PMOS current mirror - Small signal analysis - Why is Vsg = 0v?

Thread Starter

KaiL

Joined Aug 30, 2014
69
upload_2016-9-29_0-29-21.png

The one in red box , why is Vsg2 = 0V?
Vsg2 = Vs - Vg --> Vs = 0 since it is ground but what about vg?
 

AlbertHall

Joined Jun 4, 2014
8,504
As the red writing by the red box says, it is at a fixed DC voltage and there can be no AC voltage there so it acts for AC signals the same as ground or Vdd.
 

Thread Starter

KaiL

Joined Aug 30, 2014
69
upload_2016-9-29_15-39-51.png
Actually I still don't get it :(
I am trying to do a circuit analysis on the red box but I am not sure why it is 0 V for Vsg2
Vs=0 V since it is grounded but Vg isnt equal to Vout?
So Vs-Vg = Vsg2 --> Vsg = 0 - Vout???

Did I do it wrongly for analysis part?
 

Russmax

Joined Sep 3, 2015
82
Saying vsg2 = 0 means 2 things:
1) M3 and IREF can be replaced by a DC voltage source from source to gate of M2.
2) DC voltage sources are shorted for small-signal analysis.
Therefore, vsg2 = 0
 
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