Feedback Source-Follower Buffer for Over-Supply Input Voltages
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Solution Overview
Problem
Power monitoring sensors face challenges in accurately sensing input voltages greater than or close to the supply voltage due to high output impedances and low input impedances, leading to errors in voltage buffering.
Innovation Solution
A buffer amplifier comprising a source follower and a feedback amplifier is used to control the drain current of the source follower, maintaining it constant independent of the load, thereby adjusting input and output impedances to eliminate errors and provide a voltage buffer for input voltages above or below the supply voltage.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Device complexity
If a typical source follower circuit is used for voltage buffering, then the circuit structure is simple, but the output impedance is high causing gain error
Solution Approach 1:
A feedback amplifier is connected to the source follower circuit to sense the output voltage and adjust the gate voltage accordingly. This feedback mechanism reduces the output impedance by dynamically compensating for load variations, thereby minimizing gain error while maintaining reasonable circuit complexity
Solution Approach 2:
The circuit dynamically adjusts the operating parameters of the source follower by varying the gate voltage through the feedback amplifier. This parameter change allows the output impedance to be reduced from its inherently high value in a simple source follower, resolving the contradiction between simplicity and precision
2Adaptability or versatility
If a voltage buffer is designed for input voltages greater than supply voltage, then the measurement capability is improved, but the output impedance increases
Solution Approach 1:
The feedback amplifier continuously monitors the output voltage and adjusts the source follower gate to maintain proper operation even when the input voltage exceeds the supply voltage. This feedback control prevents the output impedance from increasing excessively, allowing extended voltage range capability while maintaining measurement precision
Solution Approach 2:
The circuit operates in a dynamic regime where the feedback amplifier actively adjusts circuit parameters in real-time to accommodate input voltages beyond the supply voltage. This dynamic adaptation allows the buffer to handle extended voltage ranges without suffering from increased output impedance
3Ease of operation
If the drain current of the source follower is allowed to vary with load, then the circuit operation is simple, but the input offset error increases
Solution Approach 1:
The feedback amplifier senses changes in output voltage caused by load variations and adjusts the source follower gate voltage to maintain constant drain current. This feedback control eliminates input offset error by preventing current variations while requiring minimal additional circuitry
Solution Approach 2:
The feedback amplifier automatically compensates for load-induced current variations without external intervention. The circuit self-regulates the drain current by using the output voltage information to adjust the gate voltage, thereby maintaining precision while keeping the operation simple
Data Source
AI summary
A buffer amplifier comprises a source follower and a feedback amplifier. The feedback amplifier may be configured to control a drain current of the source follower to remain substantially constant independent of a load.