The basic principle of impedance matching
1. pure resistance circuit
In secondary school physics, electricity has told such a problem: a resistance of R electrical appliances, connected to an electric potential of E, internal resistance of r battery pack, under what conditions the power output of the power supply is the largest? When the external resistance is equal to the internal resistance, the power output of the power supply to the external circuit is the largest, which is a purely resistive circuit power matching. If replaced by an AC circuit, the same must also meet the conditions of R = r circuit to match.
2. reactance circuit
Impedance circuit is more complex than the pure resistance circuit, in addition to resistance in the circuit there are capacitors and inductors. Components, and work in low-frequency or high-frequency AC circuits. In AC circuits, resistance, capacitance and inductance of alternating current obstruction is called impedance, indicated by the letter Z. Of these, the hindering effect of capacitance and inductance on the alternating current is called capacitive reactance and and inductive reactance and respectively. The value of capacitive reactance and inductive reactance is related to the frequency of the alternating current operated in addition to the size of the capacitance and inductance itself. It is worth noting that, in a reactance circuit, the value of resistance R, inductive reactance and capacitive reactance double can not be added by simple arithmetic, but commonly used impedance triangulation method to calculate. Thus, the impedance circuit to achieve matching than purely resistive circuits to be more complex, in addition to the input and output circuits in the resistive component requirements are equal, but also requires the reactance component of equal size and sign of the opposite (conjugate matching); or resistive component and reactance components are equal (non-reflective matching). Here refers to the reactance X, that is, inductive XL and capacitive reactance XC difference (only for series circuits, if the parallel circuit is more complicated to calculate). To meet the above conditions is called impedance matching, the load that can get the maximum power.
The key to impedance matching is 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, all kinds of measuring instruments and all kinds of electronic components. So what are input impedance and output impedance? The input impedance is the impedance of the circuit to the signal source. As shown in Figure 3 amplifier, its input impedance is to remove the signal source E and internal resistance r, from the AB ends into the equivalent impedance. Its value is Z = UI / I1, that is, the ratio of the input voltage and input current. For the signal source, the amplifier becomes its load. Numerically, the equivalent load value of the amplifier is the value of the input impedance. The size of the input impedance is not the same for different circuits.
For example, the higher the input impedance (called voltage sensitivity) of the voltage block of a multimeter, the smaller the shunt on the circuit under test and the smaller the measurement error. The lower the input impedance of the current block, the smaller the voltage division to 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 against the load. As in Figure 4, the power supply of the input side of the circuit is short-circuited, the output side of the load is removed, the equivalent impedance from the output side of the CD is called the output impedance. If the load impedance is not equal to the output impedance, called impedance mismatch, the load can not get the maximum power output. The ratio of output voltage U2 and output current I2 is called output impedance. The size of the output impedance depends on different circuits have different requirements.
For example, a voltage source requires a low output impedance, while a current source requires a high output impedance. For an amplifier circuit, the value of the output impedance indicates its ability to carry a load. Usually, a small output impedance results in a high load carrying capacity. If the output impedance cannot be matched to the load, a transformer or network circuit can be added to achieve the match. For example, a transistor amplifier is usually connected to an output transformer between the amplifier and the speaker, and the output impedance of the amplifier is matched with the primary impedance of the transformer, and the secondary impedance of the transformer is matched with the impedance of the speaker. The secondary impedance of the transformer is matched to the impedance of the loudspeaker. The transformer transforms the impedance ratio through the turns ratio of the primary and secondary windings. In the actual electronic circuits, often encountered with the signal source and amplifier circuit or amplifier circuit and the load impedance is not equal to the situation, so they can not be directly connected. The solution is to add a matching circuit or network between them. Finally, it should be noted that impedance matching is only applicable to electronic circuits. Because the power of the signals transmitted in electronic circuits is inherently weak, matching is needed to increase the output power. In electrical circuits, matching is generally not considered, as it can lead to excessive output current and damage to the appliance.
Application of Impedance Matching
For general high-frequency signals, such as clock signals, bus signals, and even up to several hundred megabytes of DDR signals, etc., the general device transceiver inductive and capacitive impedance is relatively small, relative resistance (i.e., the real part of the impedance) that can be ignored, and at this point, impedance matching only need to take into account the real part of the can be.
In the field of radio frequency, many devices such as antennas, amplifiers, etc., its input and output impedance is not real (not pure resistance), and its imaginary part (capacitive or inductive) is so large that it can not be ignored, then we must use the conjugate matching method.
Post time: Aug-17-2023