# High Q bandpass for 50kHz

#### sbieker

May 3, 2012
2
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

#### Wabajig

Apr 14, 2012
75
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

#### duke37

Jan 9, 2011
5,364
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.

#### Wabajig

Apr 14, 2012
75
I remember there was a limitation on op amps of Hfe*frequency for each op amp. Hope it helps.

#### Harald Kapp

##### Moderator
Moderator
Nov 17, 2011
12,516
Another source of low Q may be tolerances of the components. Try to get components that at least match.

Harald

#### sbieker

May 3, 2012
2
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?

Replies
2
Views
1K
G
Replies
6
Views
1K
Rene Tschaggelar
R
M
Replies
23
Views
2K
Dave VanHorn
D