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During i study Sallen-key active low pass filter, i wondered why formula of Q is 1/(3-A). A is Voltage Gain. So i searched from web, and i found this. enter image description here

When R1=R2 and C1=C2, the Q be 1/(3-A). OK. But, i don’t know why the formula of Q’s defined as a formula in the picture above.

Why the formula of Q’s defined as a formula in the picture above? I’d really appreciate it if you could derive this formula.