Single Supply Amplifier Ground Swing Using Segmented MOSFET Output Stage

Solution Overview

Problem

Conventional amplifiers face challenges in pulling the output voltage to ground or below using a single power supply due to the MOSFET operating in triode mode, which limits the voltage swing and increases power consumption when a negative power supply is required.

Innovation Solution

A cascode configuration with a main MOSFET and an auxiliary MOSFET, where the auxiliary MOSFET is powered by a charge pump to maintain high Vds for saturation mode, allowing the output to swing to ground or below using a single power supply, with minimal power consumption by turning on only when needed.

Engineering Contradictions & Design Principles

VSEngineering Contradiction Analysis

1Ease of operation

If the MOSFET is used in a conventional output stage with source connected to ground, then the circuit is simple and uses a single power supply, but the MOSFET operates in triode mode when Vout approaches ground, limiting the voltage swing and preventing full pull-down to ground

Engineering Contradiction:

Improvevoltage swing to groundVSAvoidoutput stage configuration

Core Design Contradiction:

Ease of operationVSDevice complexity

Solution Approach 1:

The output stage is segmented into two distinct MOSFET configurations: a main MOSFET for normal operation and a secondary MOSFET for ground-pull operation. This segmentation allows each MOSFET to be optimized for its specific function, with the secondary MOSFET specifically designed to pull Vout to ground when the main MOSFET enters triode mode, thereby resolving the voltage swing limitation without complicating the entire output stage.

Inventive Principle:

Principle #1Segmentation

Solution Approach 2:

The circuit dynamically switches between the main MOSFET and secondary MOSFET based on the operating conditions. When Vout is above the threshold voltage, the main MOSFET operates in saturation mode for efficient current sinking. When Vout approaches ground and the main MOSFET enters triode mode, the secondary MOSFET is activated to take over the ground-pull function, maintaining optimal performance across the full voltage range.

Inventive Principle:

Principle #15Dynamics

2Ease of operation

If a negative power supply is used to maintain sufficient Vds for the MOSFET to pull Vout to ground, then the voltage swing to ground is achieved, but power consumption increases and additional cost and board area are required

Engineering Contradiction:

Improvevoltage swing to groundVSAvoidpower consumption

Core Design Contradiction:

Ease of operationVSUse of energy by moving object

Solution Approach 1:

The secondary MOSFET is automatically activated only when needed—that is, when Vout approaches ground and the main MOSFET enters triode mode. This self-service mechanism ensures that the additional circuitry draws substantially no power during normal operation and activates only during the specific condition when ground-pull capability is required, thereby resolving the power consumption contradiction.

Inventive Principle:

Principle #25Self-service

Solution Approach 2:

The circuit changes its operational parameters dynamically by switching between different MOSFET configurations based on the output voltage level. When Vout is high, the main MOSFET operates with standard parameters. When Vout approaches ground, the secondary MOSFET is activated with adjusted parameters (gate voltage, channel width) to provide the necessary ground-pull capability without requiring a negative power supply, thus reducing power consumption and eliminating the need for additional power sources.

Inventive Principle:

Principle #35Parameter changes

3Use of energy by moving object

If the main MOSFET is used alone to pull Vout to ground, then the circuit is power efficient, but the MOSFET enters triode mode and cannot fully pull Vout to ground due to on-resistance

Engineering Contradiction:

Improvepower efficiencyVSAvoidvoltage swing to ground

Core Design Contradiction:

Use of energy by moving objectVSEase of operation

Solution Approach 1:

The output stage is segmented into two distinct MOSFET configurations: a main MOSFET for normal operation and a secondary MOSFET for ground-pull operation. This segmentation allows each MOSFET to be optimized for its specific function, with the secondary MOSFET specifically designed to pull Vout to ground when the main MOSFET enters triode mode, thereby resolving the voltage swing limitation without complicating the entire output stage.

Inventive Principle:

Principle #1Segmentation

Solution Approach 2:

The circuit dynamically switches between the main MOSFET and secondary MOSFET based on the operating conditions. When Vout is above the threshold voltage, the main MOSFET operates in saturation mode for efficient current sinking. When Vout approaches ground and the main MOSFET enters triode mode, the secondary MOSFET is activated to take over the ground-pull function, maintaining optimal performance across the full voltage range.

Inventive Principle:

Principle #15Dynamics

Data Source

PatentUS9000747B2Single supply amplifier with swing to ground

Publication Date: 2015.04.07 ANALOG DEVICES INT UNLTD CO

AI summary

An amplifier circuit has a voltage input terminal, for receiving Vin, and a voltage output terminal, for outputting Vout. A feedback circuit controls Vout to match Vin. A differential input stage receives Vin and Vout and generates a first output signal. An output stage comprises a pull down circuit for Vout. A main MOSFET is controlled by the first output signal to pull down Vout to match Vin when Vout is above a threshold voltage Vtrans. An auxiliary MOSFET, in parallel with the main MOSFET, is controlled by the first output signal to pull down Vout to match Vin when Vout is below Vtrans. The main MOSFET is turned substantially off when Vout is below Vtrans. A headroom generator coupled between the Vout terminal and a drain of the auxiliary MOSFET allows the auxiliary MOSFET to operate in its active region and pull Vout to ground.