GATE 2025 EC – Question 34
All the components in the bandpass filter given below are ideal. The lower $-3$ dB frequency of the filter is 1 MHz.
The upper $-3$ dB frequency (in MHz, rounded off to the nearest integer) is ________.
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Correct answer: 50
Explanation
The input branch is $R$ in series with $10C$ (a high-pass section) and the feedback branch is $2R$ in parallel with $0.1C$ (a low-pass section), so $H(s)=-Z_f/Z_{in}$ has one pole at each corner. Lower corner $f_L=\dfrac{1}{2\pi R(10C)}=1$ MHz. Upper corner $f_H=\dfrac{1}{2\pi(2R)(0.1C)}=\dfrac{1}{2\pi(0.2RC)}$. Hence $f_H/f_L=10/0.2=50$, so $f_H=50$ MHz.