# High Q bandpass for 50kHz

Discussion in 'General Electronics Discussion' started by sbieker, May 3, 2012.

1. ### sbieker

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May 3, 2012
Hi everybody,

I want to realize a bandpass for a center frequency of f0 = 50kHz with a Q factor of around 50 to 100, gain should be of order unity.

I tried with an active bandpass with multiple feedback and also with a biquad active filter (each with OP37). In principle, both designs work but Q factors are worse than expedted. (I assume someone more experienced in electronics would easily figure out the reasons..)

However, I found that National Semiconductor's AF150 seems to perfectly meet my demands. Does someone know if this circuit is still available or if there is any equal substitute? Where can I procure the device?

Any hint is appreciated... thanks in advance!
Steffen

2. ### Wabajig

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Apr 14, 2012
I remember in college we had a great result using same resistor and capacitor value in the feedback of an op amp circuit. It went resistor from output, capacitor to ground and resistor to the - input, all this in parallel with capacitor from output, resistor to ground, and then capacitor to - input. Maybe a 1k resistor for - input + input to ground, so it will invert the output. But very high Q. Can't remember the name of circuit. But it was based on the charging time constant of capacitors T=5RC math. Hope this help. John

3. ### duke37

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772
Jan 9, 2011
I have 'Electronic Filter Design Handbook' by A.B.Williams. He says that the multiple feedback circuit is suitable for Q<20.
He gives an example of a dual amplifier feedback circuit with Q=70.
He says the biquad can have Q=200, I do not know why your circuit does not behave, perhaps it is the frequency limitations of the amps.

4. ### Wabajig

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Apr 14, 2012
I remember there was a limitation on op amps of Hfe*frequency for each op amp. Hope it helps.

5. ### Harald KappModeratorModerator

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2,696
Nov 17, 2011
Another source of low Q may be tolerances of the components. Try to get components that at least match.

Harald

6. ### sbieker

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May 3, 2012
Ok thanks a lot... I will try your circuit, Wabajig. Also I will try to take care that components match, maybe that improves the Q factor.

Does anyone have an idea concerning the NS AF150?