Class AB Buffer Stage Using NPN Followers Without PNP Devices
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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 in open market foundry bipolar processes.
Innovation Solution
A buffer stage is designed using a flipped voltage follower and an emitter follower, both comprising the same type of transistors (either NPN or PNP), connected between high and low voltage rails, with a resistor linking their outputs and a bias generator for transistor biasing, enabling class AB unity gain amplification without requiring PNP transistors.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If complementary bipolar processes (Silicon Germanium BiCMOS) are used to produce both NPN and PNP transistors, then the buffer stage can achieve balanced performance, but the availability and performance of PNP transistors are limited in open market foundry bipolar processes
Solution Approach 1:
The buffer stage is divided into two separate paths: one using NPN transistors and another using PNP transistors. Each path is independently optimized, allowing the design to leverage the superior performance of NPN transistors while still incorporating PNP transistors where needed, thus resolving the contradiction between performance and manufacturing availability
Solution Approach 2:
Instead of using the conventional complementary symmetry approach where NPN and PNP transistors are used in a balanced configuration, the patent inverts the approach by using NPN transistors in both the main signal path and the feedback path, or by using NPN transistors where PNP would traditionally be used, thereby leveraging the superior performance characteristics of NPN transistors throughout the buffer stage
2Reliability
If NPN transistors are used exclusively in the buffer stage, then superior performance can be achieved, but the circuit cannot be implemented as a traditional complementary symmetry design
Solution Approach 1:
Different regions of the buffer stage are assigned different transistor types based on their specific functional requirements. The NPN transistors are used in regions where their superior performance is most beneficial, while PNP transistors are used in regions where their characteristics are more suitable, thus achieving local optimization rather than uniform design
Solution Approach 2:
The patent deliberately introduces asymmetry into the buffer stage design by using different transistor types in different paths or configurations, rather than maintaining strict complementary symmetry. This asymmetry allows the circuit to exploit the superior performance of NPN transistors in critical paths while still incorporating PNP transistors in less critical paths, thereby achieving overall performance improvement
3Reliability
If a cascaded complementary bipolar emitter follower (0002 follower or diamond follower) is used, then low distortion can be achieved, but the design requires both NPN and PNP transistors which are not readily available in open market foundry processes
Solution Approach 1:
The patent extracts the essential low-distortion functionality from the traditional complementary emitter follower design and implements it using only NPN transistors or using a reduced set of transistor types that are readily available in open market foundry processes. This extraction allows the circuit to maintain its low-distortion performance while eliminating the dependency on scarce PNP transistors
Data Source
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 includes 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.