Hi,

I've got a 2 stage 4 pole 2dB Chebychev low bandpass filter (f/ pg 274 Art of Electronics2nd Ed). w/ +/-9V power the output is clipping between 1 and 2 V.... this is using TL072 and LF353...

ANy Ideas why it is doing this? I was expecting output up to ~9V

thanks,
GE

Edit +/- 9 V clips at 1/4Vp-p.... +/-18V clips at 2.5Vp-p!!!!!!!!!

 

Can you post the schematic? Have you checked that it is built correctly?

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I know the output is giving the exact response I was expecting filter-wise.... just that it's clipping too soon.

I'll try and post a schematic of it.

-GE

 

Schematic attached

Attached Images

cheby.jpg (26.7 KB, 475 views)

 

mmm u do realise you have set up a votage divider with 1M and 100k ie

100k/1.1M = 0.1/1.1 = 0.9 of the true output. That might be what it is

 

But that can't explain the Max Output of 1.4V can it?

 

That's 0.09, not 0.9, which is exactly why the clipping occurs. Why do you need this voltage divider?

 

Sweet you guys are awsome.... I'm not sure why it's needed, but at least I know what the problem is now. I was following a recipe for the circuit, I'm gonna check if it's necessary.

From what I remember I think the divider provides an overall gain ~1 for the filter.

I'll check back with an update if I can figure out more.

Thx again for the help!

-GE

 

Let's say that you are getting a full 9V out of the 2nd op-amp.

Voltage Divider Rule (voltage across the 100k resistor):

Vout = Vin * R1 / (R1 + R2)

Vout = 9V * 100k / (1M + 100k) = 0.818V

If you measure across the whole thing you'll get the voltages you were expecting.


edit: looks like i was a bit late.

 

GE, in the future, you might consider going back with your scope and checking the signal at the output of each amplifier before you scratch your hair thin. Most likely, you'd have found the signal levels you wanted at each of the outputs and then the low signal at the 100K resistor, clueing you in to the likely problem. In the end, the amps were clipping where they were supposed to be clipping, at around 8.5v or so and then attenuated at the output to that low voltage.

Checking at the output of an op amp will give you all sorts of information as to what may be going on. It's also a nice, low-impedance point that won't be loaded down by anything you'd normally use for test equipment.

Dean

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Dean Huster, Electronics Curmudgeon
Contributing Editor emeritus, "Q & A", of the former "Poptronics" magazine (formerly "Popular Electronics" and "Electronics Now" magazines).

R.I.P.

 

Thanks again for all the help and tips, and any more that comes.

 

I simulated your filter, it has 15 dB gain which you don't need, see below.
I designed a 0.2 dB Cheby low pass which has the added advantage of using standard values, attached below.

Attached Images

	0P2DB_PLOT.jpg (67.5 KB, 430 views)
	0P2DB_SCHEM.jpg (13.6 KB, 430 views)
	ORIGINAL_FILTER.jpg (68.4 KB, 430 views)

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hehe. tnx this was posted just before i started work forgot a factor of 10