On the transmission path of an electric signal, matching the impedance on the output side with the impedance on the input side that receives it is called impedance conversion. This is important in order to achieve efficient transfer without reflection. The most efficient matching is obtained by making real number part R for both the input and output sides the same and making the imaginary part (ωL - 1 / ωC) including the transmission path null.
Impedance conversion can be done as follows.
(1) Impedance matching with resistor
When matching is performed by an LC circuit for a high-frequency circuit, since the LC circuit features a filter, the way makes matching possible only in the resonance frequency region and there is the possibility of instability due to excessive gain. In this case, matching with resistor is done.
In the case of a low-frequency circuit, reflection is not a problem and low-impedance output can be connected to a high-impedance input without problem. Conversely, connection of a high-impedance output to a low-impedance input may cause damage or distortion due to overcurrent. In such a case, matching is done by inserting a resistor in series on the input side. However, the allowable energy dissipation range must be taken into consideration.
(2) Impedance matching with coil and capacitor
Matching is done by a combination of L and C. Various configurations, including L-type, T-type, and -type, exist, depending on how L and C are connected.
(3) Impedance matching with transistor
Impedance conversion is done by exploiting the fact that the impedance on the input side and the output side differ depending on the transistor grounding method. In the last stage of the audio amplifier, a collector grounding circuit (emitter follower circuit) is often employed because a speaker (4 to 32Ω low-impedance model) is connected.
|Grounding Method||Input Impedance||Output Impedance|
(4) Impedance matching with transformer
Transformers are generally used to convert AC voltage, but they can also be used for impedance conversion. However, such transformers are used to connect speakers to vacuum tube audio amplifiers (high output impedance on the order of several kΩ), and the vacuum tube audio amplifiers themselves are rarely seen nowadays.