vivkr wrote on Jul 14th, 2008, 11:41pm:Hi Yawei,
A point which I think is not clear from my previous post:
If you are using a BJT input pair for your opamp, and you trim out the offset by whatever means (adjusting resistor, current etc.), then you will also have minimized the offset drift at the same time. This special property exists only for the BJT.
Regards
Vivek
Hi Vivek,
What you suggested as for the offset drift is true for a bipolar diff pair. However one must be very careful in order to guarantee that happens.
For example a bipolar pair with a resistive load will definitely have an offset contribution from the resistors. That will introduce a different
offset drift component which certainly be different than the one contributed by the bipolars. This is even
worse if a MOS active load is considered.
Therefore, the only way to guarantee the offset drift will be totally proportional to the offset value at the reference temperature is by using
a bipolar active load.
In this way "purely-vbe" related offsets are involved and therefore their drift can be predicted and minimized.
Regards
Tosei