Embodiments are generally related to operational amplifiers, Embodiments are additionally related to operational amplifiers with indirect Miller-type frequency compensation topologies, which employ current buffers.
Operational amplifiers are implemented in a variety of topologies including so-called indirect or Ahuja frequency compensation topologies, which employ current buffers. The most widely known introduction of operational amplifier with indirect frequency compensation of the output stage using a current buffer was derived by B. K. Ahuja. See, for example, B. K. Ahuja, “An Improved Frequency Compensation Technique for CMOS Operational Amplifiers,” IEEE J. Solid-State Circuits, vol. SC-18, No. 6, pp. 629-633, December 1983, which is incorporated herein by reference. Thus, operation amplifiers of this type are often referred to as Ahuja or Ahuja-type amplifiers, components, frequency compensation, etc.
The Ahuja frequency compensation scheme is a well-known frequency compensation for operational amplifiers. The Ahuja frequency compensation was developed to improve upon the well-known Miller compensation, in which a capacitor is connected between the drain and the gate of the output transistor operating in common source amplifier mode. The Ahuja frequency compensation implements an indirect coupling of this capacitor by providing a current buffer which delivers the capacitor feedback signal to the gate of the output transistor of the amplifier.
a) illustrates a schematic diagram of a prior art operational amplifier circuit 10 with a Miller compensation circuit. The circuit 10 shown in
b) illustrates a schematic diagram of a prior art operational amplifier circuit 50 with an Ahuja-type compensation circuit. Note that in
c) illustrates a Bode plot 90 of a prior art operational amplifier with Miller compensation.
There are many advantages to the use of Ahuja frequency compensation applications circuits.
One of the problems associated with Ahuja-type frequency compensation circuits is the use of the more complex internal feedback loop, which causes a ringing at the amplifier output. This significantly limits the use of an otherwise very effective compensation technique.
c) and 1(d) depict simulated Bode plots for these two schematics. The plot 90 of
This ringing is caused by the fact that Ahuja compensation, contrary to that of Miller compensation, has a frequency compensation internal feedback loop, which is not inherently stable. Such a loop is composed of two amplifying transistors: PMOS transistor MB (i.e., transistor 28 in
In the frequency range where 1/RL>>ωCL>>ωCm (where RL is the effective active load of the output stage, ω is radian frequency, CL is the load capacitance and Cm is the frequency compensation capacitance), the loop has a phase margin dose to 180°, but in the high frequency range where ωCL>>ωCm≧Gm(Mb) (where Gm(Mb) is the transconductance of the folded cascode transistor acting as a current buffer) two poles become dominant in the frequency response of the loop significantly deteriorating the phase margin.
In order to take full advantage of Ahuja compensation, the unity gain of this feedback loop has to be in this high frequency range. So far, it has never been an attempt to change the loop topology in order to mitigate its effect on the operational amplifier transient response but all efforts were directed upon optimization of its component parameters even in the most recent studies of the Ahuja compensation technique. As it is clearly seen in
The following summary is provided to facilitate an understanding of some of the innovative features unique to the disclosed embodiments and is not intended to be a full description. A full appreciation of the various aspects of the embodiments disclosed herein can be gained by taking the entire specification, claims, drawings, and abstract as a whole.
It is, therefore, the aspect of the disclosed embodiments to provide for an operation amplifier employing an Ahuja-type of the frequency compensation with improved stability of the internal frequency compensation feedback loop.
The aforementioned aspect and other objectives and advantages can now be achieved as described herein. An operational amplifier apparatus, method, and system are disclosed, which includes an operational amplifier circuit having at least one output node and an output stage coupled to the output node, the output stage containing an output and first MOS transistor circuit employed in a common source amplifier mode, a frequency compensation capacitor coupled between the output of the output stage and the gate of the first transistor circuit by means of a second MOS transistor employed in a common gate amplifier mode; the other node of the capacitor and the output of the output stage are coupled to the amplifier output node with a resistor.
The resistor (or resistors) improves the stability associated with the internal frequency compensation feedback loop while eliminating any ringing at the output of the operational amplifier circuit caused by a lack of a phase margin of the internal feedback loop.
Instead of MOS transistors, bipolar transistors could be equally employed in the operational amplifier. In this case the first transistor circuit is connected in a common emitter amplifier configuration while the other bipolar transistor is connected in a common base amplifier configuration.
