Active double-tuned circuit for widening passband

Active double-tuned circuit for widening passband

Function of the circuit

In order to obtain sharp attenuation characteristics, the bandpass filter adopts a larger Q value. When it is necessary to reduce the pass width and only take a certain degree of frequency band and attenuation, a double-tuned circuit can be used.

How the Circuit Works

This circuit cascades two multiple feedback bandpass filters. In order to make the characteristics have a certain flat part, the two-stage resonant frequencies are caused to deviate to a certain extent. The amount of deviation depends on the Q value and the requirements of the flat part. When the Q value increases, if the two-stage resonant frequencies FO1 and FO2 are not close, the flatness of the characteristics will deteriorate.

The parameter design is basically the same as that of the "multiple feedback bandpass filter composed of an OP amplifier ", with high FO = 1KHZ, Q = 5. If C1 = C2 = 0.01UF, RO is 15.92K, so R1 = RO.Q.AO = 78.6K (AO = 1), R2 = R2/[2Q-(AO/2Q)] = 1.6K, R3 = 2R.Q = 159K. In order to fine-tune the resonant frequency, R2 is in the form of 1.5K + 200 ohms (VR). If the resonant frequency of the two stages deviates greatly, the synthesized characteristics will change, and the gain of the passband will also decrease, so AO does not take 1, but takes the decreasing part into account and takes > 1. In addition, the resistance values ​​of R2 and R5 related to the resonant frequency must also be changed.