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[Moved]: Injected current in current mirror

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anhnha

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I asked the question in the forum. However, I haven't received a satisfactory answer yet.
Could anyone help me explain it in more detail?

Current.PNG
 

Re: Injected current in current mirror

What do You want to know?

If Iinjected is a constant DC current source, sinking 1µA of current it will result with sinking ≈3µA of current from VDD source and VD2 potential goes down to set OP point of M4 to provide ≈2µA (one for Iinjected current source and second for drain of M2 current source). Due to classic cascode biasing, VD2 with absence of Iinjected is large enough to preserve M2 in saturation after adding Iinjected (in most cases).
 

Re: Injected current in current mirror

Well, sorry I forgot to add the question.

For the circuit above as the injected current equal to -1uA the current through M4 is 1uA and there is no current through M2.
What I am confused is why there is not a case that M4 current is 2uA and M2 current is 1uA and it still satisfies KCL law.
 

Re: Injected current in current mirror

anhnha said:
For the circuit above as the injected current equal to -1uA the current through M4 is 1uA and there is no current through M2.
Click to expand...
Is it a conclusion from simulation or something?
 
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Re: Injected current in current mirror

What I am confused is why there is not a case that M4 current is 2uA and M2 current is 1uA and it still satisfies KCL law.
Click to expand...
I can't confirm your results. Id(M4) = 1.999 µA.
 
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Re: Injected current in current mirror

Thank you for these replies.
Is it a conclusion from simulation or something?
Click to expand...
The conclusion is from Jacob Baker's lecture here at 30:44.

I can't confirm your results. Id(M4) = 1.999 µA.
Click to expand...

So, could you explain why there are different results?
 

Re: Injected current in current mirror

The verbose presentation shows also Id(M4) = 1.999 µA for -1µA injected current, as expectable when using same model and transistor parameters.

Seems like you are discussing erroneous believes only.
 
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Re: Injected current in current mirror

Yeah, you are right. His initial prediction is wrong and that is confirmed by simulation result.
Another question is why the case i(M4) = 1uA and i(M2) = 0 doesn't happen?
 

Re: Injected current in current mirror

Id(M2) would be zero if you manage to pull node VD2 to ground. This probably happens at a much higher injected current than 1 µA and respectively higher Id(M4).

As a side remark. It's somehow alarming to see people jumping in conclusions about the erroneous observation claimed in your original "All about circuits" thread.
 
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    anhnha

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Re: Injected current in current mirror

Thanks. Is there a logic that higher injected current will pull node VD2 closer to ground?

As a side remark. It's somehow alarming to see people jumping in conclusions about the erroneous observation claimed in your original "All about circuits" thread.
Click to expand...
Yes, people ansewered there are experts and this make me supprised.
 

Re: Injected current in current mirror

Presume the M2 and M4 gate levels are fixed according to the current mirror input current. Pulling VD2 to ground is about doubling Vgs(M4). For the given model parameters, you need >400 µA injected current to achieve that.

One problem with the expert statements is that you have "perfectly" misguided by creating the impression that the reported results are validated simulation results. Dominik was the first one asking how they have been actually achieved.
 
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