Stacked PA Power Control via Cascode Gate Modulation
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Solution Overview
Problem
In RF amplifiers, particularly those used in GSM applications, the large size of the PMOS pass device required to prevent LDO regulator saturation results in higher die size and cost, and there is a need to either eliminate or reduce the size of the LDO regulator for efficient power control.
Innovation Solution
A cascode configuration of transistors is used where the output power is controlled by varying the gate voltages of the cascoded transistors, allowing for reduced size of the pass device and eliminating the need for a large LDO regulator, while maintaining the voltage at the drain of the output transistor fixed.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If a large PMOS pass device is used to prevent LDO regulator saturation, then power control accuracy is improved, but die size and cost increase
Solution Approach 1:
The power control function is segmented between the LDO regulator (which sets the drain voltage) and the cascoded transistor stack (which controls the output power through gate voltages). This segmentation allows the LDO regulator to use a smaller pass device while maintaining overall power control accuracy.
Solution Approach 2:
The cascoded transistor stack acts as an intermediary between the LDO regulator and the output, enabling precise power control through gate voltage modulation without requiring the LDO pass device to handle the full current burden alone.
2Measurement precision
If a large PMOS pass device is used to prevent LDO regulator saturation, then power control accuracy is improved, but device cost increases
Solution Approach 1:
The power control function is segmented between the LDO regulator and the cascoded transistor stack, allowing the use of smaller, less expensive pass devices while maintaining control accuracy through the combined action of voltage regulation and transistor gating.
Solution Approach 2:
The invention changes the control parameter from relying solely on pass device size to using gate voltage modulation of the cascoded transistors, enabling accurate power control with smaller, more cost-effective devices.
3Loss of energy
If a large pass device is removed from the main current conduction path, then power-added efficiency is improved, but power control capability must be maintained
Solution Approach 1:
The large pass device is extracted from the main current conduction path and replaced with a voltage regulation mechanism. The LDO regulator controls the drain voltage while the cascoded transistors control the current, separating the voltage and current control functions to improve efficiency.
Solution Approach 2:
The LDO regulator provides feedback control of the drain voltage to maintain precise power control capability without requiring a large pass device in the current path, thereby improving power-added efficiency while preserving control accuracy.
Data Source
AI summary
Systems, methods and apparatus for efficient power control of an RF amplifier for amplification of a constant envelope RF signal are described. A reduction in a size of a pass device of an LDO regulator is obtained by removing the pass device of the LDO regulator from a main current conduction path of the RF amplifier. Power control is provided by varying one or more gate voltages to cascoded transistors of a transistor stack of the RF amplifier according to a power control voltage. Various configurations for controlling the gate voltages are presented by way of a smaller size LDO regulator or by completely removing the LDO regulator. In a case where a supply voltage to the transistor stack varies, such as in a case of a battery, a compensation circuit is used to adjust the power control voltage in view of a variation of the supply voltage, and therefore null a corresponding drift in output power of the RF amplifier.