Wideband PA Gm Linearization for Third-Order Intermodulation
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Solution Overview
Problem
Existing power amplifiers in mobile communication systems suffer from nonlinearity, particularly in sub-micron CMOS transistors, leading to poor signal quality and limited linear output power due to nonlinear capacitance and intermodulation distortion, which is exacerbated by wide bandwidth requirements in OFDM signals.
Innovation Solution
A wideband power amplifier linearization technique using a transconductance Gm linearizer with current interpolation, employing complementary differential transistor pairs and compensating bias currents to achieve linearization over a wide bandwidth without significantly reducing gain.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Use of energy by moving object
If power amplifier is biased at lower bias condition (Class-AB or Class-B) to achieve high efficiency, then power efficiency is improved, but non-linear capacitance variation increases leading to degraded linearity
Solution Approach 1:
A compensating bias current source is introduced as an intermediary element to counteract the nonlinear capacitance variation of the main power amplifier transistor. The compensating bias current, generated through a separate transistor branch, acts as a mediator that cancels out the harmful nonlinear effects, allowing the PA to operate at lower bias conditions for high efficiency while maintaining linearity.
Solution Approach 2:
The invention dynamically adjusts the bias current parameter by adding a compensating component. The total bias current is modified by introducing a compensating term that varies with the signal amplitude, effectively linearizing the capacitance variation. This parameter change allows the system to maintain optimal operating point while compensating for nonlinear effects.
2Device complexity
If supply voltage of sub-micron CMOS transistor is reduced to one volt or below to improve integration, then device integration is improved, but linear output power and third-order intermodulation performance deteriorate
Solution Approach 1:
A compensating bias current source is introduced as an intermediary element to counteract the nonlinear capacitance variation of the main power amplifier transistor. The compensating bias current, generated through a separate transistor branch, acts as a mediator that cancels out the harmful nonlinear effects, allowing the PA to operate at lower bias conditions for high efficiency while maintaining linearity.
Solution Approach 2:
The compensating bias current acts as a counterweight to the nonlinear effects inherent in low-voltage sub-micron CMOS operation. By introducing an equal and opposite nonlinear component through the compensating branch, the harmful third-order intermodulation and limited linear output power are counterbalanced, enabling low-voltage operation without sacrificing performance.
3Reliability
If existing linearization techniques are applied to narrow bandwidth amplifiers, then linearity is improved, but the technique does not maintain linearity over wide bandwidth required for OFDM signals
Solution Approach 1:
The compensating bias current source is designed with multi-functionality to handle both narrowband and wideband signals effectively. The compensation mechanism works across a broad frequency range, making the linearization technique universally applicable to different bandwidth requirements including OFDM signals with bandwidth from 20 MHz to GHz range, without requiring separate linearization circuits for different bandwidths.
Solution Approach 2:
The linearization technique employs dynamic bias compensation that adapts to the signal characteristics in real-time. The compensating bias current dynamically adjusts to counteract nonlinear effects across the entire bandwidth, enabling the amplifier to maintain linearity whether operating at narrowband or wideband conditions, thus providing adaptability to different bandwidth requirements.
Data Source
AI summary
A wideband power amplifier (PA) linearization technique is proposed. A current interpolation technique is proposed to linearize power amplifiers over a wide bandwidth. The wideband power amplifier linearization technique employs a novel transconductance Gm linearizer using a current interpolation technique that achieves improvement in the third order intermodulation over wide bandwidth for a sub-micron CMOS differential power amplifier. By using a small amount of compensating bias into an opposite phase differential pair, linearization over wide bandwidth is achieved and can be optimized by adjusting the compensating bias.