Op-Amp Colpitt Oscillator Design & Simulation help Needed

Thread Starter

mishra87

Joined Jan 17, 2016
801
Hi Guys,

I have done some theoritical design for opamp based colpit oscillator.
Could anybody me know the reason why output is strange ?

As per my calculation Av > 10

Thanks in Advance !
 

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OBW0549

Joined Mar 2, 2015
3,454
I have done some theoritical design for opamp based colpit oscillator.
Could anybody me know the reason why output is strange ?
You need to be aware that not everyone uses LTSpice; many people use other simulation packages, and posting only an LTSpice .asc file doesn't allow them to see your circuit.

You'll likely get a lot more help if you also post a screen capture image of your schematic and waveforms, preferably in .png format.
 

Thread Starter

mishra87

Joined Jan 17, 2016
801
You need to be aware that not everyone uses LTSpice; many people use other simulation packages, and posting only an LTSpice .asc file doesn't allow them to see your circuit.

You'll likely get a lot more help if you also post a screen capture image of your schematic and waveforms, preferably in .png format.
Thanks,

Below circuit i used for simulation !
1592067205076.png
1592067243777.png
 

Thread Starter

mishra87

Joined Jan 17, 2016
801
The op-amp output is very low impedance and the inverting input is a virtual earth, so both of these load down the tuned circuit.
Try the attached.
Thanks for your reply !

May i know answer of below queery.

1. Why your simulation is not showing startup condition of oscillator ?
2. Why did you use R1 = 47K ?
3. Why the gain of amplifier is 42 ? Theoritical value is >10 so it could be 11 .
 
Last edited:

LvW

Joined Jun 13, 2013
1,056
Thanks,

Below circuit i used for simulation !
Your principle error is that you have transferred the feedback circuit from a transistor (acting as a current source) to an opamp (which is a voltage source).
As a consequence, the capacitor C2 acts as a load to the active device - thats all, but it is not part of the feedback circuit anymore.
Hence, we need a resistor between the opamps output and the feedback path.
 

Thread Starter

mishra87

Joined Jan 17, 2016
801
Your principle error is that you have transferred the feedback circuit from a transistor (acting as a current source) to an opamp (which is a voltage source).
As a consequence, the capacitor C2 acts as a load to the active device - thats all, but it is not part of the feedback circuit anymore.
Hence, we need a resistor between the opamps output and the feedback path.
Many Thanks,

Regards,
 

LvW

Joined Jun 13, 2013
1,056
@mishra87, you can mesaure/simulate nearly everything.
However, at first we need the definition of the parameter you want to measure.
What is the "bandwidth of an oscillator"?
Perhaps you mean the bandwidth of the lowpass in the feedback path?
In this case - display magnitude and phase of the loop gain function versus frequency.
 

Thread Starter

mishra87

Joined Jan 17, 2016
801
@mishra87, you can mesaure/simulate nearly everything.
However, at first we need the definition of the parameter you want to measure.
What is the "bandwidth of an oscillator"?
Perhaps you mean the bandwidth of the lowpass in the feedback path?
In this case - display magnitude and phase of the loop gain function versus frequency.
Hi ,

Thanks for reply !
I am not much aware of it.
Could you please share image file to understand it better.

People say bandwidth of oscillator but i am not much aware of this term.

Regards,
 

Alec_t

Joined Sep 17, 2013
11,414
If you mean what I think you do then run a Trans sim, plot the waveform at some desired point in the circuit, click in the plot pane, then go to View/FFT.
 

LvW

Joined Jun 13, 2013
1,056
If you mean what I think you do then run a Trans sim, plot the waveform at some desired point in the circuit, click in the plot pane, then go to View/FFT.
So - you think, that he wants to see the result of phase noise? I am not sure....
 

Thread Starter

mishra87

Joined Jan 17, 2016
801
So - you think, that he wants to see the result of phase noise? I am not sure....
The question i had was -
"how can you find the Bandwidth of this circuit using the LTSpice? What is the formula of Bandwidth for this oscillator?

I am trying to find the answer of these questions what all i have is LT Spice design file attached post#12.
 

LvW

Joined Jun 13, 2013
1,056
The question i had was -
"how can you find the Bandwidth of this circuit using the LTSpice? What is the formula of Bandwidth for this oscillator?

I am trying to find the answer of these questions what all i have is LT Spice design file attached post#12.
OK - I think, the question concernes the bandwidth of the open-loop (loop gain). This is a 3rd-order lowpass - and all you have to do is to open the circuit at the node Vout and perform an AC analysis. The magnitude of the lowpass function gives you the bandwidth.
 
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