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Two poles in oscillator circuit

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deepsetan

deepsetan

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Moved]: Frequency Response using LTSPICE

Hi,

I'm trying to plot gain and phase for this circuit using LTSPICE but I don't think my response is correct. I think maybe I am missing something during use this software. I already insert AC amplitude but it still gives me the same response. Can someone help me to figure out it? :lol:

- - - Updated - - -

Hi,

Sorry for some mistake. I re edit the circuit. Diagram3.PNG
 

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Re: Frequency Response using LTSPICE

deepsetan said:
I don't think my response is correct.
Click to expand...

I think it is correct. How can you expect an oscillation from cross-coupled (double feedback) source followers (gain<1 for both)?

Is this a fun example, from 1st April, perhaps?
 

Sorry,I forgot to update the current diagram and thanks for your "wish" erikl :popcorn:. After done some reading, I try to make some changes to the circuit. As I know, oscillator circuit need 180 phase shift (at least two poles). What I did actually is I'm connecting the first block of the circuit to another circuit block which is a common source amplifier. From my understanding, this will produce two poles (180). After done some simulation, this is what I obtained. My question is why I still get 90? I already remove the compensation capacitor. Do I need to add another RC circuit to get 180 phase shift? Please correct me if my understanding is wrong.

Diagram5.png
 

is the gate and source of pmos m5 connected to vdd?
 

Yes,I want M5 to behave like a large resistor.
 

There's no Vin, gain (transimpedance in this case) is measured with current source I1.

Seriously I have no idea how the circuit is intended to work as oscillator, respectively don't know which transimpedance value you want to achieve.
 

Actually i'm quite confused what is the difference between input voltage,Vin and also the bias voltage. Why we still need Vin if we already have the bias voltage? What is the real function of Vin in the circuit?
 

To measure frequency response in AC analysis, there must one (and only one) independent AC source, either voltage or current. You have chosen a current source, your curves respectively show transimpedance rather than voltage gain.

I thought up to now it's well considered, just wondered how's the resonator connected to form an oscillator?
 

Thanks FvM for your reply! I really appreciate your explaination but I don't quite understand your second question. If i remove the current source and replace it with an input voltage and connect the resonator between the output and this input,for sure it will produce oscillation right? Please correct me if my understanding is wrong.
 

The circuit does not necessarily oscillate, the oscillation condition must be fulfilled with the specific resonator.
 

Hi FvM

Thanks for your answer! It really helps me a lot. :)
 

The 180º phase shift must occur at the same frequency the gain crosses the 0 dB in order to oscillate.
 

CataM said:
The 180º phase shift must occur at the same frequency the gain crosses the 0 dB in order to oscillate.
Click to expand...

... or already before the gain gets ≦ 1. But this is a necessary, not always a sufficient condition, however.
 
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