Error Amplifier Transconductance Attenuation for Stable Low-Noise LDOs
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Solution Overview
Problem
Existing low-drop-out (LDO) voltage regulators face instability and increased noise due to the increase in bias current through the input pair of transistors, leading to a negative phase margin and undesirable noise levels.
Innovation Solution
Implementing a transconductance attenuator in the second stage of the error amplifier to reduce current flow through the second stage circuit, thereby maintaining stability and reducing noise by tuning the gain of the transconductance attenuator and lowering the effective transconductance.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Object-affected harmful factors
If bias current through the input pair of transistors is increased, then noise level increases, but stability deteriorates (negative phase margin)
Solution Approach 1:
A transconductance attenuator circuit is introduced as an intermediary component between the first and second stages of the error amplifier. This attenuator reduces the effective transconductance of the second stage, allowing the bias current to be optimized for low noise while preventing excessive current amplification that would cause stability issues. The attenuator acts as a mediator that decouples the noise performance from the stability constraints.
Solution Approach 2:
The patent changes the effective transconductance parameter of the second stage by introducing the transconductance attenuator. By adjusting the attenuation factor (through resistor ratios in the attenuator circuit), the effective transconductance is reduced to a value that maintains stability while allowing higher bias current for noise reduction. This parameter transformation resolves the contradiction between noise performance and stability.
2Object-affected harmful factors
If bias current is increased to reduce noise, then current consumption increases
Solution Approach 1:
The transconductance attenuator serves as an intermediary that allows the system to achieve low noise performance without proportionally increasing current consumption. By reducing the effective transconductance, the attenuator enables the use of moderate bias currents while maintaining low noise, as the attenuator compensates for the reduced current amplification capability.
Solution Approach 2:
The patent transforms the current consumption-n noise relationship by changing the effective transconductance parameter. Through the attenuator's resistance ratios, the effective transconductance is adjusted to provide an optimal balance where moderate bias currents achieve the desired noise performance without excessive current consumption.
3Stability of the object's composition
If transconductance of second stage is reduced for stability, then gain decreases
Solution Approach 1:
The error amplifier is segmented into three distinct functional blocks: the first stage (input pair), the transconductance attenuator, and the second stage. This segmentation allows independent optimization of each block - the first stage provides high gain, the attenuator provides stability control, and the second stage provides additional gain while maintaining controlled transconductance. The overall gain is the product of individual stage gains, compensating for the reduced second stage transconductance.
Solution Approach 2:
The patent changes the effective transconductance parameter of the second stage through the attenuator, but compensates for the gain loss by optimizing the first stage transconductance and the attenuator's attenuation factor. The overall voltage gain remains sufficient because the first stage operates at high transconductance and the attenuator's parameter transformation maintains the necessary gain product across stages.
Data Source
AI summary
An error amplifier includes a first transistor having a first error amplifier input and having first and second current terminals, a second transistor having a second error amplifier input and having third and fourth current terminals, a first resistor coupled between a supply voltage terminal and the first current terminal, and a second resistor coupled between the supply voltage terminal and the third current terminal. The error amplifier has a second stage circuit coupled to the first and second resistors. The second stage circuit has an error amplifier output. The second stage circuit is configured to cause less current to flow through the second stage circuit than a current that flows through either of the first or second resistors or the first or second transistors.