Class AB Super Source Follower for Wide ADC Ramp Signals
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Solution Overview
Problem
In high-pixel solid-state imaging devices, reducing power consumption and analog power supply voltage while maintaining a wide reference ramp signal amplitude and minimizing streaking and random horizontal noise is challenging due to the limitations of existing DAC output buffer technologies.
Innovation Solution
A new class AB super source follower with variable current sources and a feedback circuit that controls current supply and sinking, providing low output impedance and ensuring wide reference ramp signal amplitude, even at low power supply voltages, is used as a DAC output buffer to drive the reference ramp signal of SS-ADCs.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Use of energy by moving object
If the analog power supply voltage is reduced to lower power consumption, then power consumption is reduced, but the reference ramp signal amplitude cannot be maintained
Solution Approach 1:
The patent implements dynamic current control in the output buffer circuit, where the current sources are controlled by feedback signals that adjust their operation based on the instantaneous signal conditions. This dynamic operation allows the circuit to maintain full signal amplitude capability while consuming less average power, as the high-current state is only activated when needed for signal swing rather than being continuously active.
Solution Approach 2:
The patent changes the operating parameters of the output buffer by using variable current sources instead of fixed current sources. The current magnitude is dynamically adjusted through feedback control, allowing the circuit to operate efficiently at lower power while maintaining the ability to deliver full signal amplitude when required. This parameter modulation resolves the contradiction between power consumption and signal amplitude.
2Device complexity
If a conventional output buffer is used, then the circuit is simple, but the output impedance is high causing streaking and random horizontal noise
Solution Approach 1:
The patent introduces feedback circuits that sense the output signal and adjust the current source operation accordingly. This feedback mechanism enables the output buffer to actively compensate for loading effects and maintain low output impedance across varying signal conditions. The feedback control allows the circuit to achieve low output impedance without requiring excessively complex circuit topologies, thus resolving the contradiction between simplicity and noise suppression.
Solution Approach 2:
The output buffer employs dynamic current control where the current sources adjust their output based on real-time signal conditions detected through feedback. This dynamic adaptation allows the circuit to maintain optimally low output impedance throughout the signal cycle, effectively suppressing streaking and random horizontal noise without requiring a statically complex circuit design.
3Quantity of substance
If the number of pixels is increased, then the imaging capability is improved, but the power consumption and noise increase
Solution Approach 1:
The patent segments the output buffer functionality into multiple controlled current sources that can be independently regulated. This segmentation allows each current source to be optimized for specific operational requirements, enabling the system to support increased pixel counts and associated loading without proportionally increasing overall power consumption. The modular current control structure efficiently handles the increased demand from higher pixel counts.
Solution Approach 2:
The dynamic current control mechanism allows the output buffer to adapt its power consumption to the actual loading conditions imposed by the pixel array. As the number of pixels increases, the feedback system dynamically adjusts the current source operation to provide only the necessary drive capability, preventing excessive power consumption even when supporting large numbers of pixels. This dynamic adaptation resolves the contradiction between pixel count and power usage.
Data Source
AI summary
An output buffer of a super source follower for driving a reference ramp signal of a column-parallel single slope type ADC of a solid-state imaging device is made as a class AB feedback configuration for controlling a feedback variable current source with a signal obtained by amplifying a current fluctuation flowing through an amplification transistor by an amplifier, and thereby, the upper limit of the drain voltage of the amplification transistor is not limited by the voltage between the gate and the source of the feedback variable current source.