CMOS Differential PA Linearization Using Current Interpolation
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Solution Overview
Problem
Existing power amplifier linearization techniques are limited to narrow bandwidth, failing to maintain linearity over wide bandwidths required for mobile communication systems, particularly for RF signals with many sub-carriers at narrow frequency spacing, leading to poor signal quality and dynamic range.
Innovation Solution
A wideband power amplifier linearization technique using a novel transconductance Gm linearizer with current interpolation, employing a compensating bias current in an opposite phase differential pair to achieve linearization over a wide bandwidth, optimizing the compensating bias for improved third-order intermodulation performance.
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 especially at higher output power
Solution Approach 1:
The patent applies preliminary anti-action by introducing a compensating bias current through tail biasing transistors that preemptively counteracts the non-linear capacitance variation of the main differential pair. This compensating current is designed to cancel out the distortion components generated by the Class-AB biased amplifier, thereby maintaining linearity while operating at high efficiency bias conditions.
Solution Approach 2:
The patent uses tail biasing transistors as intermediaries that introduce a compensating bias current to the differential pair. These intermediary transistors act as a mediator between the power amplifier core and the biasing network, enabling the injection of correction currents that linearize the overall transfer characteristic without disrupting the high-efficiency Class-AB operation of the main amplifier.
2Reliability
If existing linearization techniques are used, then linearity is improved in narrow bandwidth, but linearity degrades over wide bandwidth required for mobile communication systems
Solution Approach 1:
The patent achieves universality by designing a linearization scheme that simultaneously handles multiple frequency components and wide bandwidth signals. The compensating bias current generated by the tail biasing transistors provides broadband correction that is effective across the entire operating frequency range, making the amplifier suitable for modern wideband communication standards without requiring separate linearization circuits for different bandwidths.
3Loss of energy
If supply voltage of sub-micron CMOS transistor is reduced to one volt or below, then power consumption is reduced, but linear output power and third-order intermodulation performance deteriorate
Solution Approach 1:
The patent implements feedback through the tail biasing transistors that sense the operating conditions of the differential pair and automatically adjust the compensating bias current accordingly. This feedback mechanism ensures that the linearization correction remains effective across varying supply voltages and output power levels, maintaining acceptable IM3 performance even when operating at reduced supply voltages typical of modern low-power CMOS processes.
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.