Waveform at the negative terminal of an OPAMP

Thread Starter


Joined Oct 26, 2017
I am trying to sketch the negative waveform of an OPAMP over here.

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I am not sure how to proceed with this as I have usually seen OPAMP with dual power supply. But this one have +9V and ground.
My thinking to go about this problem :
  1. The voltage level at the positive terminal of OPAMP is +4.5V
  2. So, the voltage at Vout would also be +4.5V due to the 100k between between +ve and Vout
  3. As +4.5V is present at Vout, the 10nF capacitor which is connected to ground gets charged.
  4. As it gets charged to 4.5V, the output will will become 0V?
  5. So, the output will go to 0V?
Please tell me how to proceed from this?


Joined Mar 14, 2008
The hint here is that the op amp is not operating as a linear amp, it's operating as a comparator, and thus generates a square-wave oscillation output.
The 100k resistor between the output and plus input provides positive feedback not negative, so it does not work as you stated in paragraph 2, and instead causes the oscillation.
I leave you to figure out how it works.

Bonus points if you calculate the frequency of oscillation.

In the future please label all the parts on your schematics so they can be easily referenced. ;-)
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