Now that you have some sense of how to explore and characterize a filter we can see if we can stretch this experience to active filters based on op amps. Passive filters, those we've been looking at, have only simple impedances as circuit elements. When we start including op amps there is now the possibility of more complex behavior. Hopefully this will provide some conceptual preparation for our unit on filter analysis.

We'll see how far we can go with low pass, high pass, and band pass filters in this lab. What you will need to notice is the differences between these filters and your passive ones (in terms of the data you gather). The circuits you will be exploring are documented in these two pdf files (hi pass and low pass). We will start with the filters in the top left corner that are labeled 12 dB/octave, 1 kHz, gain = 1 Butterworth filters.

Part of the reason we're exploring active filters is to give you some additional experience using op amps. The data sheets for the op amp packages (LM358, LM324, LM339, LM386, TS912) will give you the pin pattern for this device. Take your time and verify that you have the circuit wired up correctly before diving in. You will need a DC power supply from last term as well as your usual AC equipment for this lab.

Procedure:

At this point in the term I hope that you have a fairly clear concept of how to independently explore this first circuit (whether you choose to do the high pass or low pass filter). For a variety of reasons I would use a slightly lower voltage AC input (say 1V peak to peak) until you're sure you understand how the filter is behaving. This should mean that you can use +/- 5V to power the op amp.

There are a number of ways you can approach this learning opportunity. I encourage you to start with building just one filter and testing it (generate a plot of v_out vrs f) first. Go back and look at the similar plot for your passive filter of the same type. What differences do you notice?Why is it labeled as a 1 kHz filter? What do you change to move the filter "edge"? Then you can use the other op amp in the package to build and test the the 18 dB/octave version of the same filter. Then you can build the "other" filter -- hi or low depending on which you built first. Finally, if you have time, you can chain the two filters together to get a band pass filter.

A new concept I'd like you to explore is this definition of a feature called rolloff. The rolloff is calculated by comparing the output voltage a two frequecies that are an octave apart where an octave is a doubling of the frequency. Rolloff = 20*log(V2/V1)/octave. Consider several points during the transition from low to high or high to low output.

Lab Notebook:

The following information should be clearly presented in your lab "notebook" along with explanatory and insightful commentary.

I) A clear drawing of the circuit constructed including the connections to any equipment

II) Present any measurements that you made and plots of said data.

III) Clearly describe the frequency dependent behavior of this circuit based on the data you have collected. How would you describe the transition behaviors that you observe and what happens to the amplitude of the signals?