Source Follower DC 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 DC tracking circuit is introduced to equalize the drain-to-source voltage variations between the input transistor and the current source, using a configuration with operational amplifiers and resistors to maintain symmetry and clamp the second DC voltage, 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 is simple, but it is very sensitive to PVT variations causing significant second harmonic distortion
Solution Approach 1:
The source follower is divided into a differential pair section (first transistor) and a current source section (second transistor), with separate DC voltage control for each section. This segmentation allows independent optimization of each part's DC operating point to achieve symmetry in drain-to-source voltage variations.
Solution Approach 2:
DC tracking circuits are implemented to provide feedback control of the DC voltage at the drain of the first transistor and the source of the second transistor. The feedback mechanism adjusts these DC voltages to maintain symmetry and minimize second harmonic distortion.
2Device complexity
If the drain-to-source voltage variations of the input transistor and current source are not symmetric to output-common mode voltage, then the circuit configuration is simple, but second harmonic distortion becomes worse
Solution Approach 1:
The patent intentionally introduces asymmetry in the DC voltage levels of the two sections to compensate for the inherent asymmetry in the transistor characteristics. By making the DC operating points asymmetric (different DC voltages for first and second transistors), the overall drain-to-source voltage variations become symmetric with respect to the output common-mode voltage, thereby reducing second harmonic distortion.
Solution Approach 2:
The patent changes the DC voltage parameters of the transistors dynamically through DC tracking circuits. The DC voltage at the drain of the first transistor and the source of the second transistor are adjusted to track and maintain optimal symmetry, minimizing second harmonic distortion across varying operating conditions.
3Reliability
If a DC tracking circuit is introduced to equalize drain-to-source voltage variations, then second harmonic distortion is reduced, but the circuit complexity increases
Solution Approach 1:
DC tracking circuits are introduced as intermediary components between the differential pair and the current source. These intermediaries sense the output common-mode voltage and adjust the DC operating points of the transistors accordingly, acting as mediators to achieve symmetry without requiring complex direct control mechanisms.
Solution Approach 2:
The DC tracking circuits perform preliminary adjustment of the DC voltage levels before the signal processing occurs. By pre-establishing the correct DC operating points and symmetry conditions, the main signal path can operate optimally without requiring complex real-time correction during signal processing.
Data Source
Figure 1
Figure 2A~2C
Figure 3
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.