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Stability widlar bandgap reference

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jc2

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I designed a bandgap reference source. I will put an output capacitor for improving the noise specification, but the bandgap reference output presents oscilations in the transient simulations. It is the most simple bandgap reference source, this does not use an OPAMP. How could I improve the stability of the circuit?? could I measure the margin phase?? where should I cut the loop??

Thank you
 

Please show us your schematic? You can break the loop at the output gain stage.
 
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widlar1.jpg

This is the circuit.
 

It is an output capacitor of 12uF.
 

It is expected that it will oscillate, as the cap will introduce a negative phase shift to the loop response.
Besides, that is not the classical widlar bandgap.
You need the circuit, just google for it.
 

You need to apply ac signal to Gate of Q2 by braking gate bias. You must be knowing that there are two poles existing the the closed loop, if they are close to each other, And again there are existing two feedback.One Negative feedback (combination of Vbe2 and R1 and positive feedback is upper loop .
If your positive feedback is dominating to negative you circuit will oscillate. You have to either increase the effect of R1 or decrease the close loop positive feedback gain. Hope this will help.
 
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Hi,
I agree. But isnt it, due to the cap added, the frequency dependent phase shift will occur at a lower frequency, there by inverting the characteristics of the +ve loop. So, before UGB, we can get 2 possible -ve feedback. And increasing R1 will results in increasing the negative f/b magnitude, but will it try to alter the feedback characterisitcs? Please correct me if I;m wrong?
 
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there by inverting the characteristics of the +ve loop.
Click to expand...
Why it will happen?
before UGB, we can get 2 possible -ve feedback.
Click to expand...
I see only one negative feedback.
And increasing R1 will results in increasing the negative f/b magnitude, but will it try to alter the feedback characterisitcs?
Click to expand...
Yes it affects the positive loop current gain. R1 also acts as degeneration resistance. Any variation due to positive loop gain get cancelled and output current depends on voltage across R1.
 
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Due to 12uF cap, the pole will occur very well at a lower frequency, there by giving a 90 degree phase shift to the Postive feedback, given by Q1, Q2, M1 & M2. So, this phase shifted +ve loop & the original negative feedback, given by the source degenearation resistance combinedley can have 2 -ve loop before UGB.(As the source degeneration will not be affected by the cap).
 
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