Cascode LNA with Active Post-Distortion for Third-Order Linearity
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Solution Overview
Problem
Low noise figure and linearity in Low Noise Amplifiers (LNAs) lead to degraded signal-to-noise ratio (SNR) due to nonlinearity, which increases power consumption and circuit size requirements in wireless communication receivers.
Innovation Solution
The implementation of an LNA with active post-distortion (APD) using a cascode structure of N-FETs and an inductor for source degeneration, which generates and cancels third-order distortion components, improving linearity and noise performance across a wide frequency range.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If the LNA uses conventional design without active post-distortion, then the circuit is simpler and power consumption is lower, but linearity is degraded causing cross modulation distortion
Solution Approach 1:
The LNA is divided into two separate cascode pairs: the first cascode pair (first and second transistors) handles signal amplification, while the second cascode pair (third and fourth transistors) generates distortion components for cancellation. This segmentation allows each pair to be optimized for its specific function, with the distortion cancellation path independent from the main signal path.
Solution Approach 2:
The third transistor acts as an intermediary that receives the intermediate signal from the second transistor and generates the distortion components needed for cancellation. It serves as a bridge between the main amplification path and the distortion cancellation mechanism, enabling the system to generate and inject cancellation signals without directly modifying the main signal path.
2Reliability
If the LNA uses conventional design without active post-distortion, then the circuit area is smaller, but noise figure is degraded due to nonlinearity
Solution Approach 1:
The circuit is segmented into two functional blocks: the first cascode pair for amplification and the second cascode pair for distortion generation and cancellation. This segmentation allows the distortion cancellation function to be added without significantly increasing the area of the core amplification circuit, as the second cascode pair can be implemented in a dedicated cancellation path.
Solution Approach 2:
The second cascode pair serves multiple functions: it receives the intermediate signal, generates the required distortion components through its nonlinear characteristics, and provides load isolation. This multi-functionality reduces the need for additional dedicated circuits, thereby minimizing the overall area increase.
3Use of energy by moving object
If the LNA uses conventional design without active post-distortion, then power consumption is lower, but subsequent stages require higher performance increasing overall power consumption
Solution Approach 1:
The active post-distortion circuit performs preliminary distortion cancellation at the LNA output stage, before the signal proceeds to subsequent processing stages. By pre-canceling the third-order distortion components generated by the first cascode pair, the circuit prevents these distortions from propagating to later stages, thereby reducing the performance requirements and power consumption of subsequent stages.
Solution Approach 2:
The distortion cancellation mechanism uses feedback principles by monitoring the intermediate signal and generating compensating distortion components through the third transistor. These cancellation signals are injected back into the output to counteract the unwanted distortion, creating a closed-loop effect that improves linearity without requiring high-performance subsequent stages.
4Reliability
If the first transistor provides signal amplification without source degeneration, then gain is higher, but third-order distortion is increased
Solution Approach 1:
The inductor connected to the source of the first transistor changes the operating parameters of the amplification stage by providing source degeneration. This modifies the transistor's effective transconductance and output impedance, reducing third-order distortion while maintaining acceptable gain through the overall cascode configuration and load impedance selection.
Solution Approach 2:
The inductor acts as an intermediary element at the source of the first transistor, mediating between the need for high gain and the need for low distortion. It provides the necessary degeneration to reduce distortion while the cascode structure and load impedance work together to maintain the required gain level.
Data Source
Figure 1
Figure 2A~2C
Figure 3
AI summary
An amplifier, which has good linearity and noise performance, includes first, second, third, and fourth transistors and an inductor. The first and second transistors are coupled as a first cascode pair, and the third and fourth transistors are coupled as a second cascode pair. The third transistor has its gate coupled to the source of the second transistor, and the fourth transistor has its drain coupled to the drain of the second transistor. The first transistor provides signal amplification. The second transistor provides load isolation and generates an intermediate signal for the third transistor. The third transistor generates distortion components used to cancel third order distortion component generated by the first transistor. The inductor provides source degeneration for the first transistor and improves distortion cancellation. The sizes of the second and third transistors are selected to reduce gain loss and achieve good linearity for the amplifier.