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# How small a current can be used for BGR design

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#### ambreesh

##### Full Member level 5
Hi All,

Can we use just 1uA and design a BGR circuit.
What is the minimum current that should flow through the BJT diodes.

hi ambreesh,

it depends on the process whether the fab can supply u that kind of transistor.
u hav to check the beta vs Ic curve whether u can get an appropriate beta with that amount of Ic. From what i know, Ic of around 10uA is possible but then create difficulty on BGR circuit.

rgds,
chu

I think the smallest current should be around 10uA, if the current is too small, it doesn't well restrain the noise!

In my design I used 250nA trough 30 parallel BJT for bandgap design. Everything is OK. It is already in mass production.

Hi Fom,

Thanks a lot.
Are you using all devices in saturation or a certain of them in weak inversion also.
Is your reference for 1.2v or sub1volt
Is the design based on voltage summation or current summation.
And what is the total current consumed by the BGR.
For my design we have a supply of 1.5V, we were thinking of using an opamp in the BGR core. But then any extra device would come only at cost of extra power.
If it is ok with you could you give some suggestions on topology etc., it would be great help

Fom said:
In my design I used 250nA trough 30 parallel BJT for bandgap design. Everything is OK. It is already in mass production.

This is a large area !!!

U can use 1uA but u will need to increase the resistors, so its tradeoff between power consumption and area.

Hi Fom, I am curious as to what is the value of the resistors used in your PTAT generation circuit. I have tried designing a sub-1 voltage reference as shown below with a 0.18um process with a power supply of 1 V. If the circuit is to draw 1uA of current, the resistors that are connected to point A and point B will be as large as 1 Mohm. My boss feels that the resistors used is too large. Is there any trick to reduce the resistor value? Thanks.

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