Active low-pass filter with gain

It is difficult to fully understand your circuit by your description, especially the part about "output capacitor feedback between two series resistors".

I would first suggest using a a standard filter topology, suck as a Sallen Key, which will reduce the filter design to matching against a set of normalized charts and then scaling for frequency and amplitude.

Another way to look at this is to remove the frequency selective components (ie the cap), and concentrate on the amplifier portion itself. A non-inverting amplifier configuration has a gain of 1 + R_feedback / R_input. Analyze the circuit until you understand why. The opamp will TRY to drive the output such that the voltage between its input terminals is zero. This creates a 'virtual short' between the inputs which will simplify your analysis. Once you have the equation (transfer function) describing the amplifier, you can put the capacitor back in the circuit and re-analyze. You will want to use the LaPlace transform when performing your analysis to convert the differential equations caused by the inclusion of the cap to a frequency domain for use in filtering.

I think he is describing a Sallen-Key. The input signal goes to the non-inverting input of the op-amp, with the input resistor broken down into two. So you have to stages of an RC low pass filter, with the capacitor of the first stage connected to the output of the op-amp rather than ground.

He's trying to get gain from it by adjusting the gain of the op-amp, which isn't going to happen. Likely his "no output" is because the signal is swamped by oscillation, though that doesn't fully explain why he sees output when he has no negative feedback (which of course would give "near infinite" gain.

Michael

You're correct, it looks like a Sallen-Key filter (the R and C values came from the output of Microchip's FilterLab). And the output with no feedback sounds (and looks) like "near infinite" gain (severe clipping). Is there a way to prevent the oscillation problem and get some gain from this stage? I have been able to get some gain by taking the output to another LM741 stage, but I assume I need to clean things up - especially since it's on a breadboard (and not very neat either).

I'm surprised Microchip's FilterLab doesn't include the ability to set the gain as you wish. Texas Instruments FilterPro does.

--
Regards,

John Popelish

Without looking at the circuit I can't be sure, but note that the gain of the filter is often used to determine Q. Your circuit description sounds like a 2nd order low-pass, except you haven't mentioned a second capacitor to ground. Usually, if you increase gain you increase Q, and you will soon get into oscillation... not to mention putting a huge peak at the corner frequency.

Check out Don Lancaster's "Active Filter Cookbook" for some reliable circuits. (And note that the 741 is probably not a good choice any more, though it should be fine at low frequencies if low noise is not important. Consider the LF351 or TL071 series, for example, if you want drop-in parts with much better performance.)

Best regards,

Bob Masta DAQARTA v3.50 Data AcQuisition And Real-Time Analysis

Scope, Spectrum, Spectrogram, FREE Signal Generator Science with your sound card!

Thanks for the link. The program with Sallen-Key selected shows exactly what I'm trying to do, so I'm assuming oscillation is preventing the gain.

Thanks for the book reference. The library has a copy I can check out.

You have mentioned only one capacitor in your filter. The minimum filter this program generates uses two. and you should also have a small capacitor between each of the supply pins and ground, across your opamp, to stabilize it.

--
Regards,

John Popelish

I have the cap on the non-inverting input, but not those on the supply. Recently encountered this article which mentioned them:

Will apply caps and see what results.

Thanks again.