# Opamp filter - oscillator

Discussion in 'Homework Help' started by xxxyyyba, Nov 19, 2015.

1. ### xxxyyyba Thread Starter Member

Aug 7, 2012
249
2

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

2. ### Brownout Well-Known Member

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

3. ### xxxyyyba Thread Starter Member

Aug 7, 2012
249
2
We are studying opamps right now so it's probably opamp
I started thread by mistake here, it should be in homework category...

4. ### Brownout Well-Known Member

Jan 10, 2012
2,375
998
It really doesn't matter what you think it is. It's a generic voltage gain block.

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5. ### AnalogKid Distinguished Member

Aug 1, 2013
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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

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6. ### MrAl Distinguished Member

Jun 17, 2014
2,554
515
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

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7. ### xxxyyyba Thread Starter Member

Aug 7, 2012
249
2
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?

8. ### xxxyyyba Thread Starter Member

Aug 7, 2012
249
2
I got A=6.7 ...

Feb 17, 2009
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