What is impedance matching, what does it do, and what exactly is in those circuits.

Take the final amplifier output circuit of a radio as an example. In the figure, in the left frame is the equivalent circuit of the amplifier output, which is composed of the internal resistance R0 and the electromotive force E at the output of the amplifier, and the speaker load RL on the right.

According to Ohm's law for the whole circuit, i=e (r0+rl), the power on the load is: pl=rlxi 2=rlx(e 2) (r0+rl) 2;

Find the extreme value of the above equation, when r0=rl, pl is the maximum. That is, the maximum power is obtained on the load at this point.

We say that the load and the output impedance of the circuit are matched.

<> in circuits with resistance, inductance, and capacitance, the impedance that acts as a hindrance to the current in the circuit is called the impedance.

The impedance is commonly represented by z and is a complex number, the real part is called resistance, and the imaginary part is called reactance.

where the capacitance in the circuit is against the alternating current.

hindering role.

The Concept of Impedance Matching:

Impedance matching is mainly used on the transmission line, so as to achieve the purpose that all high-frequency microwave signals can be transmitted to the point of load, and there is almost no signal reflection back to the ** point. The internal impedance of the signal source is equal to the characteristic impedance of the connected transmission line and the phase is the same, or the characteristic impedance of the transmission line is equal to the size and the phase of the impedance of the connected load, which is called the input or output of the transmission line in the state of impedance matching, which is referred to as impedance matching.

Fundamentals of Impedance Matching:

1. Pure resistance circuit.

In the middle school physics of electricity, there was a problem about connecting an electrical appliance with a resistance of r to a battery pack with an electromotive force of e and an internal resistance of r. 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 the power matching of the pure resistance circuit.

If it is replaced with an AC circuit, the condition of r=r must also be met before the circuit fails to match.

2. Reactance circuit.

Reactive circuits are more complex than purely resistive circuits, and there are capacitors and inductors in the circuit in addition to resistors. components, and operate in low-frequency or high-frequency AC circuits. In an AC circuit, the resistance of resistance, capacitance, and inductance to AC is called impedance, which is denoted by the letter Z.

Among them, the obstructive effect of capacitance and inductance on alternating current is called capacitive reactance and inductive reactance respectively.

The values of capacitive and inductive reactances are related not only to the magnitude of the capacitance and inductance itself, but also to the frequency of the alternating current in which it operates. It is worth noting that in reactance circuits, the values of resistance r, inductive reactance and capacitive reactance cannot be added by simple arithmetic, but are often calculated by the impedance triangle method.

Therefore, the matching of reactance circuits is more complicated than that of pure resistance circuits, in addition to the equal resistance components in the input and output circuits, the reactive components in the middle collapse are required to be equal in size and the signs are opposite (conjugate matching); Or the resistance and reactance components are equal respectively (no reflection matching).

The relationship between impedance and resistance is: joint and several relationship. The greater the resistance of a conductor, the greater the impedance of the conductor to the current.

Resistance is a physical quantity that in physics indicates the magnitude of the conductor's effect on the electrodescent withering current. The greater the resistance of a conductor, the greater the resistance of the conductor to the current. In circuits with resistance, inductance, and capacitance, the impedance that acts as a hindrance to the current in the circuit is called.

ImpedanceIn circuits with resistance, inductance, and capacitance, the impedance that acts as a hindrance to the current in the circuit is called impedance. Impedance shock imitation is commonly represented by z, which is a complex number, which is actually called resistance, and virtual is called reactance, in which the obstructive effect of capacitance on alternating current in the circuit is called capacitive reactance, and the obstructive effect of inductance on alternating current in Ming Ying fiber circuit is called inductive reactance.

The impediment effect of capacitors and inductors on alternating current in a circuit is collectively referred to as reactance. The unit of impedance is ohms. The concept of impedance exists not only in electrical circuits, but also in mechanical vibration systems.

Impedance matching requires that the input impedance should be the same as the output impedance when the electrical equipment is connected. Electrical equipment can only achieve maximum efficiency if it meets the impedance matching.

In order to maximize the power amplifier output rated power; The supply voltage is mainly dependent on the load impedance under a given condition. Generally, in Class D switching power amplifiers, the generator transformer, primary equivalent load RL'The output power expression on is:

where vam is the magnitude of the fundamental wave at the equivalent load;

VCC is the supply voltage; VCES is the saturation pressure drop of the power amplifier tube, therefore.

In order to ensure that the system has a certain power margin (due to the loss of the output transformer, the final stage matching loop and the transistor loss resistor, PO'It is necessary to multiply by a coefficient that is approximately equal to 1 4 to 1 5. That is, the output power po is 1 5po'；

From the above equation, it can be seen that after a given supply voltage, the output power depends on the equivalent load rl'. At present, most power ultrasonic generators are loaded with piezoelectric transducers, and their impedance is about tens of ohms to hundreds of ohms, so in order to achieve the required rated power, it is necessary to convert the impedance of the transducer load RL. Transition from high impedance to low impedance.

In general, the conversion is performed by the ratio of the turns of the primary stage coil of the output transformer. If the ratio of the number of secondary primary turns of the transformer is n m, the primary resistance at the output power po.

For example, the power output of a generator on the transducer is 1000W, and the DC VCC is 220V, VCEs=10V, and the power should have a certain margin, then PO='=1500w。then the transformer is primary.

If the equivalent resistance RL 200 when the transducer is resonant, then the ratio of the primary turns of the output transformer to the secondary.

The impedance conversion referred to above is carried out by the output transformer.

The output transformer is an important part of the impedance matching and power transmission of the ultrasonic generator, and its design and winding process are very important for the working safety of the generator. It not only affects the operation of the circuit in the form of leakage inductance, excitation current, etc., but also the leakage inductance is the main reason for the formation of output voltage spikes. Therefore, in the design, high saturation materials with high magnetic flux density B, high permeability C, high resistivity C and low coercivity HC should be selected as the core.

Generally, when preventing transient saturation of high-frequency transformers, the following points should be paid attention to in the design: