Source Follower Buffer With Capacitive Bypass for Linear Switching Loads
Find Innovative SolutionsGenerate Solutions
Solution Overview
Problem
Existing source/emitter follower buffers used in semiconductor circuits to drive switching loads suffer from input-output non-linearity due to signal-dependent modulation of transconductance, leading to inaccurate digital code generation in switched-capacitor analog to digital converters.
Innovation Solution
A source/emitter follower buffer is designed with a capacitor connected between the input path and the junction of a pair of cascoded transistors, which passes input signal current directly to the switching load, minimizing signal-dependent current through the transistors and thereby reducing transconductance modulation and ensuring linear output.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If a source/emitter follower buffer is used to drive a switching load, then the buffer provides isolation and drives the load, but input-output non-linearity occurs due to signal-dependent modulation of transconductance
Solution Approach 1:
The buffer circuit is segmented into multiple functional paths: a main signal path through the source/emitter follower transistor and a separate capacitor path that bypasses the transistor. This segmentation allows the signal current to be divided, with the capacitor carrying the time-varying signal current directly to the output, while the transistor handles only the DC bias current, eliminating transconductance modulation and improving linearity.
Solution Approach 2:
A capacitor is introduced as an intermediary element between the input and output of the buffer. This capacitor acts as a mediator that transfers the signal current directly without passing through the nonlinear transistor region, thereby preserving signal linearity while still allowing the transistor to provide buffer isolation and drive the switching load.
2Ease of operation
If signal current flows through the transistor receiving the input signal, then the transistor provides buffering action, but transconductance modulation causes non-linearity
Solution Approach 1:
The current path is segmented into two separate functions: the capacitor carries the time-varying signal current providing the buffering action, while the transistor carries only the DC bias current. This segmentation eliminates the interaction between signal current and transconductance, removing the source of non-linearity while preserving the buffering function.
Solution Approach 2:
The capacitor serves as an intermediary that performs the signal transmission function, allowing the transistor to be excluded from the signal current path. This intermediary approach maintains the buffering action through the capacitor while preventing the transistor's transconductance modulation from affecting signal linearity.
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 minimizes input-output non-linearity, enabling more accurate representation of the input signal as digital codes and reducing power consumption, while supporting high input signal frequencies and preventing output transients caused by inductive effects.
Implementation Method 1
A capacitor is coupled between the input of the buffer and the junction of the cascoded current source transistors. The capacitor passes input signal current directly to a switching load connected to the output of the buffer
Data Source
AI summary
A source follower or emitter follower buffer provided according to an aspect of the present invention includes a capacitor connected between the input path and a node formed by the junction of a pair of transistors forming a cascoded current source connected to the output of the buffer. The capacitor passes input signal current directly to a switching load connected to the output of the buffer, and very little signal-dependant current flows through the transistor receiving the input signal. As a result, input-output non-linearity due to signal-dependant modulation (variation) of transconductance of the transistor receiving the input signal is minimized. When incorporated in switched-capacitor analog to digital converters, the buffer facilitates generation of digital codes that represent an input signal more accurately.