Rail-to-Rail Source Follower Circuit for Stable Bandwidth
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Solution Overview
Problem
Conventional source follower circuits only support output excursion to one voltage supply rail, leading to limitations in bandwidth and transient performance when attempting rail-to-rail excursion.
Innovation Solution
Development of rail-to-rail follower circuits that automatically configure as N-type or P-type source followers based on output levels, maintaining stable output impedance and AC behavior equivalent to basic follower stages, with an open-loop current-mode structure and level shifters to manage transitions.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Adaptability or versatility
If conventional source follower circuits are used, then the circuit structure is simple, but the output excursion is limited to one supply rail
Solution Approach 1:
The circuit is divided into two parallel branches: an N-type source follower branch and a P-type source follower branch. Each branch is responsible for driving the output to one supply rail, enabling rail-to-rail output excursion while maintaining relatively simple individual branch structures
Solution Approach 2:
The circuit uses a shared gate terminal that receives a single input voltage to control both N-type and P-type transistors. This universal input control mechanism allows the circuit to adapt its output range based on the input signal while maintaining a compact structure
2Adaptability or versatility
If previous attempts to implement rail-to-rail source followers are made, then the output excursion range is improved, but the bandwidth and transient performance are degraded
Solution Approach 1:
Each transistor branch (N-type and P-type) is optimized for its specific operating region. The N-type branch handles signals near the lower supply rail while the P-type branch handles signals near the upper supply rail, allowing each component to operate in its optimal performance region and maintain high bandwidth and transient response
Solution Approach 2:
The circuit dynamically switches between N-type and P-type transistor dominance based on the input voltage level. As the input voltage varies, the relative conduction states of the transistors change automatically, enabling the circuit to maintain optimal transient performance across the entire rail-to-rail output range
Data Source
AI summary
In some embodiments, a source follower circuit may include a first level shifter configured to receive an input voltage; an N-type Metal-Oxide-Semiconductor (NMOS) transistor having a gate terminal coupled to an output of the first level shifter; a second level shifter configured to receive the input voltage; a P-type Metal-Oxide-Semiconductor (PMOS) transistor having a gate terminal coupled to an output of the second level shifter and a source terminal coupled to a source terminal of the NMOS transistor; and an amplifier configured to receive the input voltage and to output a current at a node between the source terminal of the NMOS transistor and the source terminal of the PMOS transistor, wherein the current is determined based upon a difference between the input voltage and a reference voltage.