RF Variable Gain Amplifier Cascode Mirror for Linear 74-dB Control
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Solution Overview
Problem
Wideband Code Division Multiple Access (WCDMA) systems face challenges in achieving accurate linear-to-dB gain control over a 74-dB gain tuning range due to limited device isolation in RF variable gain amplifiers, which complicates direct conversion architectures and requires complex calibration techniques.
Innovation Solution
A CMOS RF variable gain amplifier employing a wide swing cascode mirror formed by two cascode transistors and two gain transistors, which operate in the saturation region, allowing for extended linear tuning range without relying on device reverse isolation, and utilizing a linear voltage-to-current converter to control gain.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Use of energy by moving object
If a direct conversion architecture is used with RF VGA, then image rejection and power consumption are improved, but gain control accuracy and LO leakage rejection deteriorate due to limited device isolation
Solution Approach 1:
The patent changes the operating parameters of the amplifier by using a wide swing cascode configuration that operates in the saturation region, enabling extended linear tuning range and improved gain control accuracy while maintaining the direct conversion architecture's power efficiency
Solution Approach 2:
The patent extracts and cancels the feed-forward signals that cause LO leakage by using differential cascode structures, thereby improving isolation performance without requiring complex calibration techniques or sacrificing power efficiency
2Measurement precision
If device isolation is increased to improve LO leakage rejection, then gain control accuracy is improved, but device complexity and circuit isolation requirements worsen
Solution Approach 1:
The patent employs asymmetric cascode transistor sizing and configuration where the cascode transistors are specifically designed with different dimensions to optimize the cancellation of feed-forward signals, achieving improved isolation without requiring symmetric high-isolation devices throughout the circuit
Solution Approach 2:
The patent converts the harmful feed-forward signals that cause LO leakage into a beneficial cancellation mechanism by using differential cascode structures that naturally reject these signals, turning the isolation problem into a solution that simplifies the overall circuit design
3Adaptability or versatility
If gain tuning range is extended to 74-dB, then adaptability is improved, but linearity and gain control accuracy deteriorate due to limited linear tuning range
Solution Approach 1:
The patent implements dynamic gain control by operating the cascode transistors in the saturation region where they can dynamically adjust their operating point, enabling extended linear tuning range of 74-dB while maintaining linearity through active region control rather than fixed biasing
Data Source
AI summary
A RF variable gain amplifier with an extended linear tuning range is disclosed. The variable gain amplifier employs a wide swing cascode mirror formed by two cascode transistors and two gain transistors. The two cascode transistors track each other, so are the two gain transistor. The gain transistors operate on the saturation region.