Source Follower Feedback Circuit for Low-Voltage Linearity
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Solution Overview
Problem
Source follower circuits with high output impedances and non-linearities at low input voltage levels, leading to signal distortion due to MOSFETs operating outside their saturation region.
Innovation Solution
A source follower circuit with an additional gain stage, where the drain of the first MOSFET is coupled to a difference amplifier, and the output of the amplifier is connected to the gate of a second MOSFET, providing feedback to maintain the drain-to-source voltage of the first MOSFET at a reference voltage, ensuring operation within the active linear range.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Strength
If a source follower circuit is designed to present high impedance to the sending circuit, then the sending circuit is not loaded down, but the output impedance remains high which can diminish the signal to the receiving circuit
Solution Approach 1:
The patent employs a feedback mechanism where the output of the difference amplifier connects back to the gate of the second MOSFET, which in turn affects the drain voltage of the first MOSFET. This feedback loop actively regulates the output impedance while maintaining the high impedance characteristic at the input, resolving the contradiction between impedance matching and signal transmission capability.
Solution Approach 2:
The patent changes the operating parameters of the MOSFETs by using a difference amplifier to actively control the drain voltage of the first MOSFET and the gate voltage of the second MOSFET. This dynamic parameter adjustment allows the circuit to maintain high input impedance while achieving low output impedance, thereby improving signal transmission to the receiving circuit.
2Ease of operation
If the drain voltage of M1 is set by the gate-source voltage of M2, then the circuit operates as a follower, but when the input signal approaches ground or below ground, the drain-source voltage of M1 may approach the saturation voltage driving M1 out of its saturation region and causing non-linearities
Solution Approach 1:
The difference amplifier creates a feedback mechanism that actively monitors the input signal and adjusts the drain voltage of M1 accordingly. When the input signal approaches ground or goes below ground, the feedback ensures that the drain-source voltage of M1 remains sufficiently high to maintain saturation region operation, thereby preventing non-linearities and improving linearity while preserving follower operation.
Solution Approach 2:
The circuit applies preliminary anti-action by using the difference amplifier to preemptively adjust the drain voltage of M1 before the MOSFET can be driven out of its saturation region. This proactive control prevents the harmful effect of non-linearities from occurring in the first place, maintaining reliable linear operation across the full input signal range.
3Strength
If an additional MOSFET M2 is added to the source follower circuit, then the output impedance is reduced according to the equation Rout=1/(gm1)(gm2)(ro1), but the device complexity increases
Solution Approach 1:
The difference amplifier serves multiple functions simultaneously: it provides the necessary feedback control, acts as a voltage amplifier, and regulates the operating points of both MOSFETs. By making the difference amplifier multi-functional, the patent reduces the need for additional separate components, thereby lowering device complexity while achieving the desired low output impedance through the combined action of M1 and M2.
Data Source
AI summary
A source follower circuit is disclosed with an added amplifier that extends the low input voltage linear range while providing a lower output impedance. The drain of the source follower MOSFET is coupled to a gain stage that drives a second MOSFET (or other type transistor) with its drain coupled to the follower output. High impedance current sources bias the circuitry, and the difference amplifier has a reference voltage at one input. The difference amplifier with the reference voltage provides a feedback mechanism that maintain adequate drain to source voltage across the follower MOSFET to enhance the low input voltage linearity along with reducing the follower output resistance.