Cascode Power Amplifier With Switchable Output Matching for Low Distortion
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Solution Overview
Problem
Radio-frequency power amplifiers suffer from undesirable distortion, particularly AM-AM and AM-PM nonlinear effects, which degrade system performance and are challenging to address in high-power wireless communication devices.
Innovation Solution
A radio-frequency module and power amplifier circuit with a cascode configuration and switchable output matching network, featuring a shunt arm with a switchable impedance that adjusts between high and low power modes, utilizing a capacitor and inductor in the output matching network to minimize distortion.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Adaptability or versatility
If a fixed impedance output matching network is used, then the amplifier can be optimized for either high-power or low-power mode, but it cannot effectively operate in both modes due to impedance mismatch
Solution Approach 1:
The output matching network transitions from a fixed impedance configuration to a dynamic, switchable impedance configuration. A switch element is introduced in the shunt arm of the Pi-network, allowing the network to dynamically reconfigure its impedance between high-power mode (switch open) and low-power mode (switch closed), enabling the amplifier to adapt to different operating conditions.
Solution Approach 2:
The impedance parameter of the output matching network is made variable through the introduction of a switchable element. By changing the state of the switch (open/closed), the impedance presented to the amplifier changes accordingly, allowing optimization for both high-power and low-power modes without requiring separate matching networks.
2Power
If the amplifier operates in high-power mode, then output power is high, but distortion increases due to nonlinear effects
Solution Approach 1:
The AM-PM distortion correcting capacitor is connected in parallel with the base-emitter junction of the first transistor to preemptively counteract the nonlinear phase modulation effects. This capacitor provides a predistortion mechanism that compensates for the amplitude-to-phase conversion that occurs in the transistor, thereby reducing distortion before it degrades the output signal.
Solution Approach 2:
The distortion correcting capacitor acts as an intermediary element between the input signal and the transistor's nonlinear response. By introducing this additional reactive element, the circuit creates a compensating effect that offsets the harmful nonlinear phase modulation, reducing overall distortion in the output signal.
3Power
If the amplifier operates in low-power mode, then output power is reduced, but power-added efficiency deteriorates
Solution Approach 1:
The output matching network is dynamically reconfigured for low-power operation by closing the switch in the shunt arm. This changes the impedance transformation ratio of the matching network, optimizing the load seen by the amplifier transistor for low-power conditions and thereby improving power-added efficiency at reduced output power levels.
4Reliability
If a cascode configuration is used, then stability and gain are improved, but complexity of the circuit increases
Solution Approach 1:
The amplifier is divided into two distinct transistor stages in a cascode configuration: a common-emitter first transistor for voltage gain and a common-base second transistor for current buffering and stability. This segmentation allows each transistor to perform its specialized function optimally, improving overall stability and gain while managing complexity through functional division.
Data Source
AI summary
A radio-frequency (RF) module includes a first transistor having a base, a collector, and an emitter, a radio-frequency output transmit path coupled to the collector of the first transistor at a first end and to a radio-frequency output port at a second end, and an output matching network disposed in the radio-frequency output transmit path, the output matching network including a shunt arm coupled to ground, the shunt arm including a switch that is controllable to modify an impedance of the output matching network.