When designing circuits, many people often have questions about impedance, so what is impedance? In an inductive, capacitive, and resistive circuit, the impedance that can hinder the current in the circuit is often represented by Z. The value of the impedance is determined by the interaction of the frequency of the alternating current, the resistance R, the inductance L, and the capacitance C. In practical circuits, impedance is variable at any time and can change with the change of current frequency.
1、 Introduction to Input Impedance
The input impedance is the equivalent impedance of the circuit input. Apply a voltage source U to the input terminal and measure the input current I, so that the input impedance Rin is equal to U/I. If the input terminal is at both ends of the resistor, the resistance value of the resistor is the input impedance.
The input impedance reflects the magnitude of the current blocking effect. For voltage driven circuits, the greater the input impedance, the lighter the load on the voltage source, so it is easier to drive without affecting the signal source; For current driven circuits, the smaller the input impedance, the lighter the load on the current source.
Voltage source driving, the greater the input impedance, the better;
Current source driving, the smaller the impedance, the better.
2、 Introduction to Output Impedance
Regardless of amplifier, signal source, or power supply, there will be output impedance issues. The output impedance is the internal resistance of the signal source. For an ideal voltage source, the internal resistance should all be 0, but it cannot be achieved in practice.
We often use an ideal voltage source in series with a resistor r to equivalent an actual voltage source. The resistance r in series with an ideal voltage source is the internal resistance (signal source/amplifier output/power supply). When this voltage source supplies power to a load, a current I flows through the load and generates an I on this resistor × The voltage drop of r.
A decrease in the power supply output voltage limits the maximum output power. An ideal current source has an infinite output impedance, but it is actually impossible.
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3、 Impedance matching
Impedance matching is the matching method between a signal source or transmission line and a load. Impedance matching is generally divided into high frequency and low frequency.
When capacitive or inductive impedance occurs in an AC circuit, it is required that the real parts of the signal source and load impedance are equal, and the imaginary parts are opposite to each other, which is called conjugate matching.
If a large output current is required, select a small load R;
If a large output voltage is required, select a large load R;
If you need a large output power, choose a resistor R that matches the internal resistance of the signal source.
Sometimes impedance mismatches occur, such as when the instrument output is designed under specific load conditions, and when the load conditions are changed, the original performance cannot be achieved, which is called impedance mismatch.
When the impedance does not match, what methods can be used to match it?
1. Using a transformer for impedance conversion;
2. Using series/parallel capacitors or inductors;
3. Using series/parallel resistors.
4、 Principle of impedance matching
1. Pure resistance circuit
When the external resistance is equal to the internal resistance, the power output from the external circuit of the power supply is large, which is the power matching of a pure resistance circuit. If it is replaced by an AC circuit, the circuit also needs to meet the condition of R=r to match.
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2. Reactance circuit
The case of a reactance circuit can be much more complex than a pure resistance circuit because there are not only resistors but also capacitors and inductors in the circuit. In alternating current circuits, the resistance, capacitance, and inductance that hinder alternating current is called impedance, denoted by the letter Z.
The blocking effect of capacitance and inductance on alternating current is also known as capacitive reactance and inductive reactance. The values of capacitive reactance and inductive reactance are related not only to the size of capacitance and inductance, but also to the frequency of the working alternating current.
The key to impedance matching is that the output impedance of the front stage is equal to the input impedance of the rear stage, while input impedance and output impedance exist widely in various electronic circuits and measuring instruments. So what are input impedance and output impedance?
The input impedance is the impedance of the circuit to the signal source. For a power amplifier, when the output impedance of the signal source and the input impedance of the amplifier circuit are equal, it is called impedance matching. At this time, the amplifier circuit can obtain the maximum power at the output end.
Output impedance refers to the impedance of a circuit to a load.
It should be noted that impedance matching is only applicable to electronic circuits, as the transmission signal power in electronic circuits is inherently weak and requires matching to improve output power. However, matching is not considered in electrical circuits, otherwise excessive output current may be caused and electrical appliances may be damaged.
Antai ATA-3090 power amplifier:
ATA-3090 power amplifier index parameters
Figure: ATA-3090 Power Amplifier Index Parameters
When should impedance matching be considered?
In an ordinary broadband amplifier, the output impedance is 50 Ω, so impedance matching in the power transmission circuit should be considered. However, in practice, the cable length is negligible for the signal wavelength, so impedance matching is not required.
The signal frequency is considered to be 1MHz, with a wavelength of 300m in air and about 200m in coaxial cable. In commonly used coaxial cables with a length of about 1m, it is within a completely negligible range.
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If there is impedance, there will be power consumption on the impedance, so not doing impedance matching will result in useless waste of amplifier output power.
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For pure resistance circuits, this conclusion is also applicable to low-frequency and high-frequency circuits. When an AC circuit contains capacitive or inductive impedances, the conclusion changes. It requires that the real parts of the signal source and the load impedance be equal, and the imaginary parts are opposite to each other. This is called conjugate matching.