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What is the formula for calculating the knee of a RC circuit?

April 26, 2017calculating circuit Formula knee RC

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What is the formula for calculating the knee of a RC circuit?

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From R.G. Keen: A resistor and capacitor are halfway to whatever they will do – highpass or lowpass – when the capacitive reactance equals the resistor. That is, when Xc = 1/(2*pi*Frequency)= R, the filter is at its knee. So with a little elementary algebra, Freq. = 1/(2*pi*R*C). The units are ohms, hertz and farads. For the TS series feedback loop (for example), this comes out to (1/F)= 2 * pi * (.000000047Farad) * (4700 ohms) = .001387 Sec F = 720.8Hz. You can play games with the decimal points. If you use C in uF and R in megohms, you get F = 1/(2*pi*.047uf*.004700M) = 720.8Hz But I prefer not to mess with remembering special cases of uF/Mohm, etc. Just do it in farads and ohms and keep the decimal point straight.