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Hi,
The diodes at input stage of LM2903 confuse me, seem like for ESD protection, but there is not a path to VCC or GND, anyone knows the reason of this design? Thanks.
Avalanching an emitter does hot carrier injection at low current for Vio / Iib drift beyond spec, can "zener zap" aluminum through the junction at a hundred mA for a hard fail and you can't make a 40V ESD clamp (common mode range) protect a 7V Bvebo NPN worth a damn and it'd double the die area before it got close to effective. Most linears of this vintage did not really concern themselves with ESD as we define it today, but took care over application "expected abnormal conditions" such as exceeding input differential voltage for some fault or poor design.
The cross clamping allows safe operation at moderate overrange if the user puts limiting resistors on the inputs (app note / ref design)?
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The back diodes can also greatly improve prop delay, because the PNP emitters can pull but not push so in the LH direction response to a large step would be "soggy" without them.
according to that simplified circuit - you can't have more than one Veb zener voltage ( 5 - 7 V ) and 3 diode drops between the input pins - but the data sheet says up to 36/38 V for diff input
So the simple ckt you posted is obviously some way different to the real thing ....
An older datasheet shows a slightly different structure
It avoids the input short involved with the circuit shown in post #1. Notice that most of the differential input voltage appears at Q2 respectively Q3 BE junction, so it has to be designed with a much higher breakdown voltage than standard transistors.
It's probably that the inputs are substrate PNPs and the diff pair, lateral PNPs. Both of these built with C-B junctions that can take 40V. In which case the diodes are probably about switching speed than protection.
The attached app note mentions D1 and D2 speed-up function. Maybe 'D3' and 'D4' are only there so as not force input signals into the 3.5uA current sources.
Attachments
AN-74 LM139 etc A Quad of Independently Functioning COmparators snoa654a.pdf
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