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# bandgap voltage reference design

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#### zhanghe

##### Newbie level 5
bandgap reference design

hi, in the bandgap reference design, we usually need to use a op amp, but how to connect the op amp + and - input so that the circuit can work properly? why?

thx

Re: bandgap reference design

try this....

Re: bandgap reference design

thanks , but why it is like that? i mean can the + and - terminal change place?

Re: bandgap reference design

no it cannot change..... actually feedback and proper choice of resistor is done to achieve zero thermal coefficient and hence changing the feedback would change the entire purpose of the circuit,....

Re: bandgap reference design

but how does actually the feedback work and why cannot change ? can you give some detail explaination. thx a lot

Re: bandgap reference design

current flows through R1 due to the output voltage which is then current mirrored and here R2 and R3 are selected such that the current has zero temperature coefficient and this current flowing through R3 and R2 develops a voltage which is temperature independent...

Re: bandgap reference design

yes, but why we can not change the + and - direction?

Re: bandgap reference design

if you take a good look at the diagram you can see that the +input has to be slightly greater than -input for circuit to have feedback i.e. else the output will be in negative saturation always and would turn off the transistors and so the whole circuit is off....
if you change the + and - input this is what will happen....

Re: bandgap reference design

Yes, it can not change. Srinivasan is right. For negative feedback stability considerarion.

bandgap reference design

positive feedback must be small than negative feedback

bandgap reference design

hello,A.Anand Srinivasan,I have a question when the circuit works stablely,its + and - input will have the same voltage ,however,why can they not be changed ,in a opamp design circuit,do we need to care the positive and negative input

Re: bandgap reference design

kobeghost said:
hello,A.Anand Srinivasan,I have a question when the circuit works stablely,its + and - input will have the same voltage ,however,why can they not be changed ,in a opamp design circuit,do we need to care the positive and negative input

It's definitely yes, we need to care about the positive and negative input of OPAMP.
As A.Anand Srinivasan said, the circuit is a negative feedback, and if we change the positive and negative input, the total circuit becomes to a positive feedback, and then the output of opamp will go to VDD, and the circuit does not work correctly.

Ryan

Re: bandgap reference design

in an opamp both inputs are effectively same and their signs are determined based on which of the output terminal of the differential output is grounded.... so when we interchange the entire feedback changes....

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