Confusion with Filters and Oscillators

[jp1390]

Hello,

It appears that I am having some trouble with some of the concepts with these two topics, specifically higher order filters and oscillators.

Say for instance that you had a Sallen-Key Low-pass Filter that had a relatively high Q. This means that there will be a large peak formed around the relaxation frequency w0. This frequency w0 is where the maximum peaking occurs.

My question is, if you pass in an input frequency equal to w0, what will happen? Does the signal just get a large amplification or does the system oscillate? By oscillate, does this mean that the nature of that frequency causes the RC networks to oscillate in phase with the input signal, thus making the gain massive? Will the system oscillate if the input is at w0 regardless of the value of Q (greater than 1/2)?

Though this is a S-K LPF and experiences negative feedback, isn't it still exposed to positive feedback with the RC networks connected from the output of the op-amp to the non-inverting input?

Any responses would be very helpful!

Thanks,
JP

 

[MrAl]

Hi there JP,

Most circuits like this depend on some gains somewhere. Those gains are usually the controlling factor behind whether or not the circuit oscillates. If you post a circuit we can take a closer look at this.

The theory behind it works most simply by finding the forward gain G and the feedback gain H, then computing the output:
G/(1-G*H)

From the denominator 1-G*H you can deduce if it is going to oscillate or not.

 

[audioguru]

A filter with a high Q (a narrow bandwidth) rings and slowly decays like a bell. The ringing is oscillation at the peaked frequency but it decays to zero when the input signal is stopped.