I build my first couple of oscillators using inverters and the super simple oscillator layout. Next I wanted to try a filter and built a very simple passive RC low pass. I noticed that it worked well with the inverter-based oscillators but not the super simple oscillator. I guess this is because the latter has a 100k ohms resistor on the output. So when I plug in my filter, the 100k add to the R of the filter. Since mine uses a 10k pot this makes the filter pretty useless, right?
So I need a way to decouple the filter’s input from any resistance (or capacitor) I guess that might be in series of the circuit I want to put into the filter. I guess this cannot be done with passive filters.
I found a design for a simple active inverting filter, where the capacitor is not in series with the input but on the feed-back loop of the op-amp. This would solve my problem, but now I do not know where to put the potentionmeter in a way that I can adjust the cutoff frequency without altering the gain of the opamp at the same time.
Does any of this make sense and are there any simple (one op-amp) filter layouts that can do what I need?
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It is hard to visualize your circuit but your problem is not unique and has a few different solutions.
- Can you add a simple transistor or OP-AMP buffer between the oscillator and the passive filter? The op-amp buffer is really easy and will provide a very very high input impedance and a low output impedance. The op-amp filter implementation is good but it may have a low input impedance depending on the design and may not fix your problem.
- Can you scale the values up in the filter so that filter input impedance is 1Meg instead of 100k? That may work well enough.
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Thanks, using the op-amp as buffer really solves the problem. I couldn’t try the second suggestion because I do not have the right parts.
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