voltage swing of cascode current mirror

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kishore680

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using cascode . small signal output resistance of current mirror increases and thus we get constant current over wide range of volatges.. i.e the flat output characteristics(ids vs vds) of mosfet... however voltage swing gets reduced in cascode current mirror.. how to overcome this problem ..??
 

Hi,
yes.you're right.
but we can increase the swing even in cascode.
just like this :

Note that must be v1=VT (Voltage Threshold) .using this battery swing increases to 2Vov.(without using battery it was 2Vov+VT).
also there are bettery wasy instead of using battery(This was simplest).
If you want them,Just tell me.
 
There are many cascodes.
Wide-swing, gain boosted, regulated cascodes, self-biased etc.
They all try to address the issue of increasing output swing without any or much sacrifice in output impedance, albeit with additional complexity and current consumption.
 
hey the voltage at the gate of Q3 will be 2Vov+2VT.. so in the above diagram the polarity of battery should be reversed na...to get 2Vov+Vt ????. in ur case it ll be 2Vov+3VT??.. and ya i ll be pleased to know better ways..
 

yes...
you're right.
the polarity of battery must be reversed.
Note that with this change , Vomin will be : "2Vov" (not 2Vov+Vt)
 

Also instead of using battery (as you know using battery is very difficult) we can use this circuit :

Note : All transistors are similar except M4 (Size of all transistors are W/L . M4's size is W/4L)
In this circuit Vomin=2Vov .
Hope this help you.
 

hey the battery one u suggested is unique one. i understood that thanku ... and the second one which u told M4 size is 1/4 (W/L).. this technique has been given in cmos analog ckt design by philip.allen and douglas holberg it is used in current sink or current source . he s given its not applicable for current mirror. dont know y?
 

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