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Problem:bandpass filter design

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bandpass filter design

I built a Biauad bandpass filter with TL084,the f0 is expected to be 220KHz,but when I finished the circuit, I found that it became an oscillator!
No matter how I change the value of the resistors,the only thing changed is the frequecy of the oscillator.
Can anybody tell me what is the problem and how can I avoid the oscillator?
Thank you for help.
 

nuhertz tutorial

do you have simulated your circuits?
if your circuit show oscillating, it means there is feedbacking. you must cut the feedbacking route.
then you can get what you want.
 

tl084 oscillation

I simulated my circuit in Multisim, it worked as my expected,but after I built it with real components,it ascilated.
 

band-pass filter download bmp

Of course it will become an oscillator with TL084 (jfet input opamps) I think TI warn people in datasheet.

Post the schematic to see what is wrong. Most likey you do not have any balance resistor/capacitor. It is not necessary in schematics but in real application we need them.
 

tl084 oscillating

I built the circuit according to <<Rapid Practical Designs of Active Filters>> written by DAVID E. JOHNSON,there are no balance resistors or capacitors.

since TL084 will arouse oscillating,what kind of op amps should I use?
 

active low-pass filter bode phase oscillation

Many things can cause the appearance of an oscillation. My circuit oscillates without power. Apply a signal and don't worry about it. Do you get the desired results. The power supply also has ripple and it would be hard to tell if the circuit really oscillates on it's own. A good start would be to check the frequency you are getting, if it's too high then that is a normal oscillation. The next step is to check the input to the opamp for a signal. If it's in the mV range and the output is in the V range then you definitely have an oscillation. If the output is in the mV range then there is not an oscillation.
 

nuhertz filter 用 tutorial

To Kevin Weddle:
The output of my circuit is in the V range, and the frequency is about 130KHz,at the meantime,the input is in the mv range, so I can sure it is a oscillator.Thanks for your help!

To djalli:
Here is the schematic drawn in Protel 2004:
according to the book
C=C1=C2
G=R4/R3
B=1/CR4
4*pi*pi*f0*f0=1/(R7*R8*C*C)
 

capacitor values for band pass filter

Do you short the input when you test it without signal?
The ACTIVE band pass filter depends on a zero (or very low) impedance source to drive it, so if you dont connect to it a signal you have to short the input to ground.
Does it still oscillates after shorting input?
 

bandpass active q

You said you have it simulated and the results were OK....
...so check if you have Vdd and Vss pins right decoupled, and obvious thinks like this.
 

bandpass filter oscillates

I short the input to ground, and add decoupling capacitor between Vdd and Vss,but it still oscillating..
:(
 

tutos nuhertz

The schematic in bmp format
 

bandpass filter tl084

yes,you should have simulated your circuit.Maybe you didn't use your software well enough
 

band pass filter tl084

First graph is transfer function, bode plot. Bandwidth is marked as -3dB point for the circuit.
Second graph is response of circuit which I do not like to call it bandpass filter is due to an exitation from 1Khz to 400Khz

This also can serve as active lowpass filter for 5.6kHz bandwidth but too expensive one because you can build such one with fewer components.

the circuit response may look as lowpass filter but also a very bad one, plus also a horrible response as oscillator.

pullup must redo it again.

Please pullup post something for the filter you want, bandwidth, corner frequencies, Q quality, method of implementation, gain of the filter, any load for the circuit and either me or someone else here will be able to help.
 

biquad bandpass filter

Did your simulation use the models for the op amp you used or was it an idealized model? You are operating the circuit within a factor of 10 of the amp cutoff frequency. It will have a lot of phase shift at your frequency that may be the cause of the instability.

Try substituting an amp with a much higher bandwidth in your breadboard and see if the oscillations go away.
 

bandpass op amp filter bode plot

My simulation shows that filter will oscillate if R8 is less than 5kohm. It is stable for greater resistances. Resistor value is not readable. If R8=500ohm filter oscillate at about 100kHz. Your filter is definitively bandpass. Peaking frequency is about 70kHz for R8=5kohm and 20kHz for R8=50k. So check R8, inpropper value is causing oscillations because of phase shift.
Verify that you have no capacitive load on output (there should be no long coax only oscilloscope tip) because these OPAMP's are known to oscillate at capacitive load. In this case you should connect load via 50ohm resistor.
 

    P

    pullup

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Yes, I simulate the circuit with an idea model of op amps.Now I realize it is not a good way:(

To Borber: I change the value of R8 as you mentioned,the oscillation disappear! It is amzing!
How do you make it and what software you used to simulate?
I am a boy green to every aspect.


The mission asigned by my teacher is to pick up a 220KHz signal from an environment with plentiful noise,so he suggest me to use a bandpass filter.It is my first time to design a filter, and can't get any help from my teacher,for he is too busy:(

Thanks everyone who gave me help! all of you are really nice!
 

pullup said:
The mission asigned by my teacher is to pick up a 220KHz signal from an environment with plentiful noise
Click to expand...

220kHz ±1% 10% 5% ?

Active or Passive?
Order of filter?
Filter type: Butterworth,Chebuchev, etc?
Filter implementation: Thomas, Akerberg,Frog..etc?
Gain: ?
Q quality: ?
Passband attenuation: ?
Stop band attentuation: ?
Delay: ?
Phase shift: ?

was anything mentioned?
Complete some of these or all of them and someone will help.

How do you want filter method you want it to be designed, of how you achieve to those numbers? You sure are not looking just for a pretty schematic?
 

    P

    pullup

    Points: 2
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