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Old 15th December 2008, 03:36 AM   #1
suresh is offline suresh  India
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Default Doubt about output offset

to avoid too much output offset what should be done? i think 30mv is ok.

1. suppose if output transistor is not matched ? should vbe be matched or hfe be matched.

2. what about driver transistor matching? is it required.

3. what about input differential stage transistor matching?

what are the effects for the 3 points above? it would be greatfull if some one explains me in detail.


sorry for so many questions....
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Old 15th December 2008, 03:52 AM   #2
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Just match the input differential pair.
For how to do it, search 'Leach' pages

Matching all the counterparts is good but not practical and is not needed.

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Old 15th December 2008, 05:07 AM   #3
Shaun is offline Shaun  South Africa
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Also input impedances should be matched. Less critical for MOSFET input stages.
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Old 15th December 2008, 08:36 AM   #4
suresh is offline suresh  India
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Please i need more help from this forum. i hope some senior member can spare some time for my questions...

is it all about input differential transistor matching?
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Old 15th December 2008, 10:42 AM   #5
Shaun is offline Shaun  South Africa
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Quote:
Originally posted by suresh
is it all about input differential transistor matching?
Matching transistors is a good start. Also, quiescent currents should be balanced. Bipolar transistors draw bias current via the input resistors, so the resistance looking outward from the base should be equal for inverting input and non-inverting input. Higher input impedance differential stages are less susceptible to this effect.

Furthermore, differential pairs can be thermally coupled by clamping them together physically "face to face", so that the gain relationship can remain constant with temperature. Better than this is to use a monolithic matched pair.

There is also the option of inserting a DC balance preset potentiometer where the differential pair tail currents sum into the current source.

HTH
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Old 29th December 2008, 04:50 PM   #6
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Quote:
Originally posted by suresh
is it all about input differential transistor matching?
Mostly
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