Okay, now, here is a Op Amp version of the Wah Pedal.

That, that, you, there's a, a few ideas for how to do this on, on the internet.

But, we, we, put this, this, schematic together, and prototyped it, and, and it

works. now, this is kind of a culmination of

everything you've learned about in this course.

It has a lot of A/C circuit, design and A/C circuit analysis, plus we're using Op

Amps. Now, we're introducing this one

interesting idea of feedback to the capacitor.

Which is, a new idea here and really the essence of what makes this thing work.

So I've redrawn the schematic here is the, the two resistors in series,

followed by the RLC. And the RNL are grounded.

The C is not grounded. It's attached to the output of an

amplifier. Now what you do is you take the output,

it essentially comes from this point here.

And you can put that into a simple inverting Op Amp, and you can set the

gain wherever you like. the prototypes that we've built, the

gains may be 50 to 100, so that's the ratio of this resistor to that resistor.

If you wanted a gain of 50, this resistor is 50 times that resistor.

And then, there's the output, and so then what we do is take the output and we go

through a potentiometer. And the wiper on the potentiometer then

goes to a simple buffer stage, and so this a unity game buffer built with an

ambient. And the output of that is connected to

the end of the capacitor. So, the capacitor is not grounded, it's

attached to this output. Now, what happens when you do that is,

you're, instead of grounding the capacitor.

You're using feedback on the far side of the capacitor in a way that makes the

capacitor look larger or smaller than it really is.

Now, this is a little hard to understand but give this a, a little bit of thought.

Here's a capacitor, it has a capacitance C and there's a voltage V across that

capacitor. And, here's the relationship between the

current and the voltage. So, the current is just j omega C times

the voltage, or, if you like the voltage. Is I times the impedance and the

impedance is 1 over j omega C. Now, to make this thing look like, so if

I apply a certain voltage to it, and then I ask, how much current is going to flow

through it. That's a reflection of what the

capacitance is. If I apply, say 1 volt at, say 1 kHz.

I'm going to have a current flowing through this in magnitude, that is

proportional to the size of the capacitor.

If I could somehow increase that current. Then that's like making the capacitor

look like it's bigger, for that given voltage, okay.

So that's the key thing, if I have a fixed voltage, but I can find some way to

make the current bigger than it would be. With just a grounded capacitor then I can

make that capacitor look bigger. And so what you do Is you apply feedback

to the other terminal of the capacitor. And then, what happens is, when the

voltage on this end is pulled up a little bit, the voltage on the other end of the

capacitor is pulled down. And so, notice that this is inverting and

this is non-inverting. So if this voltage here goes up a little

bit, then this voltage here goes down a little bit.

And so I'm pulling the capacitor in two different direction.

So, I'm, when I, every time I add a little voltage to it, the current should

go up a little bit. but what happens is I'm taking, and I'm

taking that voltage, amplifying it, applying it to the other terminal,

pulling the other terminal even farther away.

So I'm increasing the voltage across this capacitor.