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In practical operation, the function of the absorption circuit is to remove the signal of a certain frequency from the input signal. The absorption circuit composed of 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 series resonant frequency of fo, which is connected between the input terminal of VT1 and the ground terminal.
(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 reactor, it can be seen that there is no signal with frequency fo in the output signal.
Characteristics of series resonant circuit:
1. When the applied frequency is equal to its resonant frequency, its circuit impedance exhibits pure resistance and has a minimum value. This characteristic is called a notch filter in practical applications
2. When the external frequency is higher than its resonant frequency, the circuit impedance is inductive, equivalent to an inductive coil
3. When the applied frequency is lower than its resonant frequency, the circuit becomes capacitive, equivalent to a capacitor
Series resonant high-frequency boost circuit
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.
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.
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
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 amplified in the same way as signals at the resonant frequency