RF Power Amplifier Package Impedance Network for IMD Control
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Solution Overview
Problem
RF power amplifier packages face a trade-off between controlling impedance at second-order intermodulation distortion frequencies and overall efficiency, due to the large output capacitance of RF power transistors, which affects signal amplification and power delivery in mobile telecommunications base stations.
Innovation Solution
A circuit topology is introduced where the second capacitive element is much larger than the first, with larger electrodes, acting as a ground at relevant frequencies, and the first capacitive element uses this as an efficient ground connection, while inductive elements resonate at specific frequencies to minimize impedance mismatch and damping is used to reduce resonance peaks.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If a series resonance circuit with large capacitance C1 is used to resonate with output capacitance Cds, then the reactive part of impedance at operational frequency is reduced, but a resonance peak occurs at low frequencies due to interaction with feed inductance Lfeed
Solution Approach 1:
The patent divides the single capacitance function into two separate capacitive elements C1 and C2 with different capacitance values. C1 (smaller, e.g., 1-10 pF) handles the resonance with output capacitance at operational frequency, while C2 (larger, e.g., 10-50 pF) provides low-frequency grounding. This segmentation allows each capacitor to be optimized for its specific frequency range, eliminating the harmful resonance peak while maintaining operational frequency performance.
Solution Approach 2:
The patent applies different capacitance values to different parts of the circuit based on frequency requirements. The smaller capacitor C1 is positioned to resonate with output capacitance at the operational frequency (e.g., 800 MHz to 3.5 GHz), while the larger capacitor C2 is positioned to provide grounding at lower frequencies (e.g., DC to 100 MHz). This local differentiation of capacitance values optimizes impedance characteristics at each frequency band independently.
2Reliability
If a larger capacitance C2 is used to act as ground at RF and lower frequencies, then low-frequency impedance control is improved, but the available space for C1 is reduced and its losses increase
Solution Approach 1:
The patent segments the grounding function across two capacitors: C2 provides the primary low-frequency grounding with its larger capacitance value, while C1 provides supplementary grounding at operational frequencies. This segmentation allows C1 to have smaller size and lower losses, as it no longer needs to provide the entire grounding function across all frequencies. The series combination of C1 and C2 achieves both low-frequency grounding and operational frequency resonance without the trade-off of increased losses.
3Object-generated harmful factors
If damping resistor is added in series with L2 to dampen resonance peaks, then impedance control at IMD frequencies is improved, but RF signal dissipation occurs at operational frequency
Solution Approach 1:
The patent segments the damping function by using the inherent resistance of the series resonance circuit formed by L1 and C1, rather than adding a separate damping resistor. The small capacitance value of C1 (1-10 pF) combined with L1 creates a high-Q resonance at operational frequency that provides sufficient damping of low-frequency resonance peaks without causing significant RF signal dissipation. This segmented approach to damping avoids the trade-off between resonance peak suppression and RF signal loss.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
This topology effectively controls impedance at second-order IMD frequencies, reduces energy losses, and maintains RF performance at operational frequencies, improving the trade-off between impedance control and efficiency.
Implementation Method 1
a series resonance circuit formed by L1 and C1 is connected to the drain of the FET. L1 is chosen such that it will resonate with Cds at or close to the operational frequency.
Implementation Method 2
C2 has a terminal electrically connected to L2 and another terminal electrically connected to ground. C2 should be very large, and more in particular substantially larger than C1, and acts as a ground at RF and lower frequencies.
Implementation Method 3
a second resonance will occur at an intermediate second resonance frequency related to the resonance of L2 and C1. L2 and C1 form a parallel resonance circuit that will display a transition between a high positive and high negative reactive part near the resonance frequency of L2 and C1.
Implementation Method 4
C2 is configured to resonate with the feed inductance Lfeed at a first resonance frequency that is substantially smaller than the operational frequency.
Implementation Method 5
a damping resistor can be placed in series with L2. This resistance will dampen the resonance at the first and second resonance frequencies.
Data Source
Figure 1A~1B
Figure 2A~2B
Figure 3A~3B
AI summary
The present invention relates to a radiofrequency (RF) power transistor package. It further relates to a mobile telecommunications base station comprising such a RF power transistor package, and to an integrated passive die suitable for an RF power amplifier package. According to the present invention, an in-package impedance network is used that is connected to the output of the RF power transistor arranged inside the package. This network comprises a first inductive element having a first and second terminal, the first terminal being electrically connected to the output of the RF transistor, a resonance unit being electrically connected to second terminal of the first inductive element, and a second capacitive element electrically connected in between the resonance unit and ground. According to the present invention, the first capacitive element is arranged in series with the second capacitive element.