Buffer logic elements of the CMOS structure can be used as configuration item high frequency filters. Active filters with application such elements are of high q-factor (up to 100) and a lot of overlap range up to 50 and even up to 100.
The figure shows a diagram of the notch filter with adjustable good quality. Its resonant frequency is tuned from 200 kHz to 2 MHz the change in the coefficient of transmission elements chip 4050 adjustment voltage within Uнастр = 2,5...3,5 V. And for this purpose may be used each of the six elements.
In linear amplification mode in this regulation gain element DD1 - here it works as a voltage - to-current for transistor VT1 is substantially changed. Chain DD1,VT1,R3,R4 forms a Converter resistance having at these frequencies the inductive impedance. Capacitor C1 with this inductance forms a series resonant circuit. It setting change the supply voltage of the chip. High quality factor of the circuit is ensured thanks to the large input resistance of the chip 4050. Except it can also be adjusted by the variable resistor R2. Resistor R4 performs the role load. If you connect it between the collector of the transistor and the common wire the resistor is selected in the settings screen (for collector current 1...2 mA). Stability device mode DC is quite high due to the deep EP by voltage. The current consumed by the control circuit, is practically equal to the current base VT1.
The input capacitance of the buffer element is about 80 pF. When using other items with low input capacitance (15 pF) upper limit frequency settings can be extended to 20 MHz.
Instead of the 4050 chip in the filter you can use native analog - K561PU4. Transistor - any high frequency low power, for example, series KT315, KT3102.