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Twin T drum oscillator simulation
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Greg Lorincz 4 years ago
I have built this circuit successfully on breadboard and simulated it in circuitlab (second and third image). Somehow the same circuit doesn't work. The output is supposed to be a sinusoidal signal with decreasing amplitude (third image). The output of the circuit below is the same pulse that triggers it. What am I missing? ``` ``` ![Schematic_twin-t-w-pulse](//image.easyeda.com/pullimage/oCjmqXagvhQM1BzcBAxIbQT7vmz9k30NdyFUuTzI.png) ![twin_t.png](//image.easyeda.com/pullimage/P2C7DTQXKLG51VnniLstvU6OdUVCEL7HXwqCfpic.png) ![twin_t_plot.png](//image.easyeda.com/pullimage/6DsSAlPElU5GQWMzDR6s5VI5cCZ3cK8XmzoemH7l.png)
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andyfierman 4 years ago
Having come using CircuitLab you have to understand that a spice-based simulator is case insensitive so m=M=1e-3. For 1e6 use Meg. So, 3.9M=3.9 milliOhms whereas 3.9Meg=3.9 MegOhms. You have used these two symbols: ![image.png](//image.easyeda.com/pullimage/00kPjxfLnBYSEgOZe4CiHFhD0D6t6N8FmnxCzM0O.png) both of which represent GND or 0V so you have shorted out your negative supply to the opamp to ground. If you delete the ![image.png](//image.easyeda.com/pullimage/nVb5C3Q7rD1Klo5rCTtqgACXvMNbKsCF389abvFf.png) symbol or replace it with a VEE netlabel then your simulation should work. Note that the 100R resistor in series with the non\-inverting in the CircuitLab sim actually needs to have a value of \{\( 1/\(2\*3\.9Meg\)\+1/10Meg \)\*\*\-1\} in order for it to properly correct the voltage drop due to bias current through the very high resistances in the DC paths to the inverting input\. Also, in the CircuitLab simulation, the initial output swing of the 741 is unrealistically high. A real 741 cannot swing that close to the supply rails. To get the best out of the simulation tool in EasyEDA, please see (3) in (2) in: [https://easyeda.com/forum/topic/How-to-ask-for-help-and-get-an-answer-71b17a40d15442349eaecbfae083e46a](https://easyeda.com/forum/topic/How-to-ask-for-help-and-get-an-answer-71b17a40d15442349eaecbfae083e46a)
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Greg Lorincz 4 years ago
Great reply, so much useful info, thanks!!!
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Greg Lorincz 4 years ago
@andyfierman It's working!!! I wanted to upload a pic of the output but I can't insert .svg. Thanks again!
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