Bode plot reveals instability - solution advice sought

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All my bad! Using QSPICE .plot mag(V(2)) ph(V(2)*180/3.1459 must be added to V input as an attribute not general directive. Many thanks to everyone - it helped me problem solve an eliminate my mistake.

Cheers
 

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Thanks to the help, I've no problem with Bode plots when the VAS is a simple Diode resistor affair. However, my attempt at VBE bias, as attached, produces a flat line. 0 degrees . It's great with a Diode / Res! Tips on what to try appreciated.
 

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For now I'm still using .ac dec 10 10 100Meg for QSPICE and probing nodes OUT and NFB return to the LPT.

Along 0 degrees reads 70; middle plot starts to drop from 15 kHz and at 50kHz is down by -3.5dB
In order to keep R18 uA lower at than at R19, the res value of 604 ohms at foot of VAS remains lowish as a more typical 1K dropped R18 too much.

Can I ask is this closer to a reasonably good point for further development?

Cheers
 

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For now I'm still using .ac dec 10 10 100Meg for QSPICE and probing nodes OUT and NFB return to the LPT.
That's not the correct way of doing it. You need to insert an AC source right before the negative input of of the LTP. If we call the feedback line FB and the negative input NEG_IN, the quantity you want to plot is -V(FB)/V(NEG_IN). Let's call that magnitude A and the phase P. Look at for the point where A=0dB. Read the phase (it should be a negative number). Your phase margin is 180+P.
Here's an example from a project I'm working on. Notice V4 in the middle? I'm also attaching the ASC file.
1738072677094.png
 

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That's not the correct way of doing it. You need to insert an AC source right before the negative input of of the LTP. If we call the feedback line FB and the negative input NEG_IN, the quantity you want to plot is -V(FB)/V(NEG_IN). Let's call that magnitude A and the phase P. Look at for the point where A=0dB. Read the phase (it should be a negative number). Your phase margin is 180+P.
Here's an example from a project I'm working on. Notice V4 in the middle? I'm also attaching the ASC file.
Thanks for reading and replying. If I've measured it correctly my margin is 30 degrees so I at least know this isn't worth building.
Cheers
 
With loop broken (file attached) I can't see where A can be plotted at 0 dB
Did you insert a voltage source like I did? You can't just break the loop.

I read up a bit on the Tian method that @cabirio suggested and it's probably superior to what I'm using (which is a bit dated). Did you ever try that? The measurements I've seen from you so far seems to have been just comparing output to negative feedback, which I don't think will work. But I may have missed something.
 
Did you insert a voltage source like I did?
Yep, inserted.
It is perfectly possible that I don't get the jargon used. Open loop and closed loop, for example, and I'm doing the wrong thing. For closed loop I leave the NFB attached to OUT / open loop - I detach it. This may be wrong - I don't see how the V source is going to be of any use with NFB detached! Likely my misunderstanding.