Active low pass filter. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Knots of amateur radio equipment. Filters and matching devices
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The proposed filter (see figure) differs from those already known by the use of a field-effect transistor, which provides an increased input impedance. The filter can be used in transmitter modulators and low-frequency amplifiers of communication receivers when working with a telephone.
The device is designed to isolate signals with a frequency of 300-3400 Hz. With a voltage gain of 0,65, the filter provides an attenuation of 22 dB per octave beyond a cutoff frequency of 3400 Hz with flatness and a bandwidth of 0,9 dB.
A signal with a voltage of up to 1 V can be applied to the filter input.
With serviceable parts, the filter starts to work without adjustment. Field-effect transistor V1 can also be of types KP1102, KP103, transistor V2 - KT312. KT315 (all - with any indices).
Capacitors C2-C5 and resistors R1-R3. R5 is desirable to apply with a tolerance of 5%.
To obtain greater attenuation, similar links are added to this filter, between which amplifier stages can be connected.
Author: M. Plakhotnikov, Voronezh; Publication: N. Bolshakov, rf.atnn.ru
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