Welcome to EDAboard.com

Welcome to our site! EDAboard.com is an international Electronic 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.

Register Log in

A question about bandgap voltage reference.

Status
Not open for further replies.

holddreams

Full Member level 6
Joined
Aug 2, 2005
Messages
351
Helped
15
Reputation
30
Reaction score
7
Trophy points
1,298
Location
Shanghai
Activity points
4,243
A question about bandgap voltage reference.
I have run a pre-simulation of a bandgap circuit, run the transient analysis,

.tran 1n 1u sweep temp 0 85 17

and give the power supply a pwl signal,

vdd vdd 0 pwl 0 100n 3.3v

The output voltage of the bandgap is 1.35 at 0 degree.and 1.39 at 85 degree.

So my question is

What have effected the output voltage changing from 1.35 to 1.39v?
1.39-1.35=40mv seems to be big!
Is there any way to solve this problem?

Thanks.
 

jwfan

Member level 4
Joined
Nov 13, 2005
Messages
70
Helped
5
Reputation
10
Reaction score
2
Trophy points
1,288
Location
Earth
Activity points
1,741
You can change the ratio of resistors in your bandgap. There is a PTAT current flow through the resistor and a negative temperature coefficient on your BE junction. your output is the sum of both of them. Let the temperature coefficient of them to be cancal out at about 60C.
 

holddreams

Full Member level 6
Joined
Aug 2, 2005
Messages
351
Helped
15
Reputation
30
Reaction score
7
Trophy points
1,298
Location
Shanghai
Activity points
4,243
How to realize “Let the temperature coefficient of them to be cancal out at about 60C.”?
 

fnlee

Newbie level 5
Joined
Dec 19, 2005
Messages
8
Helped
0
Reputation
0
Reaction score
0
Trophy points
1,281
Activity points
1,335
chang resistors ratio m(bandgap circuit), you will cancel it.
 

ambreesh

Full Member level 5
Joined
Feb 21, 2005
Messages
319
Helped
21
Reputation
42
Reaction score
4
Trophy points
1,298
Activity points
4,347
Hi Holddreams

What does your DC simulation give results for the BGR output at various temperature.
Does it show a PTAT nature. If the BGR behaves as you expect it to in DC sweep, then we need to have a look at you architecture and find the bug.
I would also suggest the you incerease your simulation time. Donot sweep the supply as fast as you are doing.
Kindly share what you have found out, and all of us can proceed from their
 

chu

Junior Member level 3
Joined
Jul 25, 2005
Messages
29
Helped
0
Reputation
0
Reaction score
0
Trophy points
1,281
Activity points
1,588
jwfan said:
You can change the ratio of resistors in your bandgap. There is a PTAT current flow through the resistor and a negative temperature coefficient on your BE junction. your output is the sum of both of them. Let the temperature coefficient of them to be cancal out at about 60C.
hi holddreams,

Vref=KVt + Vbe ; K a function of resistors and bjt size

if Vt TC=+0.085mV/C and Vbe=-2mV/C, we find K which will cancel out each other. let say K=24

assume we increase temp from 25C to 60C, delta is 35C,
for Vbe--> decrease abt 70mV (from 35×2) from original
for Vt--> increase 71.4mV (from 35×24×0.085) from original
both this cancel out each other no matter in what temp, provided that the transistor is not breakdown at too high temp.

so u hav to scale the K (ratio of resistor and transistor size) to a good value to get stable temp independant.
hope this will help.
and guys, also let me know if i'm wrong.


rgds,
chu
 

Status
Not open for further replies.
Toggle Sidebar

Part and Inventory Search

Welcome to EDABoard.com

Sponsor

Top