I. The basic principle of impedance matching
1. Pure resistance circuit
Physics and electricity have told such a problem: a resistance of R appliances, connected to a potential E, internal resistance of r battery pack, in what conditions the power output of the power supply is the maximum? When the external resistance is equal to the internal resistance, the power output from the power supply to the external circuit is the maximum, which is the power matching of pure resistance circuit. If replaced by an AC circuit, the same must also meet R = r this condition circuit to match.
2. Reactance circuit
Reactance circuits are more complex than pure resistive circuits, in addition to resistors, there are capacitors and inductors in the circuit. Components, and work in low-frequency or high-frequency AC circuits. In the AC circuit, the impedance of resistance, capacitance and inductance to AC is called impedance, indicated by the letter Z. Among them, capacitance and inductance to the impedance of AC, respectively, is called capacitive resistance and and inductive resistance and. In addition to the capacitance and inductance itself, the value of the capacitive and inductive resistance is related to the size of the frequency of the alternating current at work. It is worth noting that in a reactance circuit, the values of resistance R, inductance and capacitance cannot be calculated by simple arithmetic summation, but by the impedance triangle method. Therefore, the reactance circuit is more complex to match than the pure resistance circuit, in addition to the resistive components of the input and output circuits are required to be equal, but also require the reactance components of equal size and opposite sign (conjugate matching); or the resistive and reactance components are equal (non-reflective matching). Here the reactance X is the difference between the inductive resistance XL and capacitive resistance XC (only for series circuits, if the parallel circuit is more complicated to calculate). Meet the above conditions is called impedance matching, the load that can get the maximum power.
The key to impedance matching is that the output impedance of the front stage is equal to the input impedance of the back stage. The input impedance and output impedance are widely used in electronic circuits at all levels, various measuring instruments and various electronic components. So what is the input impedance and output impedance? The input impedance is the impedance of the circuit to the signal source.
For example: the higher the input impedance (called voltage sensitivity) of the voltage block in the multimeter, the smaller the shunt to the circuit under test, the smaller the measurement error. And the lower the input impedance of the current block, the smaller the voltage division of the circuit under test, and thus the smaller the measurement error. For power amplifiers, when the output impedance of the signal source is equal to the input impedance of the amplifier circuit, it is called impedance matching, and then the amplifier circuit can obtain the maximum power at the output. Output impedance is the impedance of the circuit speaking to the load.
For example: a voltage source requires a low output impedance, while a current source requires a high output impedance. For an amplifying circuit, the value of output impedance indicates its ability to bear the load. Usually, if the output impedance is small, the ability to bear the load is strong. If the output impedance and the load cannot be matched, a transformer or network circuit can be added to achieve the match. For example, the output transformer is usually connected between the transistor amplifier and the loudspeaker, and the output impedance of the amplifier matches the primary impedance of the transformer, and the secondary impedance of the transformer matches the impedance of the loudspeaker. The transformer transforms the impedance ratio by the turns ratio of the primary and secondary windings. In the actual electronic circuit, often encounter signal source and amplifier circuit or amplifier circuit and load impedance is not equal to the case, so they can not be directly connected. The solution is to add a matching circuit or matching network between them. As a final note, impedance matching is only applicable to electronic circuits. Because the signal power transmitted in electronic circuits is inherently weak, matching is required to increase the output power. In electrical circuits, matching is generally not considered, otherwise the output current will be too high and damage the appliance.
II. the application of impedance matching
For general high-frequency signal field, such as clock signals, bus signals, and even up to several hundred megabytes of DDR signals, etc., the general device transceiver inductive and capacitive resistance are relatively small, relative to the resistance (i.e., the real part of the impedance) can be ignored, at this time, impedance matching only need to consider the real part on it.
In the field of RF, many devices such as antennas, amplifiers, etc., their input and output impedances are non-real (not pure resistance), and their imaginary part (capacitive or inductive resistance) is so large that it cannot be ignored, and then the conjugate matching method should be used.
