The present invention relates generally to improving the slew rate of a folded cascode amplifier while also maintaining low noise operation.
The “slew rate” of an amplifier is a measure of how fast the amplifier can charge up a large capacitor that is connected to an output conductor of the amplifier in response to a large, rapid increase or decrease (such as a step function increase or decrease) of the input voltage applied to the amplifier. (More generally, the slew rate is a measure of the maximum rate of change of the output voltage in response to an input step function, and is normally, but not necessarily, limited by charging the compensation capacitors.) A high slew rate generally is a desirable characteristic of an amplifier, especially an operational amplifier, and especially a high-speed CMOS operational amplifier.
One technique for increasing the slew rate of an amplifier is to increase the bias current of the input stage, but that has a tendency to increase the bandwidth of the amplifier and leads to a need to increase the compensation capacitance of the amplifier to improve circuit stability, which tends to decrease the slew rate. Moreover, in the folded cascode amplifier, increasing the input stage bias current requires a commensurate increase in the second stage current (to avoid turning the second stage off, which in turn adds to the total input referred noise.
Providing both a high slew rate and a low noise level in a two stage folded cascode amplifier puts conflicting constraints on bias currents of the input stage and the second stage of the amplifier, because keeping the second stage noise contributions low generally requires keeping the operating bias currents low in the second stage. However, that ordinarily results in a low slew rate of the two stage folded cascode amplifier.
There is a very large market for improved, low-cost operational amplifiers with high slew rate capability. Although there are many operational amplifier designs capable of providing high slew rates, they unfortunately have various problems, including high noise generation, high power dissipation, ineffective operation at low power supply voltages, poorly controlled operational parameters over a range of power supply voltages, and/or redundant circuitry.
The closest prior art is believed to include commonly assigned U.S. Pat. No. 6,359,512 entitled “Slew Rate Boost Circuitry and Method” issued Mar. 19, 2002 to Ivanov et al. and commonly assigned U.S. Pat. No. 6,437,645 entitled “Slew Rate Boost Circuitry and Method” issued Aug. 20, 2002 to Ivanov et al., both of which relate to boosting slew rates of differential amplifiers and operational amplifiers. The closest prior art is believed to also include U.S. Pat. No. 4,570,128 entitled “Class AB Output Circuit with Large Swing” issued Feb. 11, 1986 to Monticelli, which discloses details of a class AB output circuit which can be used in conjunction with the present invention.
FIG. 1 is a schematic diagram of an operational amplifier including a conventional simple folded cascode input stage 19A which drives a class AB output stage 20A similar to the one described in the above-mentioned Monticelli U.S. Pat. No. 4,570,128. FIG. 2 is a schematic diagram of another operational amplifier including a conventional complementary folded cascode input stage 19 which drives a class AB output stage 20 that is slightly different than class AB output stage 20A shown in FIG. 1.
Thus, there is an unmet need for a low cost, low noise amplifier having a high slew rate.
There also is an unmet need for a low cost, low noise amplifier having the ability to increase the slew rate of various already existing amplifier designs without altering the signal path architecture and without adding additional quiescent power consumption.