Class-AB Operational Amplifier Output Stage With Drive-Node Decoupling
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Solution Overview
Problem
Conventional operational amplifiers experience output swing coupling back to the push-pull stage, causing short current and voltage fluctuations in the output stage, particularly during frequent voltage transitions, which affects AC current conservation.
Innovation Solution
The operational amplifier design includes a series connection of a p-channel and n-channel output transistors between power nodes, with a decoupling capacitor circuit and Miller capacitors to stabilize the output stage, reducing short current and voltage fluctuations by dynamically adjusting voltages at drive nodes.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Power
If a class-AB circuit is used as the output buffer, then the operational amplifier can provide high gain and wide bandwidth, but output swing couples back to the push-pull stage causing short current and voltage fluctuation
Solution Approach 1:
The patent introduces a decoupling capacitor circuit as an intermediary element between the push-pull stage and the load. This capacitor (CP) acts as a mediator that blocks the harmful feedback path while allowing the beneficial signal transmission, thereby resolving the contradiction between maintaining high gain/bandwidth and preventing output swing coupling back to cause short current and voltage fluctuation.
2Productivity
If voltage transition occurs more frequently, then more AC current is conserved, but voltage fluctuation increases in the output stage
Solution Approach 1:
The decoupling capacitor is pre-charged to a specific voltage level before voltage transitions occur. This preliminary charging action prepares the capacitor to compensate for voltage fluctuations that will occur during subsequent transitions, allowing the system to maintain both high AC current conservation and voltage stability without the harmful feedback effects.
3Speed
If the output stage is designed for high speed operation, then bandwidth is improved, but short current increases due to output swing coupling
Solution Approach 1:
The decoupling capacitor serves as a frequency-selective intermediary that allows high-frequency signals (providing bandwidth) to pass while blocking the lower-frequency components that cause short current. This resolves the contradiction by enabling high-speed operation without the energy loss associated with short current in the push-pull stage.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
This configuration effectively stabilizes the output stage, decreases short current, and suppresses voltage fluctuations, enhancing AC current conservation and overall operational stability.
Implementation Method 1
The decoupling capacitor circuit is electrically connected between the first drive node and the second drive node
Data Source
AI summary
An operational amplifier includes a first output transistor and a second output transistor connected in series between two power nodes, the second output transistor having a semiconductor type opposite to the first output transistor, the first output transistor and the second output transistor being electrically coupled at an output node, and gates of the first output transistor and the second output transistor being connected to a first drive node and a second drive node respectively; and a decoupling capacitor circuit electrically connected between the first drive node and the second drive node.