Homework Help: Help needed Analyzing a real op amp with both positive and negative feedback

1. Apr 30, 2012

Rio20

Hi
this is my first subject in this forum and i hope you can help
1.we have been given an assignment to analyze this non-ideal op amp with both positive and negative feedback and to be honest i don't even know where to start

i know i should post an attempt of trying to solve but honestly i don't know how to ,, so please give me hints or ideas and i will go through them

thanks forum

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2. Apr 30, 2012

Staff: Mentor

Welcome to the PF.

Is there more to the question? What are the initial conditions of the circuit? Is there some Vi(t) somewhere, or just the circuit?

And what is the 1uF cap doing on the output of the "equivalent circuit" on the right?

3. May 1, 2012

Antiphon

It's going to oscillate with a frequency determined by the pot.

Ignore the positive feedback and consider the negative. Draw the circuit as an ideal OpAmp with a capacitor on the output. You'll see that the OpAmp charges the cap to a point where the output flips and then reverses so that the cap is discharging.

The positive feedback determines the trip points and therefore the frequency.

4. May 1, 2012

Rio20

thnx for welcoming me berkema :D .

there is no Vi(t) and the equivalent circuit is drawn next to it ,, we asked asked to analyze the following op amp in a non ideal way and get the voltage that will be on the capacitor and draw it ,,, it's more like an really very very complex RC circuit

5. May 1, 2012

Rio20

thnx antiphon and i think i got ur point ,, but can u show me some equations so i can fully understand ur point and thnx

6. May 1, 2012

Antiphon

Sorry I can't take the time to work it out.

Assume that the output is clamped to the power supply levels when the inputs are not balanced. This simplifies your analysis to an RC circuit and the voltage dividers on the inputs.

The OpAmp will act like a comparator.