Electrical – Op-Amp High Pass Filter – Cutoff Frequency derivation

active-filterfilteroperational-amplifier

I am trying to derive an equation describing the cutoff frequency of a high pass filter op amp, as seen below:
High-Pass Op-Amp

I have tried deriving the cut off frequency from it's transfer function:
transfer function

But I can't get the intended result as indicated at the bottom of working? Is anyone able to spot where I am going wrong in my working or method of attack please?

working

Best Answer

For this circuit we have

$$H_{(s)} = - \frac{s R_2 C_1}{1 + s R_1 C_1} $$

So we have one Pole at

$$-\frac{1}{R_1 C_1}$$

And one Zero at the origin.

All this means that for low frequency the circuit behaves like an ordinary op-amp based differentiator.

With the gain $$A_V = \omega R_2C_1$$

And the gain reachs \$1 V/V\$ when sinal frequancy is equal to \$Fo=\frac{1}{2 \pi R_2 C_1}\$

As signal frequency increases \$Xc\$ drops and when \$Xc = R_1\$ we have a Pole:

$$\omega = \frac{1}{R_1 C_1}$$

$$Fp=\frac{1}{2 \pi R_1 C_1}$$.

And the the magnitude of a transfer function (voltage gain vs frequency) is equal $$A = \frac{\omega R_2 C_1}{\sqrt{1 + \left ( \omega R_1 C_1 \right )^2}}$$