Folded-Cascode Class AB Amplifier for Feedforward-Stable Compensation

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Solution Overview

Problem

Class AB folded-cascode amplifiers face degradation in frequency response and instability due to feedforward paths in simple Miller compensation, which is not effectively mitigated by existing compensation techniques, and requires large capacitors consuming significant die area.

Innovation Solution

The implementation of embedded cascode compensation, which includes feedback paths that couple complementary signals from the output stage to the differential input circuit without signal inversion, and the use of symmetric or asymmetric embedded cascode compensation to stabilize the amplifier, reducing feedforward effects and capacitor size.

Engineering Contradictions & Design Principles

VSEngineering Contradiction Analysis

1Stability of the object's composition

If simple Miller compensation is used, then the amplifier is stabilized through RC compensation networks, but feedforward paths cause degradation in frequency response and instability

Engineering Contradiction:

Improveamplifier stabilityVSAvoidfrequency response degradation

Core Design Contradiction:

Stability of the object's compositionVSReliability

Solution Approach 1:

The patent extracts and removes the harmful feedforward path from the compensation network by configuring the compensation capacitor to connect only to the inverting input node, excluding the noninverting signal path that causes feedforward degradation

Inventive Principle:

Principle #2Taking out (Extraction)

Solution Approach 2:

The compensation network is segmented into separate paths: a Miller compensation path for stabilization and a feedforward path elimination, allowing independent optimization of stability and frequency response

Inventive Principle:

Principle #1Segmentation

2Object-affected harmful factors

If nulling resistors are increased in size to reduce feedforward signal magnitude, then feedforward is mitigated to limited extent, but the introduced LHP zero moves closer to crossover frequency degrading gain margin

Engineering Contradiction:

Improvefeedforward signal magnitudeVSAvoidgain margin

Core Design Contradiction:

Object-affected harmful factorsVSStability of the object's composition

Solution Approach 1:

The patent removes the need for nulling resistors by extracting the feedforward path entirely through asymmetric capacitor configuration, eliminating the trade-off between feedforward reduction and gain margin degradation

Inventive Principle:

Principle #2Taking out (Extraction)

Solution Approach 2:

The compensation capacitor serves as an intermediary element that provides both stabilization through Miller effect and feedforward path elimination, replacing the dual-function requirement of nulling resistors

Inventive Principle:

Principle #24Intermediary (Mediator)

3Stability of the object's composition

If Miller compensation capacitors are increased in size, then compensation effectiveness is improved, but large die area is consumed

Engineering Contradiction:

Improvecompensation effectivenessVSAvoiddie area

Core Design Contradiction:

Stability of the object's compositionVSArea of stationary object

Solution Approach 1:

The patent applies different capacitor values to different sides of the differential input circuit, with the noninverting side having zero or minimal capacitance and the inverting side having the full compensation capacitance, optimizing both effectiveness and area utilization

Inventive Principle:

Principle #3Local quality

Data Source

PatentUS7639078B2Class AB folded-cascode amplifier having cascode compensation

Publication Date: 2009.12.29 ANALOG DEVICES INT UNLTD CO

AI summary

A class AB folded-cascode amplifier having improved gain-bandwidth product, comprises a differential input circuit including a differential transistor pair coupled to a source of tail current and responsive to a differential input signal for conducting a first current, a cascode circuit coupled to the differential input circuit for supplying a second current thereto, and a class AB output stage. A compensation circuit is configured for feeding back mutually complementary compensation signals from an output node to the differential input circuit. Another compensation circuit is configured for feeding back a signal from the output of the output stage to the input of the output stage.