Impedance Matching in RF Circuits
Key Takeaways
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In electronics, maximum power is transferred when the source impedance matches the load impedance.
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Impedance matching involves the design of a circuit to be inserted between the source and load for maximum power transfer.
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When applications demand impedance matching over a wide frequency range, wideband matching networks involving four or more elements are chosen.
Smith charts are one of the traditional methods used for developing impedance-matching networks for RF circuits
Consider an RF energy harvesting system that supplies energy for portable electronic circuit operations. In such systems, an antenna transfers the received RF signal to impedance-matching circuits to maximize power transfer and convert it to DC voltage using a rectifier. Impedance matching in RF circuits is ubiquitous, as it brings the maximum power transfer concept into RF applications. In this article, we will discuss the importance of impedance matching in RF circuits.
RF circuit
Impedance Matching in RF Circuits
In electronics, maximum power is transferred when the source impedance matches the load impedance. When source resistance equals load resistance, maximum power is transferred. If there is a reactive component in the source impedance, then the load impedance should be a complex conjugate of the source impedance for maximum power transfer. That means the source and load resistances match up and the imaginary reactive part of the load impedance will be negative of the imaginary reactive part of the source impedance.
Impedance matching is significant in RF circuit design. Impedance matching involves the design of a circuit to be inserted between the source and load to achieve maximum power transfer. Impedance matching is not always about maximum power transfer; it can be used to trade off gain requirements, bandwidth, and noise in wideband amplifiers and low-noise amplifiers.
RF energy harvesting system
Where Is Impedance Matching Used in RF Circuits?
RF devices incorporate impedance-matching circuits for better power transfer and performance. RF energy harvesting systems use impedance matching circuits before rectification to deliver optimum power to the load. Usually, impedance matching is used in RF energy harvesting systems to match the impedance of the rectifier (load side) with the antenna (source side) impedance of 50 for transferring maximum power.
Antenna Feed Lines
The impedance of the power amplifier should be matched with the antenna for more powerful signal transfer. Impedance matching can be used in antenna feed lines for coupling maximum power transfer from the power amplifier to the antenna.
Low-Noise Amplifiers
Impedance matching in low-noise amplifiers is not for maximum power transfer, but for low or minimum noise figures. There is an optimum source impedance associated with the amplifier for achieving a minimum noise figure. By using impedance circuits, the input impedance of the amplifier is matched to the optimum value. There is a trade-off made between power and noise figures in such applications by using impedance-matching circuits.
Power Dividers
Power dividers are used in RF circuits for dividing power. For example, Wilkinson power dividers split or combine the power equally in RF circuits. Impedance matching contributes to the maximum power transfer from the power divider to the load.
Impedance Matching Networks in RF Circuits
There are different ways in which impedance matching can be implemented in RF circuits. The following section will introduce you to a few impedance-matching methods.
1) Two-Element L Networks
L networks can be incorporated into circuits for impedance matching; either inverted L-section networks or reverse L-section networks. As the name suggests, the matching network is oriented such that it forms an ‘L’ shape. It is the simplest and easiest impedance-matching network to design. Low component loss is the major advantage that keeps the L network superior to other matching circuits.
2) Three-Element Network
In impedance matching networks, the quality factor of the network defines the bandwidth of the matching network. When applications require a wideband matching network, two-element L networks will work. However, for band-limited applications requiring high Q-matching networks, three-element network architecture needs to be incorporated.
Three-element networks provide flexibility in choosing the practical value of ‘Q’, which is greater than that achieved with two-element Lnetworks.
Two types of three-element impedance networks are:
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Pi networks - The network elements are arranged in the form of the greek letter 𝚷.
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T networks - The matching network elements are oriented in the form of the Latin alphabet T.
3) Wideband Matching Networks With Four or More Elements
When applications demand impedance matching over a wide frequency range, wideband matching networks involving four or more elements are chosen. Generally, L network configurations are cascaded to achieve wideband impedance matching network architecture; these are called low-Q networks.
Methods Aiding Impedance Matching in RF Circuits
Smith charts are one of the traditional methods used in developing impedance-matching networks for RF circuits. Computer-aided methods can be utilized, enabling the easy and fast realization of impedance matching in RF circuits. Cadence’s PCB design software can help you construct impedance-matching networks with minimum input and output losses.
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