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Opamp Based Bandgap Stability issue (Read 1208 times)
surreyian
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Re: Opamp Based Bandgap Stability issue
Reply #15 - Jul 04th, 2012, 9:26am
 
raja.cedt wrote on Nov 27th, 2009, 11:12pm:
hi Yawei,
            ya it's correct, but you cann't say that n1 is sinal ground because i feel we have to say that it should be represented by it's small signal equivalent (dynamic resistance)..but you are correct compared to n1 n2 should be high impedance node.

@loose-electron: i did't understand why you are saying that it's single loop..because wht i feel you are feedback to both terminals of the opamp.

Thanks,
Rajasekhar.




hello
i dont quite understand why n2 is high impedance node.
Can someone help me understand how we decide which input of the op-amp to connect to the bandgap circuit.

is it because the impedance seen is (R1+R2)/(R1+R2+R3), compared to the impedance seen at n1 R1/R2. Assuming all have the same impedance.
So high impedance = high gain, is that why it is tied to the inverting input.
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Horror Vacui
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Re: Opamp Based Bandgap Stability issue
Reply #16 - Jul 9th, 2012, 3:26am
 
Think at the gains from the amps output!
G1=1/(1+gmd*R1)
G2=(R3+1/[m*gmd])/(R1/m+R3+1/[m*gmd])=(1+m*gmd*R3)/(1+gmd*R1+m*gmd*R3)

where gmd: gm of the diode; m: the ratio of the currents in the two legs.

G1<G2 always, since 1/n=1/n*(n+x)/(n+x)=1/(n+x)+(x/n)/(n+x)<(1+x)/(n+x)

Therefore G2 has to be connected in a way to form the negative gain loop: inverting terminal of the amp.

Simple, right?
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Kevin Aylward
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Re: Opamp Based Bandgap Stability issue
Reply #17 - Jul 13th, 2013, 9:23am
 
An AC analyses of a band gap is misleading and does not actually explain what is happening. There is a DC voltage/current that is technically always a “negative” loop in the sense that if too high it makes an output less, and too low it makes an output higher. However, it is in a notional positive feedback loop. To correctly understand this, the delta Vbe circuit must be correctly understood, as shown here. http://www.kevinaylward.co.uk/ee/ptat/PTAT.xht . It’s absolutely fundamental to understanding how a really bandgap works.
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Shajo
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Re: Opamp Based Bandgap Stability issue
Reply #18 - Feb 16th, 2016, 4:25am
 
Hi all,

I'm having a tough time deriving equation 3.17 from the circuit in the attachment. Please do point me in the right direction.


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1_001.PNG
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blue111
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Re: Opamp Based Bandgap Stability issue
Reply #19 - Dec 12th, 2016, 12:51am
 
hi, Shajo,

may I know from which book the screenshot is taken from ?

Thanks!
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