Slew rate enhancement circuitry for folded cascode amplifier

Abstract

A folded-cascode operational amplifier including a differential input stage (19) and a class AB output stage (20) includes a first slew boost current mirror (13) and a second slew boost current mirror (14) having inputs connected to drains of the input transistors, respectively. Each current mirror amplifies excess tail current steered into it as a result of a large, rapid input signal transition. The amplified excess tail current is used to boost the slew rate of the class AB output stage. in accordance with a first polarity of the difference between the first (Vin+) and second (Vin−) input voltages. The drains of the input transistors are maintained at a voltage less than a transistor threshold voltage above the ground except during slewing operation of the operational amplifier to effectively isolate the current mirrors except during slewing operation.

Description

BACKGROUND OF THE INVENTION [0001] The present invention relates generally to improving the slew rate of a folded cascode amplifier while also maintaining low noise operation. [0002] 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. [0003] One technique for increasing the slew rate of an amplifier is to increase the bias current of the input stage, ...

AI Technical Summary

Benefits of technology

[0010] It is an object of the present invention to provide a low cost, low noise amplifier having a high slew rate.

[0011] It is another object of the invention to provide 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.

[0012] Briefly described, and in accordance with one embodiment, the present invention increases a slew rate of a folded-cascode operational amplifier by applying a large, rapid transition of an input signal between the bases of a first input transistor having a gate coupled to a first input voltage (Vin+), and emitter coupled to a tail current source (15), and a collector (12) coupled to a collector of a first current source transistor and an emitter of a first cascode transistor and a second input transistor having a base coupled to a second input voltage (Vin−), and emitter coupled to the tail current source (15), and a collector (11) coupled to a collector of a second current source transistor and an emitter of a second cascode transistor and amplifying an excess of tail current steered by the first input transistor into the input of a first slew boost circuit (13) over a current flowing in a first current source transistor and applying the amplified excess current to boost the slew rate of a class AB output stage in accordance with a first polarity of the difference between the first (Vin+) and second (Vin−) input voltages.

Problems solved by technology

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.

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.

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