Problems with Cascading First order active filters

[Robbinz]

Hi everyone, wooo first post

I am designing a passband filter using a high and low pass filters. When I model it using Tina8 with ideal op amps the filter works beautifully as it should.

Whenever I put in real op amps I get some irregularities. It spikes at the end then the output gain is off. I have searched and searched but cannot find out why.

I believe it is possibly Harmonic Distortion but am not 100% sure.

I have attached both what I wish it to look like, what it does look like and my circuit.

If anyone could give me a hand recognising the problem I would greatly appretiate it.

Thanks you

 

[Gary B]

I may be barking up the wrong tree but the first thing I would suspect is an unsimulated feedback path. Layout would be the primary suspect and a lack of bypass capacitors on the power supply or at least the opamp power input leads would be my next concern. It looks like you have a positive feedback path operating around 700K that may even need some interstage shielding.

 

[crutschow]

Harmonic distortion is not likely to be causing the problem.

I suspect the gain-bandwidth of the op amps may also be contributing. Rerun your simulations with an op amp with a GBW (Unity gain bandwidth) of 4MHz as the TL071C has.

 

[MrAl]

Hi everyone, wooo first post

I am designing a passband filter using a high and low pass filters. When I model it using Tina8 with ideal op amps the filter works beautifully as it should.

Whenever I put in real op amps I get some irregularities. It spikes at the end then the output gain is off. I have searched and searched but cannot find out why.

I believe it is possibly Harmonic Distortion but am not 100% sure.

I have attached both what I wish it to look like, what it does look like and my circuit.

If anyone could give me a hand recognising the problem I would greatly appretiate it.

Thanks you

Hello,


What is your input signal level? Did you check to see that are within the common mode input range of the op amps?