Source Follower Bias Tracking for Low Second Harmonic Distortion
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Solution Overview
Problem
Conventional source followers are sensitive to PVT variations, leading to significant second harmonic distortion due to asymmetry in drain-to-source voltage variations, which degrades their linearity and performance.
Innovation Solution
A source follower design incorporating a DC tracking circuit that sets a second DC voltage at a specific level relative to a first DC voltage, using transistors and operational amplifiers to maintain symmetry in drain-to-source voltage variations, thereby reducing second harmonic distortion.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Device complexity
If a conventional single-input-ended source follower is used, then the circuit structure is simple, but the second harmonic distortion is significant due to asymmetry in drain-to-source voltage variations
Solution Approach 1:
The patent introduces a DC tracking circuit that deliberately creates asymmetric voltage tracking between the input and output nodes. The circuit uses operational amplifiers and transistors to generate a second DC voltage that is twice the first DC voltage, compensating for the inherent asymmetry in the source follower's drain-to-source voltage variations. This controlled asymmetry cancels out the harmful second harmonic distortion while maintaining overall circuit symmetry in terms of component count and complexity.
2Ease of operation
If the drain-to-source voltage variations are not symmetric to output-common mode voltage, then the circuit operation is straightforward, but the linearity is degraded due to second harmonic distortion
Solution Approach 1:
The DC tracking circuit implements a feedback mechanism where the first DC voltage at the first node is continuously monitored and used to control the second DC voltage at the second node. The operational amplifiers compare the voltages and adjust the biasing conditions to maintain the relationship V2 = 2*V1, ensuring that the drain-to-source voltage variations remain symmetric with respect to the output common-mode voltage. This feedback control improves linearity without complicating the basic source follower operation.
3Object-generated harmful factors
If a DC tracking circuit is added to set the second DC voltage, then the second harmonic distortion is suppressed, but the device complexity increases
Solution Approach 1:
The DC tracking circuit is designed to perform multiple functions simultaneously: it generates the second DC voltage, tracks the first DC voltage, provides biasing for the transistors, and suppresses second harmonic distortion. The operational amplifiers serve both as voltage followers and as error amplifiers for distortion cancellation. By making the added components multi-functional, the patent minimizes the net increase in device complexity while achieving distortion suppression.
Data Source
AI summary
A source follower with an input node and an output node includes a first transistor, a second transistor, and a DC (Direct Current) tracking circuit. The first transistor has a control terminal, a first terminal coupled to a first node, and a second terminal coupled to a second node. The second transistor has a control terminal, a first terminal coupled to a ground voltage, and a second terminal coupled to the first node. The DC tracking circuit sets the second DC voltage at the second node to a specific level. The specific level is determined according to the first DC voltage at the first node. The output node of the source follower is coupled to the first node.