Preamplifier Common-Mode Feedback for Low-Noise ADC Front Ends
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Solution Overview
Problem
Existing preamplifier designs for analog-to-digital converters face challenges in maintaining stable common-mode voltage, particularly when using NMOS transistors, leading to variations in output common-mode voltage and increased noise, which affects the input referred offset and noise performance.
Innovation Solution
A preamplifier design incorporating an operational transconductance amplifier (OTA) with specific MOS transistor configurations and feedback circuits that regulate common-mode voltage without introducing extra noise or affecting gain, using either NMOS or PMOS transistors to control the common-mode voltage differentially, ensuring stable output common-mode voltage.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Stability of the object's composition
If common-mode feedback is introduced using an OTA to control NMOS transistors M1 and M2, then output common-mode voltage stability is improved, but extra noise is introduced at the preamplifier outputs
Solution Approach 1:
The patent introduces a new common-mode control node (ncm) as an intermediary point in the preamplifier circuit. By placing the controlling device M1 at this intermediate node rather than directly controlling M2 and M3, the feedback mechanism can regulate common-mode voltage without injecting noise directly into the signal paths of M2 and M3, thus resolving the contradiction between stability improvement and noise introduction
Solution Approach 2:
The patent extracts the common-mode control function from the signal path by creating a separate common-mode control node (ncm) that is electrically connected to the common-mode point but does not directly interfere with the differential signal paths. This separation allows common-mode voltage regulation to occur independently without adding noise to the differential output signals
2Stability of the object's composition
If the preamplifier output common-mode voltage is controlled using existing methods, then common-mode stability is improved, but the controlling device introduces noise that degrades input referred noise performance
Solution Approach 1:
The patent uses the common-mode control node (ncm) as an intermediary that allows the controlling device M1 to regulate common-mode voltage without directly coupling noise into the differential signal paths. The node ncm serves as a buffer that separates the control function from the signal paths, preventing noise degradation of input referred noise performance
Solution Approach 2:
The patent segments the preamplifier circuit into distinct functional regions: the differential signal path (M2, M3, R2, R3) and the common-mode control path (M1, ncm, R1). This segmentation allows independent optimization of each function, enabling common-mode voltage control without compromising the noise performance of the differential signal path
3Stability of the object's composition
If bias current is increased to maintain stable common-mode voltage, then common-mode control is improved, but power consumption increases
Solution Approach 1:
The patent implements dynamic common-mode voltage control by using an OTA with feedback that continuously adjusts the gate voltage of controlling device M1 based on the actual common-mode voltage at node ncm. This dynamic adjustment allows the circuit to maintain stable common-mode voltage without requiring excessive bias current, as the system adapts in real-time to maintain the desired operating point
Solution Approach 2:
The patent employs a feedback mechanism where the common-mode voltage at node ncm is monitored and used to adjust the gate voltage of controlling device M1 through the OTA. This feedback loop enables precise common-mode voltage control with minimal power consumption, as the system only uses enough current to maintain the feedback signal and make small adjustments to M1's gate voltage rather than continuously driving large currents
Data Source
AI summary
A preamplifier includes an operational transconductance amplifier, a first-type metal-oxide-semiconductor (MOS) transistor have a gate electrically coupled to the OTA output; a first second-type MOS transistor; a second second-type MOS transistor electrically connected in parallel with the first second-type MOS transistor; a first load resistor electrically connected in series with a drain of the first second-type MOS transistor that has a first output voltage; a second load resistor electrically connected in series with a drain of the second second-type MOS transistor that has a second output voltage; a tail node electrically connected to a source of the first second-type MOS transistor, a source of the second second-type MOS transistor, and a drain of a third second-type MOS transistor; a common-mode feedback circuit electrically coupled to the first and second output voltages and to a first OTA input; and a reference voltage electrically coupled to a second OTA input.