction is known as Miller compensation.
FIG. 1
is a schematic representation of an operational amplifier with a compensation circuit which is used for frequency response correction. Here, the operational amplifier is designated by the reference OPAMP, and the compensation circuit is designated by the reference KOMP. The compensation circuit KOMP is formed in the example in question of a capacitor C
0
whose terminals are connected to selected points inside the operational amplifier OPAMP.
The compensation circuit KOMP ensures that the operational amplifier is operated with a sufficiently high phase reserve, and thus operates in a stable fashion under all circumstances.
However, the compensation circuit KOMP also has effects on other parameters of the operational amplifier. The (compensation) capacitance of the capacitor C
0
acts, inter alia, on the slew rate and the gain-bandwidth product of the operational amplifier.
The slew rate is generally proportional to I/C, largely independently of the topology of the circuit, and the gain-bandwidth product is proportional to Gm/C, I being the available current, Gm being the steepness of the transistors contained in the operational amplifier and C the compensation capacitance, also largely independently of the topology of the circuit. As the compensation capacitance C becomes larger, the slew rate and the amplification-bandwidth product therefore drop. This cannot generally be compensated by an increase in I or Gm because the technology is usually already operating at the limit of what is feasible.
Good (high) values for the slew rate and the gain-bandwidth product can therefore be achieved only if the compensation capacitance C is not too high. On the other hand, the compensation capacitance C must, however, not be too small because otherwise there is the risk of the operational amplifier becoming unstable.
For this reason, there is an optimum value or value range for the compensation capacitance C, and this value or value range should not, or must not, be significantly undershot or exceeded.
It becomes apparent that this is very difficult in particular if the capacitor is integrated into an integrated circuit which forms or contains the operational amplifier. The capacitances of the capacitors which are integrated into integrated circuits can in fact have very high tolerances (up to ±40%). Although the capacitors can be adjusted to the desired capacitance, for example by trimming with a laser, this process is very complex and expensive. A possible alternative to this is to provide the capacitor outside the integrated circuit which forms or contains the operational amplifier. However, it is then complicated and costly to use the operational amplifier.
Such problems and similar problems may occur not only in the case of frequency response correction but also when other undesired properties of the operational amplifier are attenuated or eliminated.
SUMMARY OF THE INVENTION
It is accordingly an object of the invention to provide a compensation circuit for attenuating or eliminating undesired properties of an operational amplifier and a method for attenuating or eliminating undesired properties of an operational amplifier which overcome the above-mentioned disadvantages of the heretofore-known circuits and methods of this general type and which reduce or eliminate the undesired properties of the operational amplifier as required in a reliable and easy fashion and which involve only little outlay.
With the foregoing and other objects in view there is provided, in accordance with the invention, a compensation circuit for attenuating or eliminating undesired properties of an operational amplifier, including:
a plurality of compensation circuit elements; and
at least some of the compensation circuit elements having at least one property selected from the group consisting of being selectively activatable and deactivatable, being connectable to the operational amplifier, and being connectable in different ways to respective other ones of the compensation circuit elements.
In other words, the compensation circuit according to the invention is defined by the fact that its elements can be activated and deactivated at least partly selectively and/or can be connected in different ways to other elements of the compensation circuit and/or to the operational amplifier.
As a result it is possible to ensure that the operational amplifier always operates in a fast and stable fashion as requested under all circumstances even if the elements which are required for compensation can be manufactured only with large tolerances. It is even possible to perform dynamic adaptation to the respective current conditions and requirements.
According to another feature of the invention, the operational amplifier has a given frequency response, and the compensation circuit elements correct the given frequency response of the operational amplifier.
According to yet another feature of the invention, a regulator is operatively connected to at least some of the compensation circuit elements, and the regulator configures the compensation circuit according to given specifications.
According to a further feature of the invention, the regulator configures the compensation circuit such that the operational amplifier operates in a fast and stable manner according to given requirements.
According to another feature of the invention, the operational amplifier has a transient response with a given duration and a given profile, and the regulator configures the compensation circuit as a function of at least one characteristic selected from the group consisting of the given duration and the given profile of the transient response of the operational amplifier.
According to yet another feature of the invention, the operational amplifier has input terminals, and the regulator determines the given duration and the given profile of the transient response from a ratio of magnitudes of voltages established at the input terminals of the operational amplifier.
According to another feature of the invention, the regulator operates based on the assumption that the transient response is still occurring if the voltages established at the input terminals of the operational amplifier are not of a substantially same magnitude.
According to a further feature
Greenberg Laurence A.
Infineon - Technologies AG
Lam Tuan T.
Locher Ralph E.
Stemer Werner H.
LandOfFree
Circuit and method for attenuating or eliminating undesired... does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Circuit and method for attenuating or eliminating undesired..., we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Circuit and method for attenuating or eliminating undesired... will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-3012830