Operational Amplifier Tail Current Compensation for High-Frequency PSRR
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Solution Overview
Problem
Existing operational amplifiers using Miller compensation or cascode compensation suffer from power supply rejection deterioration, especially at higher frequencies, due to direct coupling of power noise through compensation capacitors, which degrades their performance in radio-frequency circuits.
Innovation Solution
The operational amplifier incorporates a tail current compensation circuit that connects to both the tail current source and the output terminal of a second-stage gain circuit, introducing a left half-plane zero to improve phase margin and stability, while a positive feedback tuning circuit rejects positive feedback signals to prevent power supply rejection deterioration.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Stability of the object's composition
If Miller compensation or cascode compensation is used to improve the stability of the operational amplifier, then the stability is improved, but power supply rejection deteriorates at higher frequencies
Solution Approach 1:
The patent segments the compensation function by introducing a dedicated tail current compensation circuit separate from the main signal path. This circuit independently compensates for power supply variations at the tail current source, isolating the power supply rejection function from the Miller or cascode compensation capacitors, thereby resolving the contradiction between stability improvement and power supply rejection deterioration.
Solution Approach 2:
The tail current compensation circuit acts as an intermediary between the power supply and the tail current source. It introduces a compensating signal that counteracts power supply variations before they affect the differential pair, thereby improving power supply rejection without compromising the stability provided by the main compensation capacitors.
2Object-affected harmful factors
If additional power supply rejection circuits are used to improve power supply rejection characteristic, then power supply rejection is improved, but device complexity increases
Solution Approach 1:
The patent merges the power supply rejection function with the existing tail current source by introducing a compensation circuit that uses the same tail current node. This integration approach allows power supply rejection improvement without adding separate complex rejection circuits, as the compensation is embedded within the existing current source structure.
Solution Approach 2:
The tail current compensation circuit implements feedback by sensing the output signal and using it to generate a compensating signal that is applied to the tail current source. This feedback mechanism automatically counteracts power supply variations, improving power supply rejection through a relatively simple circuit configuration rather than complex additional rejection circuits.
Data Source
AI summary
Disclosed is an operational amplifier, including a first-stage gain circuit, a second-stage gain circuit, and a tail current compensation circuit. The first-stage gain circuit is connected to the second-stage gain circuit, the first-stage gain circuit is provided with an input terminal, the second-stage gain circuit is provided with an output terminal. The first-stage gain circuit at least includes a tail current source, a first terminal of the tail current compensation circuit is connected to the tail current source, and a second terminal of the tail current compensation circuit is connected to the output terminal of the second-stage gain circuit. The tail current compensation circuit is configured to compensate the tail current source with an output signal of the output terminal of the second-stage gain circuit.