# Chebyshev filter - theoretical vs simulation gain

Question is about the gain peak (or maybe more suitable would be growth) ocurring in higher frequencies (>20kHz) in Chebyshev filter 4-th order in simulation (simulated using Bode Analyzer in NI Multisim) - what's the reason for this? Theory shows that gain is going still down even up to -150dB, however simulation shows minimum at about -120dB. Is that because of some capacitance effects ocurring in op-amps or something else? I found similar problem to mine, but the grounding there and op-amp was quite different (Unexpected behavior of the Sallen-Key second order filter). Below theoretical gain based on formula: $$A(f)_{dB}=20\log{\left\lvert\frac{k_0}{(1 + a_1(\frac{j(2\pi f)}{\omega_g}) + b_1(\frac{j(2\pi f)}{\omega_g})^2)(1 + a_2(\frac{j(2\pi f)}{\omega_g}) + b_2(\frac{j(2\pi f)}{\omega_g})^2)}\right\rvert}$$ and chart for this: The result of the simulation is shown below. In higher frequencies like 1MHz gain continues to go up. The circuit was constructed based on the Sallen-Key scheme - schematics of the cicuit below. • Welcome to the real world, where opamps have finite I/O impedances, limited bandwidths, even RLC elements behave strangely with frequency. Not to mention the methods of measuring, or the questionable use of that particular opamp. – a concerned citizen Jun 2 '20 at 11:03