Differential Post-Mixer Amplifier Topology for Low-Noise SDR Linearity
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Solution Overview
Problem
Current amplifier systems for software-defined radio (SDR) applications face challenges in achieving low noise, high linearity, and low power consumption, particularly due to issues with current-mode topology, high output impedance, and the use of inverting op-amp configurations which introduce noise and increase power consumption.
Innovation Solution
A fully differential mixer and class AB post-mixer amplifier system using bipolar junction transistors and voltage-mode amplifiers with a unique current-feedback topology, replacing PMOS transistors with bipolar transistors to reduce flicker noise and using resistors instead of FETs to minimize noise, and employing a 'magic resistor' and 'folded magic resistor' configurations to maintain high linearity and low power consumption.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Object-affected harmful factors
If current-mode topology is used with high output impedance LNA, then low noise is achieved, but linearity degrades at high frequencies due to compression
Solution Approach 1:
The patent changes the impedance parameter of the LNA output stage from high to low, and changes the PMA input impedance from low to high. This parameter transformation resolves the contradiction by allowing the LNA to operate in voltage mode with low output impedance, driving the passive mixer which presents high input impedance to the LNA, thereby maintaining both low noise and high linearity at high frequencies
Solution Approach 2:
The patent inverts the conventional current-mode topology by adopting voltage-mode operation. Instead of using a current-mode LNA with high output impedance driving a low impedance PMA, the invention uses a voltage-mode LNA with low output impedance driving a high impedance PMA, fundamentally reversing the conventional approach to resolve the linearity degradation issue
2Use of energy by moving object
If inverting op-amp configuration is used to achieve high input impedance, then low power is consumed, but thermal noise increases significantly
Solution Approach 1:
The patent inverts the conventional inverting op-amp configuration by using a non-inverting topology. The non-inverting PMA configuration achieves high input impedance without requiring very large resistors, thereby avoiding the significant thermal noise that would be generated. This inversion of the conventional approach allows simultaneous achievement of low power consumption and low thermal noise
3Object-affected harmful factors
If bipolar transistors are used at PMA inputs to ensure low close-in noise, then noise is reduced, but DC current through resistors creates flicker noise
Solution Approach 1:
The patent changes the biasing parameters and operating point of the bipolar transistors to minimize DC current flow through the input resistors. By optimizing the bias conditions and using the non-inverting configuration, the design reduces the DC current component that would otherwise generate flicker noise, while maintaining the low close-in noise benefit of bipolar transistors
4Stress or pressure
If large resistors are used to realize large Zin in inverting op-amp, then input impedance is increased, but thermal noise and surface area increase
Solution Approach 1:
The patent inverts the conventional approach by using a non-inverting op-amp configuration which naturally provides high input impedance without requiring very large resistor values. This inversion allows the use of smaller resistors that generate less thermal noise while still achieving the required high input impedance level
Solution Approach 2:
The patent changes the configuration parameters of the PMA from inverting to non-inverting, which fundamentally alters the input impedance characteristics. The non-inverting configuration provides high input impedance through the op-amp's virtual ground concept rather than relying on large physical resistor values, thereby reducing thermal noise generation
5Reliability
If dual-input differential pairs with CMFB structure are used, then fully differential operation is achieved, but noise, complexity, and power consumption increase
Solution Approach 1:
The patent extracts and removes the CMFB structure from the fully differential PMA design. By eliminating the CMFB circuitry, the design reduces the additional noise sources and complexity that the CMFB structure introduces, while maintaining fully differential operation through alternative design techniques that do not require separate common-mode feedback paths
Data Source
AI summary
A post-mixer amplifier device which receives and processes signals for use in a software-defined radio integrated circuit is provided. The post-mixer amplifier device includes but is not limited to a voltage amplifier having first and second inputs and a first output, a positive signal output connected with the first output of the voltage amplifier, and a positive signal input connected with a first bipolar junction transistor along a first pathway. The first bipolar junction transistor includes but is not limited to a first collector connected with a the first input of the voltage amplifier and a first emitter connected with an second output of the push-pull unity gain follower and forming a first current feedback pathway. The first bipolar junction transistor is driven with a passive resistive load.