OpAmp general rules-of-thumb (educated guess) on filter type

[adamaero]

Homework Statement

For example, what type of filter is this:

How can one tell, as a general guideline, without finding the transfer function? I think finding the transfer function is the only real way to tell for sure, but what is a quick way to make an educated guess?

The Attempt at a Solution

It is a high pass filter. (Found the transfer function.)

• Capacitors of some formation at the input?

 

[berkeman]

Homework Statement

For example, what type of filter is this:
View attachment 220752

How can one tell, as a general guideline, without finding the transfer function? I think finding the transfer function is the only real way to tell for sure, but what is a quick way to make an educated guess?

The Attempt at a Solution

It is a high pass filter. (Found the transfer function.)

• Capacitors of some formation at the input?

Do you mean what type of polynomial, or just LPF/BPF/HPF?

 

[adamaero]

Do you mean what type of polynomial, or just LPF/BPF/HPF?

Just if it is low-pass, high-pass, BP, notch. When taking an exam I want an alternative way to nearly confirm that I wrote the correct transfer function, and not made an algebraic mistake or something.

 

[berkeman]

I don't know if I'm an expert at that, but think about the rules for analyzing an ideal opamp circuit. what can you say about the input impedances of the inputs and the output impedance of the output? How would those help you to get an intuitive feel for what happens in the circuit you posted?

 

[adamaero]

I don't know if I'm an expert at that, but think about the rules for analyzing an ideal opamp circuit. what can you say about the input impedances of the inputs and the output impedance of the output? How would those help you to get an intuitive feel for what happens in the circuit you posted?

That is not what I'm asking. I'm asking about filter characterization without finding the transfer function.

 

[berkeman]

That is not what I'm asking

But I thought it was what you were asking...

Just if it is low-pass, high-pass, BP, notch.

It's going to be hard to do a quick evaluation of the circuit if you don't keep the opamp properties in mind. Take your circuit that you posted -- do you see both of the LPF stages? Why are they HPF stages, and not some other type?

 

[donpacino]

Here is a trick that was taught to me back during my undergrad days. we used to have to complete a "dasc" for everyone homework problem. it stands for dimensional analysis and sanity check.

dimensional analysis: you analyze the units of your answer, along with the units of your work, and make sure they make sense. ex. if you're solving for a voltage and you get 17A, you did something wrong. This isn't relevant to this problem in particular, but it can be helpful in general.

Sanity check: find a way to do 'back of the envelope' calculations to check your answer. is it the right order of magnitude, etc.
To sanity check these filter problems, evaluate what happens at dc and high frequencies.

at dc current, a capacitor is an open circuit and an inductor is a short circuit.
at high frequencies, a capacitor is a short circuit and an inductor is an open circuit.

So evaluate your circuit at dc, what happens.
then evaluate at high frequencies, what happens.
hint: first open the capacitors, no current goes through them, so at dc the gain is zero. then short them, what happens.

You can also use this to check transfer functions by plugin in zero and inf for frequency values and seeing if your transfer function matches the simplified circuits.

 

[LvW]

Topology: The filter contains an active part (fixed-gain amplifier) equipped with an R-C feedback path.
There is only one single feedback element and, therefore, it is a second-oder filter stage which is called "Sallen-Key"-Filter - named after its inventors R.P. Sallen and E.R. Key.

Filter-Type: For very low frequencies, the input signal cannot pass through the capacitors - hence, low frequencies are attenuated.
In contrary, very large frequencies can pass both capacitors with only a small attenuation and reach the amplifier input node.
Hence, it is a second-oder highpass filter.

Filter characteristics: It is the task of the feedback resistor to enhance the magnitude of the filter in the cut-off region to allow for different transfer characteristics (Bessel, Butterworth, Chebyshev).