LM2902-Q1: Common mode input voltage problem

Hi Ronald,

Thanks for your reply!

I remember the common mode voltage's formula is  as follows, isn't right?

Thanks & Best Regards,

Sherry

i Clemens,

Could you please help explain why I will get phase reversal when the input voltage is above VCC-2V? The LM2902's schematic is as below:

Thanks & Best Regards,

Sherry

Sherry,

There might be phase reversal.  The key point is that the input transistors are cutoff therefore the input voltage doesn't not control output anymore. The output will be based on what the amplifier naturally does. Maybe high maybe low, although the majority will be one or the other.

2V from input to VCC is plenty to support 2 base emitter junctions and a current source. Higher than than that will eventually cause transistors to cutoff, turn off.

Hi Ronald,

Thanks for your reply!

You mean when Vdiff is large (>0.1V) , for example, V₊-V_->0.1, then I2 will get all the internal current flow, and I1=0, so the higher voltage input can be set any value as long as lower than the maximum VCC rating. Is my understanding right?

Thanks & Best Regards,

Sherry

Hi Ronald,

Thanks for your reply!

Now the condition is V-=VCC-2.4<VCC-2, so the negative input side transistors are on (as below). Due to V+>VCC-2, the IN+ cannot control output, and V+'s value is not sure, maybe high maybe low, so the comparator's output maybe 1 maybe 0, and this phenomenon is phase reversal, right?

Sherry,

Regarding: You mean when Vdiff is large (>0.1V) , for example, V+-V->0.1, then I2 will get all the internal current flow, and I1=0, so the higher voltage input can be set any value as long as lower than the maximum VCC rating. Is my understanding right?

Yes.