专利汇可以提供Amplifier circuits and methods for cancelling Miller capacitance专利检索,专利查询,专利分析的服务。并且An amplifier circuit has an input stage, a current mirror stage, and an output stage. The output stage has a transistor for which a non-linear and/or linear Miller capacitance exists across the transistor. A capacitive element, referred to herein as a “negative Miller capacitor,” is coupled between an input node of the current mirror stage and the transistor's collector or drain causing the current flowing through the negative Miller capacitor to be inverted, supplying the current taken by the usual Miller capacitance of the output stage. Thus, the negative Miller capacitor cancels the usual Miller capacitance across the transistor of the output stage, and such cancellation occurs without significantly increasing the amplifier's input power and costs. In some embodiments, both linear and non-linear components of the usual Miller capacitor are cancelled. Further, cancellation of the Miller capacitance generally enhances bandwidth and reduces distortion, thereby improving the performance of the operational amplifier.,下面是Amplifier circuits and methods for cancelling Miller capacitance专利的具体信息内容。
Now, therefore, the following is claimed:
This application claims priority to U.S. Provisional Patent Application No. 61/362,548, entitled “Negative Miller Capacitor for Bandwidth Enhancement and Distortion Reduction” and filed on Jul. 8, 2010, which is incorporated herein by reference.
An operational amplifier (op-amp) is a well-known device used to provide a voltage and/or current gain to an input signal. Such an amplifier typically has an input stage, a current mirror stage, and an output stage. A transistor of the output stage has a natural non-linear Miller capacitance across its base and collector, for a bipolar transistor, or across its gate and drain, for a field effect transistor (although the non-linearity of Cgd in a metal-oxide-semiconductor field-effect transistor (MOSFET) is typically much smaller than in a bipolar transistor). The presence of this Miller capacitance limits the bandwidth of the output stage. That bandwidth varies with the non-linear capacitance, causing distortion. Often, the non-linear capacitor is supplemented by a parallel linear capacitor to make the bandwidth more predictable and reduce distortion. A capacitor positioned at such location is typically referred to as a “Miller capacitor.”
Previous attempts to cancel the portion of the Miller capacitance built into the semiconductor devices have various shortcomings. In particular, they use an additional inverting output stage, which consumes extra power, and/or they are unable to cancel the non-linear components of the Miller capacitance. If the non-linear components of the Miller capacitance can be cancelled without adding an additional inverting output stage, then it is possible to make a wider bandwidth amplifier having less distortion for a given amount of power consumption. Moreover, techniques for compensating for the non-linear and/or linear Miller capacitance in the output stage of an operational amplifier without adversely affecting the amplifier's cost or performance, in terms of input power, speed, and area, are generally desired.
The disclosure can be better understood with reference to the following drawings. The elements of the drawings are not necessarily to scale relative to each other, emphasis instead being placed upon clearly illustrating the principles of the disclosure. Furthermore, like reference numerals designate corresponding parts throughout the several views.
The present disclosure generally pertains to amplifier circuits for cancelling Miller capacitance. In one exemplary embodiment, a circuit for an operational amplifier has an input stage, a current mirror stage, and an output stage. The output stage has a transistor for which a non-linear and/or linear Miller capacitance exists across the transistor. A capacitive element, referred to herein as a “negative Miller capacitor,” is coupled between the transistor's collector or drain and the input of the current mirror stage. The voltages across both the Miller and the Negative Miller capacitors are substantially the same. However, the inversion of the current mirror causes all the current taken by the Miller capacitor to be supplied by the action of the current mirror in concert with the negative Miller capacitor. This nullifies the effect of the Miller capacitor of the output stage. If the current mirror has unity gain and the negative Miller capacitor and the Miller capacitor have similar capacitance versus voltage characteristics, then the negative Miller capacitor can cancel linear and/or non-linear components of the Miller capacitor. Such cancellation occurs without significantly increasing the amplifier's input power and costs. Further, cancellation of the Miller capacitance of the output stage generally enhances bandwidth and reduces distortion, thereby improving the performance of the operational amplifier.
