Source-Follower Amplifier Load Circuit for Solid-State Imaging
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Solution Overview
Problem
Conventional solid-state imaging devices face challenges in achieving high gain and preventing imaging characteristic deterioration due to high drain conductance and hot-carrier generation in the load transistor, which results in false signals and reduced image quality.
Innovation Solution
The implementation of a source-follower amplifier with a load circuit comprising a first MOS transistor and a load device connected to its source, where the load device can be a resistive element or a MOS transistor, helps reduce the source-drain voltage of the MOS transistor, thereby reducing conductance and preventing hot-carrier generation, and includes a cascode-connected structure with multiple stages and current mirror circuits to further enhance gain and reduce conductance.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Device complexity
If a conventional load transistor is used in the output amplifier, then the circuit structure is simple, but the drain conductance is high which deteriorates the voltage gain
Solution Approach 1:
The load circuit is segmented into two parts: a first MOS transistor (M12) with fixed gate potential and a load device (LD1) connected to its source. This segmentation allows the first MOS transistor to act as a buffer that reduces the source-drain voltage across the load device, thereby reducing its conductance and improving voltage gain without significantly complicating the overall circuit structure.
2Power
If high voltage is applied to the load transistor to achieve high output voltage, then the power supply efficiency is improved, but hot-carrier generation occurs which deteriorates imaging characteristics
Solution Approach 1:
The first MOS transistor (M12) serves as an intermediary element between the power supply and the load device. It buffers the high voltage from the power supply, ensuring that the source-drain voltage across the load device remains low (thereby preventing hot-carrier generation) while still allowing the circuit to achieve the required high output voltage through its fixed gate potential configuration.
3Object-affected harmful factors
If the source-drain voltage of the load transistor is reduced to prevent hot-carrier generation, then imaging characteristics are improved, but the voltage gain of the amplifier decreases
Solution Approach 1:
The invention changes the operating parameters of the load circuit by introducing a first MOS transistor with fixed gate potential. This configuration allows the source-drain voltage across the load device to be reduced (preventing hot-carrier generation) while the fixed gate potential of the first MOS transistor provides the necessary voltage amplification, thereby maintaining high voltage gain despite the reduced source-drain voltage across the load device.
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 approach improves the gain of the source-follower amplifier, reduces the occurrence of hot-carrier-induced light emission, and prevents deterioration of imaging characteristics, while also optimizing the device area and design flexibility by using resistive elements or MOS transistors in the load circuit.
Implementation Method 1
there is a possibility of a hot-carrier being generated from the increase in the electrical field in the vicinity of the drain within the channel of the load transistor
Implementation Method 2
the problem of light emission can occur in connection with the impact ionization of the carrier
Implementation Method 3
a photoelectric conversion unit, a charge transfer unit, and an output amplifier which converts signal charge into voltage signal
Data Source
AI summary
A solid-state imaging device includes a source-follower amplifier having a driver transistor and a load circuit connected to the driver transistor. The load circuit is structured by a first MOS transistor having a gate with a fixed potential, and a load device connected to a source of the first MOS transistor. Furthermore, the load circuit of the source-follower amplifier is structured by a plurality of serially connected MOS transistors. Furthermore, by adapting a structure using a current mirror circuit, load conductance is reduced.