The accompanying figures, in which like reference numerals refer to identical or functionally-similar elements throughout the separate views and which are incorporated in and form a part of the specification, further illustrate the present invention and, together with the detailed description of the invention, serve to explain the principles of the present invention.
a) illustrates a schematic diagram of a prior art operational amplifier with a Miller compensation circuit;
b) illustrates a schematic diagram of a prior art operational amplifier with an Abuja-type compensation circuit;
c) illustrates a Bode plot of a prior art operational amplifier with Miller compensation;
d) illustrates a Bode plot of a prior art operational amplifier with Ahuja-type compensation;
e) illustrates a plot of the transient response of a prior art operational amplifier with Miller compensation;
f) illustrates a plot of the transient response of a prior art operational amplifier with Ahuja-type compensation;
The particular values and configurations discussed in these non-limiting examples can be varied and are cited merely to illustrate at least one embodiment and are not intended to limit the scope thereof.
The embodiments will now be described more fully hereinafter with reference to the accompanying drawings, in which illustrative embodiments of the invention are shown. The embodiments disclosed herein can be embodied in many different forms and should not be construed as limited to the embodiments set forth herein; rather, these embodiments are provided so that this disclosure will be thorough and complete and will fully convey the scope of the invention to those skilled in the art. Like numbers refer to like elements throughout. As used herein, the term “and/or” includes any and all combinations of one or more of the associated listed items.
The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the invention. As used herein, the singular forms “a”, “an” and “the” are intended to include the plural forms as well, unless the context clearly indicates otherwise. It will be further understood that the terms “comprises” and/or “comprising,” when used in this specification, specify the presence of stated features, integers, steps, operations, elements, and/or components, but do not preclude the presence or addition of one or more other features, integers, steps, operations, elements, components, and/or groups thereof.
One goal of the disclosed embodiments is to eliminate ringing at the operational amplifier output caused by the lack of the phase margin of the internal frequency compensation loop. This goal can be achieved by introducing a resistor between output node of the operational amplifier and the output of the output stage, which is the common connection point of the drain node of the output transistor operating in common source mode and the local feedback capacitor. Two possible ways of connecting such a resistor are shown in
In circuit 113 the active load transistor of the output stage (e.g., PMOS transistor 36, being a “third transistor circuit”) can be directly connected to the output of the output stage. In this situation, the current associated with the load transistor does not flow through the additional resistor 138. In circuit 111, the active load transistor 36 is coupled to the output node of the operational amplifier by resistor 138 and the current associated with the load transistor creates an additional voltage drop across newly introduced resistor 138. The choice of the resistor connection option is based on the required output signal dynamic range of the operational amplifier. If the range should be as close to the power supply midpoint as possible, the option shown in schematic 111 is preferable. The option of schematic 113 is better suited, if the range should be close to the VDD.
The resistor (e.g. resistors 138 and 148) introduce a zero into the transfer function of the internal feedback loop, which mitigates the effect of the two dominant poles and improves the phase margin of the loop around the loop unity gain frequency. If the resistor value satisfies the following expression: R>=Cm/(CL*Gm(Mb)), the internal feedback loop will be stable enough not to create complex conjugated poles in the whole operational amplifier transfer function. The additional zero in the local feedback transfer function acts as another non-dominant pole of the whole amplifier transfer function. This pole and the zero, which is introduced by Ahuja-type compensation at the frequency ω=Gm(Mb)/Cm, mutually cancel each other resulting in a whole operational amplifier classical transfer function with one dominant pole and one non-dominant pole as it can be seen from the Bode plot 210 shown in
Finally, a plot 220 of the transient response of the operational amplifier global loop according to the disclosed embodiments is shown in
The improvement of an operational amplifier phase margin by including a resistor between the operational amplifier output and significantly capacitive load is a common and widely used approach. In this case, however, the global feedback is connected to the amplifier output, but not directly to the load. The voltage drop across an additional resistor creates an error at the load since only the operational amplifier output node has the accurate voltage.
In the disclosed embodiments, the additional resistor (e.g., resistors 138, 140 shown in
It will be appreciated that variations of the above-disclosed and other features and functions, or alternatives thereof, may be desirably combined into many other different systems or applications. Also, that various presently unforeseen or unanticipated alternatives, modifications, variations or improvements therein may be subsequently made by those skilled in the art which are also intended to be encompassed by the following claims.
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Entry |
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Ahuja, B. K., “An Improved Frequency Compensation Technique for CMOS Operational Amplifiers,” IEEE Journal of Solid-State Circuits (1983) SC-18(6):629-633. |
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Number | Date | Country | |
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20140043100 A1 | Feb 2014 | US |