Stacked Pre-Driver Amplifier for High-Voltage RF PA Efficiency
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Solution Overview
Problem
Radio frequency power amplifiers face efficiency losses due to the need to reduce battery voltage to a maximum allowed voltage for transistors, leading to power dissipation as heat in existing solutions like DC-DC converters, resistor dividers, and variable transistor bias, violating reliability rules and reducing overall efficiency.
Innovation Solution
A stacked pre-driver amplifier stage with multiple pre-drivers in series, each comprising MOS transistors, operates within the maximum allowed voltage, minimizing voltage drop across the regulator, allowing the use of higher battery voltages without wasting power as heat, and combining outputs efficiently.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If battery voltage is reduced to maximum transistor voltage using DC-DC converter, then transistor reliability is improved, but power efficiency deteriorates due to conversion losses
Solution Approach 1:
The pre-driver stage is segmented into multiple cascaded amplifier stages (first amplifying stage with multiple amplifying circuits in series, second amplifying stage), allowing progressive voltage reduction across stages rather than single-stage conversion. This segmentation enables reliable transistor operation at lower voltages while distributing power loss across multiple efficient amplification stages.
2Reliability
If resistor divider is used to reduce voltage from battery to use voltage, then transistor voltage limit is satisfied, but power efficiency deteriorates due to heat dissipation
Solution Approach 1:
The passive resistor divider is replaced with active MOS transistor-based amplifying circuits that provide voltage reduction through controlled signal amplification and regulation. The amplifying circuits use MOS transistors in saturation region to actively regulate output voltage, substituting passive dissipative components with active regulatory components that minimize power loss.
3Reliability
If variable bias transistor is used to provide voltage drop, then voltage regulation is achieved, but power efficiency deteriorates due to heat dissipation in transistor
Solution Approach 1:
The system uses dynamic bias control where the second amplifying stage receives control signals from the first amplifying stage to dynamically adjust operating points. MOS transistors operate in saturation region with dynamically adjusted gate voltages, enabling adaptive voltage regulation that optimizes power efficiency across varying signal conditions rather than static bias points.
4Device complexity
If single pre-driver stage is used, then circuit complexity is reduced, but transistor voltage stress exceeds maximum allowed voltage
Solution Approach 1:
The pre-driver is segmented into a first amplifying stage with multiple series-connected amplifying circuits and a second amplifying stage, distributing voltage stress across multiple devices. Each transistor experiences only a fraction of the total battery voltage, ensuring Vds remains below maximum allowed voltage while maintaining functional pre-driver capability.
5Reliability
If multiple amplifying circuits are connected in series in first amplifying stage, then voltage distribution across transistors is improved, but device complexity increases
Solution Approach 1:
Multiple amplifying circuits are merged into a cascaded configuration where the output of one stage feeds the input of the next. The first and second amplifying stages are combined with the PA stage, creating an integrated multi-stage system that achieves voltage distribution through functional merging rather than separate independent circuits.
Data Source
AI summary
A stacked pre-driver stage and a power amplifier including the stacked pre-driver stage are described. The stacked pre-driver stage comprises stacked pre-drivers arranged in series between a supply voltage and a reference voltage. Each pre-driver includes a pre-driving amplifier, together with MOS transistors. Each pre-driver is subject, in operation, to a voltage difference which is inferior to a maximum allowed use voltage of the MOS transistors with a largely reduced voltage drop across the regulator included in the power amplifier.