RF Switch Circuit Using Impedance Transformers to Cut Distortion
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Solution Overview
Problem
Existing switches for high frequency signals, such as those using high electron mobility transistors (HEMT) or heterojunction bipolar transistors (HBT), face challenges with high costs due to expensive compound semiconductor wafers and special processing requirements, leading to power leakage and distortion issues.
Innovation Solution
A switch design incorporating two switching units with impedance transformers and bipolar transistors, where the first switching unit has an emitter-grounded and collector-connected configuration, and the second switching unit has a collector-grounded and emitter-connected configuration, using a SiGe BiCMOS process to reduce distortion and insertion loss while maintaining isolation.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If a compound semiconductor HEMT is used for high frequency signal switching, then switching performance is improved, but manufacturing cost increases due to expensive wafers and limited aperture size
Solution Approach 1:
The patent replaces expensive compound semiconductor HEMTs with inexpensive silicon-based bipolar transistors. Although bipolar transistors have shorter lifespan and higher power consumption, they provide adequate switching performance for the application while dramatically reducing manufacturing costs and improving wafer aperture availability.
Solution Approach 2:
The patent changes the fundamental material parameter from compound semiconductor to silicon, and changes the transistor type from FET to bipolar transistor. This parameter change enables cost-effective manufacturing while maintaining acceptable switching performance through careful circuit design including impedance matching networks and biasing schemes.
2Reliability
If special processes such as SOS or SOI are used to prevent power leakage in CMOS switches, then switching characteristics are improved, but manufacturing cost increases
Solution Approach 1:
The patent avoids expensive special processes like SOS or SOI by using standard silicon bipolar transistor technology. While bipolar transistors consume more power than CMOS, they inherently provide better switching characteristics without requiring additional manufacturing steps, thereby eliminating the need for costly special processes.
3Ease of manufacture
If a bipolar transistor is used in a switch, then manufacturing cost is reduced, but distortion with respect to high frequency signal amplitude increases due to asymmetric forward and reverse characteristics
Solution Approach 1:
The patent acknowledges the asymmetric characteristics of bipolar transistors but uses this asymmetry beneficially through careful circuit design. By implementing impedance matching networks and designing the switching circuit to operate in a specific mode, the patent minimizes the impact of asymmetric forward and reverse characteristics on signal distortion while maintaining cost advantages.
Solution Approach 2:
The patent introduces impedance matching networks as intermediary components between the bipolar transistor and the high frequency signal path. These networks act as mediators that transform the asymmetric bipolar transistor characteristics into symmetric input-output characteristics, thereby reducing signal distortion while maintaining the cost benefits of using bipolar transistors.
Data Source
AI summary
A switch is capable of performing switching, while reducing distortion with respect to amplitude of a high frequency signal. The switch includes: an input terminal to which a high frequency signal is input; a first switching unit connected between the input terminal and a first output terminal and selectively outputting the high frequency signal through the first output terminal; and a second switching unit connected between the input terminal and a second output terminal and selectively outputting the high frequency signal through the second output terminal. Each switching unit includes an impedance transformer installed on a signal line, a bipolar transistor having an emitter grounded, a collector connected to the signal line, and a base receiving current according to a control voltage applied thereto; and a bipolar transistor having a collector grounded, an emitter connected to the signal line, and a base receiving current according to the control voltage.