Class AB Buffer Stage Using NPN Followers for Low Distortion

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Solution Overview

Problem

The availability of complementary bipolar processes, such as Silicon Germanium BiCMOS, is limited for producing both NPN and PNP transistors, leading to performance disparities in unity gain buffers where NPN transistors outperform complementary process transistors.

Innovation Solution

A buffer stage design utilizing a flipped voltage follower and an emitter follower, both comprising the same type of transistors (either NPN or PNP) without the opposite type, connected between high and low voltage rails, with a resistor linking their outputs and a bias generator for transistor biasing, operating as a class AB unity gain amplifier.

Engineering Contradictions & Design Principles

VSEngineering Contradiction Analysis

1Adaptability or versatility

If complementary bipolar processes (NPN and PNP transistors) are used to implement unity gain buffers, then both transistor types can be utilized for balanced push-pull operation, but the PNP transistors available in open market foundry processes have inferior performance compared to NPN transistors

Engineering Contradiction:

Improveability to use both NPN and PNP transistorsVSAvoidtransistor performance consistency

Core Design Contradiction:

Adaptability or versatilityVSReliability

Solution Approach 1:

The invention extracts and removes the PNP transistor from the complementary pair, using only NPN transistors in both the flipped voltage follower and emitter follower stages. This eliminates the performance disparity between NPN and PNP transistors while maintaining the ability to implement class AB operation through the combination of these two NPN-based stages.

Inventive Principle:

Principle #2Taking out (Extraction)

2Reliability

If a flipped voltage follower and emitter follower both using the same transistor type are connected in parallel, then low distortion and high performance are achieved through equal output impedances and negative feedback, but the design complexity increases compared to traditional complementary designs

Engineering Contradiction:

Improvelow distortion performanceVSAvoidcircuit configuration complexity

Core Design Contradiction:

ReliabilityVSDevice complexity

Solution Approach 1:

The invention changes the configuration parameters of the voltage follower stage by flipping it (inverting the signal path), which transforms the output impedance characteristics. This parameter change allows the flipped voltage follower to present equal output impedance to the emitter follower, enabling negative feedback operation and achieving low distortion performance.

Inventive Principle:

Principle #35Parameter changes

Data Source

PatentUS8120424B2Class AB output stages and amplifiers including class AB output stages

Publication Date: 2012.02.21 INTERSIL AMERICAS INC

AI summary

A buffer stage includes a flipped voltage follower and an emitter follower. The flipped voltage follower is connected between a high voltage rail and a low voltage rail and include an input and an output. The emitter follower is also connected between the high voltage rail and the low voltage rail and includes an input and an output. A resistor connects the output of the flipped voltage follower to the output of the emitter follower. The input of the flipped voltage follower and the input of the emitter follower are connected together and provide an input of the buffer stage. The output of the emitter follower provides an output of the buffer stage. A differential buffer stage can be implemented using a pair of such buffer stages. Such a differential buffer stage can provide the output stage for a fully differential operational amplifier.