Component value sensitivity is extremely critical, as are temperature coefficients and matching of the components. Best performance is attained when perfectly matched components are used and when the gain of the amplifier is unity.

Schematic of 4.5 MHZ Notch Filter

To illustrate, the quality factor Q is very high as amplifier gain approaches 1 with all components matched (in fact, theoretically it approaches infinity) but decreases to about 12.5 with the amplifier gain at 0.98.