When I put the following narrowband active bandpass circuit on a breadboard, using a MCP601 single-supply CMOS op-amp, it just oscillates at about 339 kHz with a Vpp of about 1.36 V at a supply of 5 V. When I apply a signal generator (sine wave) to the input, sweeping between 100 and 900 kHz, it doesn't seem to do much at the output other than mix with the oscillation sine wave. How do I solve the oscillation problem and get a working bandpass filter? The objective is to have an active bandpass filter with a center somewhere in the low to mid kilohertz range. Right now I don't care about precision of the center frequency nor precision of the bandwidth. Tuning that can come later once I get a better grasp on the oscillation issue.
I realize I can design a passive bandpass and amplify the output of that and call it a day, but I'd prefer first to understand what I'm doing wrong with this active design before I give up on it.
Side note: I don't have a working LTSpice MCP601 model but when I simulate with other single-supply CMOS op-amp models that come with LTSpice, I get good results (i.e, it acts as a bandpass in kHz range without oscillating).
UPDATE: I made the mistake of using R6 to model the next stage of the circuitry on my breadboard, which was in actuality a higher impedance than 5k ohms. Once I removed that circuitry and replaced it with an actual 5k resistor, I had better luck. Perhaps the lower-Z load is what solves the oscillation problem? Although I'm not entirely sure why. So if that's the case, I think the question is still valid in terms of understanding how to properly size the output load on this active filter to avoid oscillation (where next stage is high-Z)?
UPDATE #2: Just swapping various resistors out for R6, the best seemed to be R6=10k (which, notably, is the same as R3). I say best because it didn't oscillate at 10k and there was least amount of voltage attenuation.
- I have discovered this circuit is called a Multiple Feedback bandpass filter. Reference: Linear Circuit Design Handbook, 2008, Analog Devices, Ch. 8, Analog Filters, pp. 8.75-8.76 and p. 8.94. There are design equations on p. 8.94.
- There is unanimous consensus that the breadboard is one of the primary problems.
- The accepted answer mentions the TI Filter designer. I found this tool to be helpful in looking at the design of this kind of filter. It shows minimum op-amp specs needed in order to achieve results for the desired filter response.
- The TI Filter designer shows that the MCP601 op-amp has a GBW specification that is not even close to the minimum needed for the specifications I provided in the comments section. The accepted answer mentions the possibility of cascading lower Q filters to achieve results but I think another reasonable conclusion is that a multiple feedback filter is not the right approach for the Q at the center frequency that I need. The accepted answer mentions more realistic filter types, such as ceramic filters or crystal filters.
- Despite the hunt n' peck approach being not ideal, it's still notable that I was able to solve the oscillation problem with a low enough resistor-to-ground (R6) on the output. However, as stated above, even though it's not oscillating anymore, clearly I will still have problems with the filter. Rather than hunt n' peck, it's better to design with equations and for that I found equations for multiple feedback filters in the Analog Devices Linear Circuit Design Handbook, cited above.