High pass second order filter

[Englewood]

I have built this high pass filter on a bread board but it seems to be very unstable.
Is there any mistakes on the circuit

 

[FvM]

Not shown in the schematic, but you have built the circuit with single supply, R1 connected to OP negative supply. This doesn't work. If dual supply isn't an option, you can use a 2x20k voltage divider between V+ and V- in place of R1.

 

[Englewood]

of course a op amp needs a dual supply DOH!!!

Is anyone kind enough to do a schematic for dual supply? or where do I put the +/-

 

[ads-ee]

Hey I'm not an analog guy (I work on FPGAs), but this is a that doesn't have any filter circuits!

 

[Audioguru]

ANY opamp works from a single polarity supply if its input is biased at about half the supply voltage.

 

[kam1787]

of course a op amp needs a dual supply DOH!!!

Is anyone kind enough to do a schematic for dual supply? or where do I put the +/-

Take a look at the datasheet. It will clearly state which pins are for V+ and V-.

Don't feel too bad. I remember [many years ago] a popular electronics magazine publishing a circuit. Many readers wrote in, complaining it did not work. The reason was that the V+ and V- were not shown on the schematic, and readers had not attached a power supply!!!

 

[Audioguru]

where do I put the +/- ?

You correctly put +9V on pin 8 but you wrongly put 0V on pin 4 instead of a second battery that is -9V.

 

[Surendhar M]

You just connect half of the supply voltage instead of ground at R1. That should work

 

[Englewood]

Pin 7 is +v and pin 4 is -v.

8 is the NC

 

[Englewood]

Can anyone design this circuit in a breadboard for me .
Struggling at the moment can't seem to get a out put

 

[FvM]

I believe the single supply circuit has been suggested in post #2 and #8.

 

[Englewood]

Thanks.

I have some results.

How do I work the gain out :-S

 

[FvM]

The gain is +1 by design, but with passband ripple according to your selected (Chebychev, about 1.2 dB) filter characteristic.

 

[Audioguru]

The input and output signal levels drop when the frequency increases. Why?
How are you measuring the signal levels? A cheap multimeter is accurate only at 50Hz and 60Hz then its reading drops at higher frequencies.

The gain of this circuit is 1 because the opamp is a follower with a gain of 1. If the gain is increased to about 1.25 then it peaks at the cutoff frequency and if the gain is higher than about 1.6 then it oscillates. The ratio of the two capacitor values affects peaking or oscillation when the opamp gain is increased. Add another opamp if you want some gain.

 

[Englewood]

I was using a cheap multimeter yes.

 

[Audioguru]

You do not need to work out the gain or loss at each frequency. It is second order highpass filter so its output drops -12dB (to one-quarter) for each octave (half the frequency).
At 1500Hz its output is -3dB (0.707 times) of frequencies when its output levels are flat.
Then at 750Hz its output is 1/4 of frequencies when its output levels are flat.
Then at 375Hz its output is 1/16th.
Then at 187.5Hz its output is 1/64th.
Then at 93.75Hz its output is 1/256th (if the input is 2V then the output is 0.0078V). The gain (loss) is 12dB + 12dB + 12dB + 12dB= -48dB.

Get a better multimeter, an audio mV meter or oscilloscope.

 

[Englewood]

Needs to be like this example, so I can plot a graph

 

[FvM]

The gain of this circuit is 1 because the opamp is a follower with a gain of 1. If the gain is increased to about 1.25 then it peaks at the cutoff frequency and if the gain is higher than about 1.6 then it oscillates. The ratio of the two capacitor values affects peaking or oscillation when the opamp gain is increased.

You can actually achieve any filter Q and respective peaking in the Sallen Key topology with gain = 1.

 

[Englewood]

Okay all sorted.

Now how do I work out the critical frequency
?

 

[Audioguru]

Now how do I work out the critical frequency?

What is a "critical frequency"? Highpass and lowpass filters have a "cutoff frequency" where it is just beginning to filter and the output level is -3dB from the input level.
Look in Google for this type of filter called Sallen-Key Filter and you will see many articles showing the equations.