Cascode PA Bias Circuit for AM-PM Phase Linearization
Find Innovative SolutionsGenerate Solutions
Solution Overview
Problem
Power amplifiers in mobile communication devices, particularly those using 5G-NR and millimeter wave signals, face challenges in maintaining phase linearity across multiple power levels due to cascode transistor operation in saturation and triode modes, leading to significant phase distortion and non-linear phase behavior.
Innovation Solution
A bias circuit is implemented to fix the bias of cascode transistors within power amplifiers, allowing them to operate at a fixed duty cycle between saturation and triode modes, resulting in constant capacitance and negligible phase distortion across a wide signal level range.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Power
If cascode transistors operate in saturation and triode modes to amplify signals across multiple power levels, then power amplification capability is improved, but phase linearity deteriorates due to variable capacitance
Solution Approach 1:
The bias circuit dynamically adjusts the operating point of the cascode transistor to maintain a fixed duty cycle between saturation and triode modes across varying signal power levels. This dynamic control ensures that the transistor spends equal time in each mode during RF cycles, stabilizing the average capacitance and thereby maintaining phase linearity while preserving broad power amplification capability
Solution Approach 2:
The invention changes the bias voltage parameter applied to the cascode transistor gate to hold it at a specific voltage level that corresponds to the threshold voltage. This parameter change fixes the transistor's switching behavior between saturation and triode modes, creating a stable duty cycle that results in constant average capacitance and improved phase linearity across multiple power levels
2Adaptability or versatility
If cascode transistors switch between saturation and triode modes, then power level flexibility is improved, but phase distortion increases due to variable capacitance
Solution Approach 1:
The bias circuit dynamically maintains a fixed duty cycle operation of the cascode transistor across different power levels. By continuously adjusting the bias conditions to ensure the transistor operates in saturation for exactly 50% of the RF cycle and in triode mode for the other 50%, the system achieves both power level flexibility and reduced phase distortion through stabilized average capacitance
Solution Approach 2:
The bias circuit implements feedback control by monitoring the transistor's operating state and adjusting the gate bias voltage to maintain the fixed duty cycle. This feedback mechanism ensures that regardless of the signal power level, the transistor switching behavior remains consistent, thereby minimizing phase distortion while preserving adaptability across power levels
Data Source
Figure 1A~1B
Figure 2~3
Figure 4~5
AI summary
Amplitude modulation-phase modulation (AM-PM) linearization in a power amplifier (200) using bias circuitry, which fixes a bias of a cascode transistor within a power amplifier is disclosed. In particular, the cascode transistor may switch between operation in a saturation mode and a triode mode. The bias is set such that the cascode transistor operates at a fixed duty cycle in the triode mode relative to the saturation mode for a wide range of signal levels from small-signal to large-signal. An example duty cycle is fifty percent (50%), although other duty cycles may be used. This bias will result in a constant capacitance contributed by the cascode device to the power amplifier over a wide signal level range.