What is the principle of LC series resonant circuit? Wuhan UHV specializes in producing series resonance, with a wide range of product options and 15 years of professional electrical testing experience. When looking for series resonance, choose Huatian Power.
1. LC series resonant absorption circuit
The function of the absorption circuit is to remove the signal of a certain frequency from the input signal. The absorption circuit shown in Figure 1 is composed of an LC series resonant circuit. VT1 in the circuit constitutes a first stage amplifier, where U is the input signal and U is the output signal of this amplifier. Ll and Cl form an LC series resonant absorption circuit with a resonant frequency of fo, which is connected between the input terminal of VT1 and the ground terminal.
Figure 1
(1) The input signal frequency is fo. For the signal with frequency fo in the input signal, due to the same resonant frequency as Ll and Cl, the series circuit of Ll and Cl has a small impedance to it. The input signal with frequency five is bypassed to ground by Ll and Cl and cannot be added to the base of VT1. VT1 cannot amplify the signal, and of course, there is no signal with frequency fo in the output signal.
(2) The input signal frequency is higher or lower than the stone. For signals with frequencies higher or lower than fo in the input signal, due to the unequal resonance frequencies with Ll and Cl, the series circuit of Ll and Cl becomes detuned, resulting in a high impedance. The input signal is not bypassed to ground by Ll and Cl, but is added to the base of VT1, amplified by VT1, and output.
From the frequency response characteristics of this amplifier, it can be seen that there is no signal with frequency fo present in the output signal.
2. Series resonant high-frequency boost circuit
Figure 2 shows a high-frequency boost circuit composed of LC series circuits. VT1 in the circuit forms a first stage common emitter amplifier, while Ll and C4 form an LC series resonant circuit to enhance high-frequency signals. The resonant frequency of the series resonant circuit of Ll and C4 is five, which is higher than the highest frequency of the operating signal of this amplifier.
Figure 2
Due to the minimum impedance of the Ll and C4 circuits during resonance, the negative feedback resistance is minimized when connected in parallel with the emitter negative feedback resistor R4, resulting in the maximum amplification factor at this time. In this way, the high-frequency signal close to fo is improved, as shown in the frequency response characteristic curve of the amplifier in the figure. The high-frequency response curve without Ll and C4 is a dashed line, while the response curve with Ll and C4 is a solid line. Obviously, the high-frequency response of the solid line is better than that of the dashed line.
For input signals with frequencies much lower than fo, the Ll and C4 circuits have no boosting effect on them. Because the Ll and C4 circuits are in a detuned state, their impedance is high, and the negative feedback resistance at this time is R4.
3. Summary of the Working Principle Analysis of LC Resonant Circuit
(1) Master the impedance characteristics. Understanding some of the main characteristics of these two resonant circuits is the basis for analyzing their application circuits, among which the most important is the impedance characteristics of the two resonant circuits, because in the analysis of the working principles of various circuits, the analysis of the circuit is mainly based on the impedance of the circuit. When the LC parallel resonant circuit resonates, the impedance is the highest, while the LC series resonant circuit is the lowest. It is easier to remember to correspond them.
(2) The LC series resonant circuit has the minimum impedance during resonance. The precautions to be taken when analyzing LC series resonant circuits are the same as those for parallel resonant circuits, except that the impedance of the circuit is minimized during series resonance and maximized during parallel resonance. For LC series resonant circuits, when the circuit is detuned, the impedance of the circuit is large. At this time, for signals with frequencies lower than the resonant frequency, the main reason is that the capacitance impedance of capacitor Cl is large. For signals with frequencies higher than the resonant frequency, the main reason is that the inductance impedance of inductor Ll is large.
(3) When the LC parallel resonant circuit loses resonance, the impedance is small. For LC parallel resonant circuits, when the circuit is detuned, the impedance of the circuit is very small. At this time, signals with frequencies lower than the resonant frequency mainly pass through the inductor Ll branch, while signals with frequencies higher than the resonant frequency mainly pass through the capacitor Cl branch.
(4) The input signal frequency is divided into two situations. When analyzing the application circuits of these two LC resonant circuits, the input signal frequency should be divided into two situations: the circuit operation when the input signal frequency is equal to the resonant frequency and the circuit operation when the input signal frequency is not equal to the resonant frequency.
(5) Damping resistor effect. The purpose of adding damping resistors in parallel resonant circuits is to obtain the required bandwidth. The smaller the resistance value of the added resistor, the wider the frequency band, and vice versa.
The frequency of the signal input to the LC parallel resonant circuit is very wide, including signals with frequencies equal to the resonant frequency. Among numerous input signals of different frequencies, the circuit only resonates with signals at the resonant frequency, at which point the impedance of the circuit is at its maximum. A resonant circuit has a bandwidth. In circuit analysis, it can be considered that signals within the frequency band are amplified or processed in the same way as signals at the resonant frequency; But for signals with frequencies deviating from the resonant frequency, it is important to understand. The width of the frequency band is related to the size of the Q value. If the Q value is large, it is considered that it has not been amplified or processed, which is why circuit analysis requires a narrow frequency band; Small Q value and wide frequency band.