Amplifiers – With semiconductor amplifying device – Including differential amplifier
Reexamination Certificate
1999-08-16
2001-08-28
Pascal, Robert (Department: 2817)
Amplifiers
With semiconductor amplifying device
Including differential amplifier
C330S259000, C330S292000
Reexamination Certificate
active
06281751
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This application relates to operational amplifier methods and systems and more particularly to methods and systems for frequency compensation of class AB operational amplifiers (i.e., “opamps”) with fully-differential input stages.
2. Description of Related Art
Some prevalent operational amplifier designs require frequency compensation to meet predetermined operational standards. In particular, class AB operational amplifiers having a single ended output and fully-differential input stages require frequency compensation to operate at an acceptable level. Single-ended class AB operational amplifiers have fully-differential input stages in some instances. This is because the non-linearity of the amplifier is dominated by the non-linearity of the input stage in many cases. A fully differential input stage is a balanced structure which is very linear. The non-linearity of the second stage is divided by the gain of the input stage.
A common way of compensating two-stage operational amplifiers is to connect a pole-splitting capacitor between a first stage high-impedance node and a second stage high-impedance node. The two stages are inverting with respect to each other. This causes one of the poles to move to a low frequency level, with the pole becoming dominant, while the other pole moves to high frequency. According to Paul Gray and Robert Meyer in
Analysis and Design of Analog Integrated Circuits
(Wiley), if the new high frequency pole is above the crossover frequency of the operational amplifier, the stability of the operational amplifier is not detrimentally affected.
Operational amplifiers are configured as line drivers in digital subscriber lines (DSL) and as preamplifiers in sensing systems having an input sensor connected to a preamplifier which in turn drives an analog to digital converter.
ADSL and DSL Technologies
by Walter Goralski (McGraw-Hill 1998) describes DSL systems in which such operational amplifiers can be used.
A major technical problem in the design of single-ended output and fully-differential input operational amplifiers is degraded performance due to lack of balanced frequency compensation. Accordingly, circuit designers avoid using such operational amplifiers because of their inadequate performance characteristics. Accordingly, it is desired to develop more refined operational amplifier designs that will enable class AB amplifiers to operate at enhanced levels without suffering from degraded performance.
SUMMARY OF THE INVENTION
According to the present invention, an operational amplifier has a single-ended output and fully-differential input stages, respectively an output connection and first and second input connections. The operational amplifier includes a first operational amplifier stage including first and second output connections, or first and second intermediate differential nodes, i.e., respectively nodes P and Q. The operational amplifier further includes first and second intermediate stage inverting and non-inverting amplifiers connected at their respective inputs to respective ones of inputs P and Q. The operational amplifier according to the present invention further includes first and second output transistors connected at a common node, including a single-ended output for said operational amplifier, each of said first and second output transistors having a gate connected to the output of said first and second intermediate stage inverting and non-inverting amplifiers. The operational amplifier further includes frequency compensating feedback circuitry including a compensation capacitor connected to said single-ended output of the operational amplifier. According to one embodiment of the present invention, the frequency compensating feedback circuitry further includes a current mirror circuit which is connected to a selected one of the first and second intermediate differential nodes P and Q.
According to one embodiment of the present invention, a class AB operational amplifier having first and second intermediate differential nodes respectively driving intermediate stage inverting and non-inverting amplifiers to produce a single-ended output voltage is provided with a frequency compensating feedback signal at a selected one of the first and second intermediate differential nodes, which is derived from the single-ended output of the operational amplifier subject to frequency compensation. The frequency compensating feedback signal is generated with feedback circuitry according to the present invention. The feedback circuitry according to one embodiment includes a compensation capacitor connected to the single-ended output and a current mirror circuit connected to the selected one of the first and second intermediate differential nodes of the operational amplifier.
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Choe Henry
Cirrus Logic Inc.
Pascal Robert
LandOfFree
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