# Opamp filter - oscillator

#### xxxyyyba

Joined Aug 7, 2012
289

How to solve this? I'm confused because of only one input in opamp.

#### Brownout

Joined Jan 10, 2012
2,390
Not necessarily an opamp; it's just an amplifier with a gain of "A."

#### xxxyyyba

Joined Aug 7, 2012
289
We are studying opamps right now so it's probably opamp
I started thread by mistake here, it should be in homework category...

#### Brownout

Joined Jan 10, 2012
2,390
We are studying opamps right now so it's probably opamp
It really doesn't matter what you think it is. It's a generic voltage gain block.

#### AnalogKid

Joined Aug 1, 2013
8,832
If it helps you visualize the circuit, think of it as a non-inverting opamp stage, with the R2-C2 node connected to the + input.

ak

#### MrAl

Joined Jun 17, 2014
7,895

View attachment 94943

How to solve this? I'm confused because of only one input in opamp.
Hi,

What you would do is first write out the transfer equation which of course includes the gain A, then solve for the A that makes one or more of the poles cross the jw axis into the right hand plane of the complex frequency plane. The input doesnt even matter that much but you probably assume that is a stable input, like a unit step.

I have to assume that you know how to write the transfer equation or i could not see how this question could have been asked of you, unless you missed a lecture or something

#### xxxyyyba

Joined Aug 7, 2012
289
To find loop gain, we cut feedback and apply test generator like this:

Then we use conditions Re{Abeta} >=1, Im{Abeta}=0, to find A, right?

#### xxxyyyba

Joined Aug 7, 2012
289
I got A=6.7 ...

#### Jony130

Joined Feb 17, 2009
5,212

#### xxxyyyba

Joined Aug 7, 2012
289
Of course.