Band-Pass Filter Help

[BobK]

An opamp has a parameter called the GBW gain bandwidth product, expressed as a frequency. This is the frequency at which the gain has fallen to 1, If you want to get a gain of 750 for audio frequencies of up to 20KHz, you would need an opamp with a GBW of 15MHz. I don't know what an LM324 is, but I doubt that it is that high.

And, as Harald said, opamps do not shift freqency, but they do shift phase, is that what you are talking about? I,e, the peaks in the output are not at the same place as the peaks in the input?

Edit: Corrected my acronym usage.
Bob

 

[EBrown]

I guess the frequency shifting I am seeing is the low-pass filter that the op-amp is acting as.

Thanks,
EBrown

 

[EBrown]

An opamp has a parameter called the GPW gain power bndwith, expressed as a frequency. This is the frequency at which the gain has fallen to 1, If you want to get a gain of 750 for audio frequencies of up to 20KHz, you would need an opamp with a GPB of 15MHz. I don't know what an LM324 is, but I doubt that it is that high.

And, as Harald said, opamps do not shift freqency, but they do shift phase, is that what you are talking about? I,e, the peaks in the output are not at the same place as the peaks in the input?

Bob

That makes much more sense. That was the response I was looking for. When I selected an op-amp with a much higher GBP I obtained the result I wanted for all of my frequencies.

It looks like this particular issue is solved, thanks a bunch everyone!

Thanks,
EBrown

 

[EBrown]

Op-Amp Refuses to Amplify

So I have a new issue now. It seems that the second amplifier I have is not amplifying. I don't actually know what it is doing or why.

It should have the same GBP as the other Op-Amp, so that shouldn't be an issue. The gain I have it setup for is only supposed to amplify the .502V signal to around 11V, and it has 20V input. Can anyone explain why this is acting this way?

I've attached a picture to describe the problem. The blue is pre-amp, the green is post-amp. It's hard to tell with the way this forum converts images. The U16 Op-Amp is not amplifying like it should. As you can see it is dropping my signal. Is there a reason for this? Is there something I am overlooking again?

Thanks,
EBrown

 

[Harald Kapp]

The feedback resistors are way too low. Instead of 1 Ohm, 22 Ohm and 357 Ohm use 1k, 22k, 357k.
Even if this is only a simulation, any real opamp will be overloaded by such small resistances. I don't know how good the simulation models are in reacting to overload, but I simulated a single stage using
a) 22 Ohm/1Ohm feedback
b) 2200 Ohm/100Ohm feedback
The 22Ohm/1Ohm simulation gave grossly distorted signals, whereas the 2200Ohm/100Ohm simulation showed a clear sine wave.

Also you seem to use only a single supply voltage (20V). That does not work with the circuit you have If you want single supply operation, you need a virtual ground somewhere at approx. 1/2 Vcc. That way the input signal can swing around that virtual ground (Google this). With a single supply, the signal can only be positive. Any negative signal will be clipped (set to 0).
For simplicity I suggest you use a dual supply (+-20V) in your simulation.

 

[EBrown]

The feedback resistors are way too low. Instead of 1 Ohm, 22 Ohm and 357 Ohm use 1k, 22k, 357k.
Even if this is only a simulation, any real opamp will be overloaded by such small resistances. I don't know how good the simulation models are in reacting to overload, but I simulated a single stage using
a) 22 Ohm/1Ohm feedback
b) 2200 Ohm/100Ohm feedback
The 22Ohm/1Ohm simulation gave grossly distorted signals, whereas the 2200Ohm/100Ohm simulation showed a clear sine wave.

Also you seem to use only a single supply voltage (20V). That does not work with the circuit you have If you want single supply operation, you need a virtual ground somewhere at approx. 1/2 Vcc. That way the input signal can swing around that virtual ground (Google this). With a single supply, the signal can only be positive. Any negative signal will be clipped (set to 0).
For simplicity I suggest you use a dual supply (+-20V) in your simulation.

Ok, so if I am to use dual supplies, what nodes do I connect the -20V to? I guess my question is what grounds do I set to -20V instead of Ground? Is it just the ones on the filters? Or do I need to set the ones on my Op-Amps?

Thanks,
EBrown

 

[Harald Kapp]

Ground is the reference for the signal input etc.
-20V are connected to the negative suply of the opamp.s

 

[EBrown]

Ground is the reference for the signal input etc.
-20V are connected to the negative suply of the opamp.s

When I do that I still get the Op-Amp not amplifying issue. It just strengthens the amount of attenuation the op-amp has on my signal.

I've got to be doing something wrong here.

In the picture V(n005) is the line from the point of U2 to the + of U16, and V(n006) is from the point of U16. Although that dark blue is hard to see.

Thanks,
EBrown

 

[duke37]

If you are using LTspice you can fatten the lines so they can be seen.

 

[Harald Kapp]

Get rid of the whole filter part and get the opamp up and running first. You may also post the ASC file as an attachment. If I find the time I can give it a try.

 

[EBrown]

Get rid of the whole filter part and get the opamp up and running first. You may also post the ASC file as an attachment. If I find the time I can give it a try.

So here is a simulation I setup, to expand upon what the Op-Amps are doing.

The .asc on the left is just the Op-Amp and the voltage sources. The .asc on the right is the filter and Op-Amp. They are both identical at the Op-Amp section, so why is the second Op-Amp on the filter not working correctly?

Thanks,
EBrown