Electronics > Beginners
Simulating Bandpass Filter
Coceth:
I'm simulating a MFB bandpass filter and given my part selection, I should be getting a center frequency of 421.1kHz according to Okawa's filter calculator. An AC sweep in LTSpice shows a center frequency of 187kHz though. I'm certain I wired the circuit up right, but the simulator doesn't lie... Can someone take a peek?
LvW:
I don`t know where the errror might be - however, try another (better) design and start with R2 infinite.
As you can see, R2=20 ohm is extremely low in comparison to the other resistors.
More than that, because of the high Q-value, another variation of the filter structure is recommended:
Some positive feedback (resistive voltage divider to the pos. oamp input). For high Q values, this modification was recommended by Deliyannis long time ago.
Also in this case: R2 infinite !
As a consequence, this modification gives a higher midband gain - is this acceptable?
Hint: This MFB alternative is considered in some filter design packages - perhaps also in Okinawa`s program?
ogden:
You definitely shall try this: http://www.analog.com/designtools/en/filterwizard/
and/or this: http://www.ti.com/design-tools/signal-chain-design/webench-filters.html
Coceth:
I appreciate your advice LvW. I agree that R2 is very low (nearly a short). I'll play with the values a try make it >100 ohms at least.
Also thanks for directing me to the modified MFB topology - I'll look into it!
T3sl4co1l:
What opamp is that?
Tim
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