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How to Reduce BJT Base Current Mismatch

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ljp2706

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Are there any other circuit solutions besides emitter degeneration and the darlington configuration to reduce a BJT differential input base current variation across monte carlo? My system PSR spec is taking a hit across monte carlo, and I think it's due to mismatch in the amplifier base current.
 

Device size of course drives down mismatch.

You might first look at whether it's mismatch or process
(and/or temp) that accounts for base current difference.
Systematic design offsets can be highlighted at process
corners.

Degeneration helps when resistor variation matters less
(or is less, %-wise) than transistor variation.

Be sure the mismatch distribution that the MC setup
lays on you, is borne out by data. Modeling dudes
love to make stuff up and once the kit is done, have
zero motivation to go back and put real data in.

Make sure that bases see identical driving impedances,
this is where base current (hFE) mismatch turns into
voltage mismatch.
 
I'm inclined to think that it's mismatched impedances that's causing the issue. I have an amplifier in a feedback network driving an active+resistive load. I have approximated the negative node impedance to that of the positive node. I think its this impedance that's varying across process/monte carlo. I'll focus my efforts on minimizing that variation.



Thanks!
 

Did you match and verify layout parasitics for the two matched devices - in terms of resistance, capacitance, RC, etc.?
 

timof said:
Did you match and verify layout parasitics for the two matched devices - in terms of resistance, capacitance, RC, etc.?
Click to expand...

I haven't gotten to layout yet, but I'm designing with layout in mind. I will be using the usual techniques such as common centroiding.
 

Layout has several different implications.
Making design independent of process or thermal gradients is one of them (device placement).
Matching layout parasitics is another one (metal / via routing).
 

Right, thanks! I was planning on ensuring that the R and C of the matched traces were equivalent.
 

I don't recall doing it on a diff pair, but I have used small base resistors on power devices that swamp out any parasitic interconnect resistances.
 

rangermad said:
I don't recall doing it on a diff pair, but I have used small base resistors on power devices that swamp out any parasitic interconnect resistances.
Click to expand...

I didn't do that because I don't like adding base resistance in that way. I usually increase the effective base resistance through emitter degeneration.
 

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