Welcome to EDAboard.com

Welcome to our site! EDAboard.com is an international Electronics Discussion Forum focused on EDA software, circuits, schematics, books, theory, papers, asic, pld, 8051, DSP, Network, RF, Analog Design, PCB, Service Manuals... and a whole lot more! To participate you need to register. Registration is free. Click here to register now.

bandgap current problem?

Status
Not open for further replies.

iamxo

Full Member level 4
Joined
Mar 22, 2006
Messages
227
Helped
16
Reputation
32
Reaction score
4
Trophy points
1,298
Location
Southeast Asia
Activity points
2,495
as in the pic, we know that when the current through R2 R3 increases, the negative terminal voltage of the opamp increase quickly than the positive terminal, that is

I*R1+Vt*ln(I/nIs)>Vt*ln(I/Is), here neglect the Is mismatch and current mismatch.

so we can get the current should satisfy the following condition:

I>Vt*ln(n)/R1,

so is there this limit of current value in real bandgap circuit?

please help, thanks.
 

winsonpku

Advanced Member level 4
Joined
Jul 7, 2005
Messages
119
Helped
6
Reputation
12
Reaction score
0
Trophy points
1,296
Activity points
2,351
In fact, the current flows through R2 and R3 is almost same
and the value is I=Vt*Ln(n)/R1 ----(1)
the pic is a closed loop, and the loop is stable if those values are proper

So the current is only the same as equation (1), no limit as you said.

iamxo said:
as in the pic, we know that when the current through R2 R3 increases, the negative terminal voltage of the opamp increase quickly than the positive terminal, that is

I*R1+Vt*ln(I/nIs)>Vt*ln(I/Is), here neglect the Is mismatch and current mismatch.

so we can get the current should satisfy the following condition:

I>Vt*ln(n)/R1,

so is there this limit of current value in real bandgap circuit?

please help, thanks.
 

pfd001

Advanced Member level 1
Joined
Jul 23, 2005
Messages
410
Helped
45
Reputation
90
Reaction score
10
Trophy points
1,298
Activity points
3,214
what kind of current limet you want to know?
 

iamxo

Full Member level 4
Joined
Mar 22, 2006
Messages
227
Helped
16
Reputation
32
Reaction score
4
Trophy points
1,298
Location
Southeast Asia
Activity points
2,495
In fact, the current flows through R2 and R3 is almost same
and the value is I=Vt*Ln(n)/R1 ----(1)
the pic is a closed loop, and the loop is stable if those values are proper

So the current is only the same as equation (1), no limit as you said.


so, what do you mean of proper value?

what kind of current limet you want to know?

I want to know whether the current has a minimum value to insure the bandgap circuit work properly.
 

engronger

Member level 2
Joined
Feb 19, 2006
Messages
46
Helped
3
Reputation
6
Reaction score
1
Trophy points
1,288
Activity points
1,585
when the bandgap work stable , the current is the equation you give
but ,i think you can not say it is the limitation


iamxo said:
as in the pic, we know that when the current through R2 R3 increases, the negative terminal voltage of the opamp increase quickly than the positive terminal, that is

I*R1+Vt*ln(I/nIs)>Vt*ln(I/Is), here neglect the Is mismatch and current mismatch.

so we can get the current should satisfy the following condition:

I>Vt*ln(n)/R1,

so is there this limit of current value in real bandgap circuit?

please help, thanks.
 

edwintsu

Member level 4
Joined
Mar 24, 2005
Messages
73
Helped
9
Reputation
18
Reaction score
3
Trophy points
1,288
Location
China
Activity points
1,677
iamxo said:
what kind of current limet you want to know?

I want to know whether the current has a minimum value to insure the bandgap circuit work properly.

U can get the curve of BJT beta value vs. collect current by simulation, as below.If the collect current is too small or large, the beta will be changed. For the stable bandgap reference, u need keep the beta value level with different temp, corner and power supply. Typical i use 1uA at .5u process.

B/R
Edwin
 

winsonpku

Advanced Member level 4
Joined
Jul 7, 2005
Messages
119
Helped
6
Reputation
12
Reaction score
0
Trophy points
1,296
Activity points
2,351
I think you doesn't understant the principle of the bandgap.
You will find the anwser if you read Chapter 11 of razavi's book.



what kind of current limet you want to know?

I want to know whether the current has a minimum value to insure the bandgap circuit work properly.[/quote]
 

Status
Not open for further replies.

Similar threads

Part and Inventory Search

Welcome to EDABoard.com

Sponsor

Top