This question follows a previous one I asked, where I accidentally achieved undesirable results from an opamp. Since then I have read up on active opamp filters (here and here) and refreshed my knowledge of LC filters.
My goal is to add some low-pass filtering to the circuit below (features in this question). The input is from a loop of 24AWG wire approx. 1ft x 4 ft, through a balun then coax. As well as the 5Mhz signal from my transmitter this understandably picks up a lot of other RF (like the local radio ham on the 2 metre band).
I simulated four different circuits, bode plots below:
- The circuit with no filtering (all parts taken out)
- Just the 62p/30p capacitors in the feedback loops
- Just the LC filter between amplifiers
- Both the opamp filtering and LC filter
My question is: Is one of these low pass filtering methods more suitable than the other for real world circuits?
Edit: I opened a specific question for the application here. However I would still be interested in generic answers to when active opamp filtering is better/worse than an LC filter, if this is a sensible question.
After some experimentation I found the 62p/30p capacitors allow roll-off around 5Mhz. However these seem like very small capacitors, and I worry that 5% parts leave a lot of room for the -3dB point to shift.
The LC filter was designed with ELSIE, simulation results look good and it rolls off fairly sharply. However I don't know if it is unwise to use it in this way.
1. Bode plot - no filtering
2. Bode plot - active opamp filter only
3. Bode plot - LC filter only
4. Bode plot - active opamp filtering and LC