Electronics > Beginners
Simulating Bandpass Filter
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LvW:
If you select the Deliyannis Modifikation (with pos. feedback, factor k) the design is very simple:

R2>>>infinite
C1=C2=C; R3=R1=R
k=Ro/Rf (Rf: Feedback resistor to the pos. input, Ro between pos. input and ground)

Midfrequency wo=1/RC;
Quality factor: Q=1/(2-k)
Midband gain: Ao=Q(1+k)

As you can see (formula for Q) the circuit is rather sensitive to the factor k (tolerances) - in particular for High Q values (as in your case).
Therefore, it would be wise to use one of the other bandpass topologies which are better suited for large Q values - for example a GIC structure (if you can afford to use two opamps).
Coceth:
It's "UniversalOpamp2". I've subbed existing generic op amps in Analog Devices' catalog, and the bandwidth gets slightly worse. So my simulations were best-case I guess.
LvW:

--- Quote from: Coceth on October 17, 2018, 03:34:10 pm ---It's "UniversalOpamp2". I've subbed existing generic op amps in Analog Devices' catalog, and the bandwidth gets slightly worse. So my simulations were best-case I guess.

--- End quote ---

I hope you are aware that - for hardware realizations - the opamp must be selected very carefully with respect to GBW as well as slew rate (which very often is the limiting quantity).
Wimberleytech:

--- Quote from: LvW on October 17, 2018, 04:15:58 pm ---
--- Quote from: Coceth on October 17, 2018, 03:34:10 pm ---It's "UniversalOpamp2". I've subbed existing generic op amps in Analog Devices' catalog, and the bandwidth gets slightly worse. So my simulations were best-case I guess.

--- End quote ---

I hope you are aware that - for hardware realizations - the opamp must be selected very carefully with respect to GBW as well as slew rate (which very often is the limiting quantity).

--- End quote ---

Correct.  I ran a simulation using a VCVS and got close to the correct response. Did not investigate the additional peaking however (did you design it for 6dB gain at midband?).

update...took another look and zoomed in to the passband peak.  Looks like a little over 9dB at the peak.
Coceth:
Thank you for all the responses! It helped me realize that "UniversalOpamp2" has a default GBP of 10MHz, which may be too low. I turned up the GBP, and the higher I made it, the the more the center frequency converged to the calculated value. I'm lowering the Q and gain of the filter so the GBP of the opamp doesn't have to be so high.

Thanks again  :-+
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