Differential Amplifier Tail Referencing for Low Output Skew
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Solution Overview
Problem
Differential amplifier circuits exhibit significant skew due to differing transition times between low-to-high and high-to-low output transitions, particularly at high frequencies, which is exacerbated by variations in tail voltage affecting the output buffer's ground-referenced inverter design.
Innovation Solution
The solution involves connecting the TAIL node of the differential amplifier to the source of the N-Channel transistor in the first inverter stage, allowing the switching point of the inverter to track the output swing midpoint of the differential amplifier, thereby compensating for voltage level variations and providing regenerative feedback for faster switching.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Device complexity
If a ground referenced ratioed inverter is used for receiving the output of the differential amplifier stage, then the circuit structure is simple, but the skew of the output buffer changes when the tail voltage changes over operating conditions
Solution Approach 1:
The patent changes the reference parameter from ground to the tail voltage node. By connecting the inverter's source to the tail voltage instead of ground, the inverter's switching threshold dynamically tracks with the tail voltage changes, maintaining consistent skew performance across varying operating conditions (voltage, temperature, input common mode range).
Solution Approach 2:
The patent implements feedback by connecting the tail voltage node to the inverter stage. This creates a feedback mechanism where the inverter automatically adjusts its operating point based on the tail voltage level, compensating for variations without requiring external control circuits.
2Power
If the tail voltage increases and the VOL of the differential stage increases, then the output level swing changes, but the trip point of the receiving inverter remains fixed causing skew
Solution Approach 1:
The patent dynamically changes the inverter's trip point parameter by connecting it to the tail voltage node. When tail voltage increases causing VOL to increase, the inverter's source voltage also increases, automatically raising its trip point to match the new output level swing, thereby maintaining symmetric transition times.
3Reliability
If additional circuitry is added to reduce skew, then skew performance improves, but device complexity increases
Solution Approach 1:
The patent applies self-service by using the existing tail voltage node to automatically adjust the inverter's operating point. The circuit serves itself by utilizing its own internal voltage variations to compensate for performance degradation, eliminating the need for external skew compensation circuits.
Solution Approach 2:
The tail voltage node serves multiple functions: it provides the bias current for the differential pair and simultaneously serves as the reference voltage for the inverter stage. This multi-functionality reduces the need for separate reference voltage generation circuitry.
Data Source
AI summary
Using the tail level referencing for an inverter stage immediately following a differential amplifier provides trip point tracking with the variations in magnitude of the output level swings on the differential amplifier stage output over the operating range of the circuit. When the tail voltage increases and the VOL of the differential stage increases, the trip point of the receiving inverter also increases. When the tail voltage decreases and the VOL of the differential amplifier goes lower, the trip point of the inverter decreases. An additional benefit is provided by the tail connection to the inverter. Faster switching of current from the right side to the left side of the differential amplifier occurs due to the tail node voltage being raised momentarily by a transistor in the inverter stage when the input of the inverter stage transitions high.