Switched RC Gate Network for LNA Noise and High-Frequency Stability
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Solution Overview
Problem
Related-art low-noise amplifiers (LNAs) face instability issues at high frequencies due to capacitive impedance at the gate of transistors, leading to potential oscillations and poor stability, particularly above 10 GHz, which are exacerbated by process, voltage, and temperature variations.
Innovation Solution
The RF amplifier incorporates series-connected capacitors and resistors with controlled shunt and grounding switches, managed by a controller, to maintain stability and minimize noise figure degradation across varying conditions.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If additional capacitors are added between gate and source node to improve noise figure performance and input matching, then noise figure and input matching are improved, but stability issues occur at frequencies greater than 10 GHz due to capacitive impedance presenting negative real part at gate
Solution Approach 1:
A resistor is introduced as an intermediary element in series with the capacitor between gate and source. This resistor mediates the impedance characteristics, preventing the capacitive network from presenting a negative real part at the gate at high frequencies, thereby maintaining stability while preserving noise figure improvement.
Solution Approach 2:
The impedance characteristics of the gate-source network are modified by adding a resistor in series with the capacitor. This changes the overall impedance parameter to ensure it maintains a positive real part across the frequency range, eliminating stability issues while retaining noise figure benefits.
2Area of stationary object
If one common source degeneration inductor is shared among different bands to save layout area, then layout area is reduced, but stability issues arise when transistors are switched on and off at high frequencies
Solution Approach 1:
Resistors are added as intermediary elements in the gate-source path of each transistor. These resistors act as mediators that prevent instability when transistors are switched on and off, working in conjunction with the shared inductor to maintain stability across frequency bands while preserving layout area savings.
3Measurement precision
If transistor size and capacitance and inductance values are selected to optimize performance, then noise figure and gain are optimized, but potential for stability issues increases at frequencies greater than 10 GHz
Solution Approach 1:
The impedance parameters of the transistor gate network are modified by adding series resistors with the gate-source capacitors. This parameter change ensures that the real part of the impedance remains positive at high frequencies, preventing stability issues while allowing optimization of transistor size, capacitance, and inductance for noise figure and gain performance.
Data Source
AI summary
A radio frequency (RF) amplifier has a first transistor with a first gate coupled to a first input, and a first series-connected capacitor-resistor combination coupled between the first gate and a first source. A first shunt switch is in parallel with the first resistor and controlled by a first signal. A grounding switch is between the first gate and ground, controlled by a second signal. A controller commands the switches to select an on-state for amplification at the first input or off-state to prevent RF signal amplification. A second transistor with a second gate is coupled to a second input, and a second series-connected capacitor-resistor combination is coupled between the second gate and a second source. A second shunt switch is in parallel with the second resistor and controlled by a third signal. A grounding switch is between the first gate and ground and is controlled by a fourth signal.