67. Active Band-pass Filters

Two active band-pass filter topologies are examined, Sallen Key and Multi Feedback. A little theory is covered followed by a circuit analysis and experiments to prove that the darn things actually work like we expect.

[@DefekCs]

People like You in KZbin are PURE GOLD! Thank You for Your videos!!!

[@SusanAmberBruce]

Lovely just what I was looking for thanks

[@alibehrouz9833]

nice job. again one great video and explanation.

[@PelDaddy]

I've found some crazy out of tolerance ceramic caps from eBay as well. Worth always testing them. Thanks for the great videos!

[@chronobot2001]

excellent video!

[@MrNedelcuBogdan]

Great video!

[@tzimmermann]

For the MFB circuit, I think that you need to assume that C1=C2=C in order to compute the values the way you do. If C1 is not equal to C2, the gain at f0 will be modified: H = - Rf*C1/(R1(C1+C2)), and so will be the quality factor: Q = 2*pi*f0*Rf*C1*C2/(C1+C2). Then the formulas will need to be adapted or you won't control the gain correctly. But yeah, in practice you will more often than not set C1 equal to C2.

[@sebastiandelgado6824]

Thanks a lot for this video , can i ask you something ? ... Can you share the documentation about this filters pls

[@phillipneal8194]

Great job. Thank you. Can these active filter Sallen-Key types be used at RF levels of 20 Mhz ?

[@mrjohhhnnnyyy5797]

That multi feedback circuit will be my choice if I need a bandpass. I'll use Sallen-Key for LP or HP only, making BP with them requires more parts without giving better performance. Or is it?

[@TheOffsetVolt]

Yes, that 's right. You can also get quad op amps and make 4 different multi-feedback BPs. Space each BP about an octave apart and you have an equalizer. Thanks for watching!

[@mrjohhhnnnyyy5797]

The Offset Volt One more question for you :) In the video about common base amps, you mentioned that it's hard to predict gain. So, by putting a constant current sink instead of the emitter resistor emitter current will be the same, and the gain should be easy to calculate. Or maybe the high impedance of the current sink will mess it up?

[@TheOffsetVolt]

Hell again, I wouldn't use a current source or sink with the common base. The CBs high voltage gain us due to the low r'e value compared to the resistance on the collectors output. Current doesn't figure into this.

[@mrjohhhnnnyyy5797]

The Offset Volt OK, thanks. That's the answer I was looking for.

[@lustrup]

"R" and "C" value question: I want to make a one or two stage multifeedback bandpass filter for radio astronomy. The slow alternating dc "wavelenght" I will be detecting from radiostars Sources will be in the order from 4 to 7 minutes. This is very very low frequency! Anyone know the value of "C" an "R" I need to use. I'm not able to calculate this myself.

[@raygosper1656]

Hi, was going through the calculations for the Multi-feedback Band pass Filter for C. I'm getting 8.2462nF not 82.462nF as you have calculated, could you confirm for me.

[@roelandriemens]

at 6.50 you also show shorter formula when both caps and resistors are equal. I think it should be 1 / 2.pi.r.c without the square root sign?

[@miexnotes1114]

Hello . Im working with a Multiple feedback filter with fc of around 1kHz and a Q of 20 . My question is what is the best or good op amp IC for you?. I am using lm324 ic but its not working. Can i have your suggestion regarding to this problem?

[@TheOffsetVolt]

Hi - You may be exceeding the bandwidth gain product. You can either lower the gain or use an op amp with a higher slew rate. It's hard to say much more without knowing the circuit particulars. Thanks for watching and good luck with the circuit.

[@miexnotes1114]

Thanks for the reply. Since i am using only breadboard type connection. Do you think there is also an issue with breadboard compared with PCB type circuit? and also the distances of the feedback components from the device?.

[@engner]

i need document guide of this design , please

[@TheOffsetVolt]

Hello, I am not sure what you need exactly. Is it the schematic, data sheets, etc. Thanks for watching.

[@davidluther3408]

GOOD PRESENTATION BUT,ITS ASHAME YOU DO NOT HAVE A SPECTRUM ANYALIZER TO SHOW THE BODE PLOT.