As shown by
There exists a natural non-linear Miller capacitance across the base and collector of the transistor 52 for the output stage 29. The non-linear nature of such capacitance (Cm) undesirably results in changing bandwidth with changing output voltage, which can be swamped by connecting the base and collector via a Miller capacitor 55 having linear Miller capacitance (CL). The total capacitance across the base and collector of the transistor 52 is equal to Cm+CL.
In this regard, the negative Miller capacitor 66 is coupled between the output connection 25 (and, hence, the collector of the transistor 52) and a node 72, referred to hereafter as “input node,” of the current mirror 28.
Notably, all the capacitors under discussion (including the Miller capacitor 55, the negative Miller capacitor 66, and the capacitive components of the transistor 52) have the same connection on one side, that being output connection 25. The other end of each capacitor connects to voltages very much smaller than the output voltage 25. Thus, all the capacitors have substantially the same voltage across them. Mirror 28 inverts the current flowing through the negative Miller capacitor (81 in
Since the transistor 81 of the negative Miller capacitor 66 has similar characteristics as the output transistor 52 and via the action of the mirror 28 results in an inverted current relative to the current fed to the output transistor 52, any change in the non-linear Miller capacitance Cm of the output transistor 52 due to a change in the collector-to-base voltage (VCB) of the output transistor 52 or otherwise should be reflected in the non-linear Miller capacitance Cn of the negative Miller capacitor 66. Accordingly, the complementary behavior of the negative Miller capacitor 66 ensures cancellation across a range of voltages.
Note that in some cases, it may be desirable to limit the amount of Miller capacitance that is cancelled. As an example, in a unity gain stable operational amplifier, it may be desirable to have a small difference between Cm and Cn to provide enough feedback current to achieve a desired pole splitting behavior. Such a difference can be achieved by configuring the transistor 81 of the negative Miller capacitor 66 to have a slightly different size relative to the output transistor 52.
In some other cases, one may choose to cancel all the nonlinear Miller capacitance Cm, but still place a modest amount of linear capacitance via the Miller capacitor 55. The result will have less bandwidth variation with output voltage that a similar stage having uncancelled non-linear Miller capacitance.
In addition, in some cases, such as a high bandwidth or gain application, it may be desirable to remove the Miller capacitor 55 such that cancellation of the non-linear Miller capacitance Cm effectively drives the capacitance across the base and collector of the output transistor 52 to approximately zero. Such an embodiment generally provides maximum bandwidth for a given amount of input current to the amplifier circuit 63.
In the exemplary embodiments described above, the transistors 44, 45, 48, 49, 52, and 81 are implemented as bipolar transistors. However, it is possible for such transistors to be implemented as field effect transistors, or with combinations of both bipolar and field effect transistors.
Note that when the output transistor 52 is a field effect transistor, its drain-to-gate capacitance is substantially linear. In such case, the capacitance of the negative Miller capacitor 66 is substantially linear as well.
In some embodiments, it is possible for the Miller capacitance of the output stage 29 to have both linear and non-linear components, and it may be desirable to cancel both types of capacitances.
The negative Miller capacitor 66 can be implemented via a field effect transistor (not shown) having a drain coupled to the output connection 25 and a gate coupled to the input node 72. In such case, the source may be left open similar to the emitter of the transistor 81 in
If desired, the negative Miller capacitors 66 and 99 may be configured to cancel all of the linear and non-linear Miller capacitances of the output stage 29 thereby driving the total Miller capacitance of the output stage 29 to zero. In such an embodiment, the circuit 63 may be implemented without the Miller capacitor 55. However, in an alternative embodiment, the Miller capacitor 55 may used in order to control the bandwidth of the circuit 63, as may be desired. In such an embodiment, the negative Miller capacitors 66 and 99 may cancel all of the natural Miller capacitances built into the devices used to implement the output stage 29, and the Miller capacitor 55 may be added to provide precise and predictable control of the circuit bandwidth.
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