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Design >> Analog Design >> How does cascoding helps in supply rejection?
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Message started by hchanda on Feb 14^th, 2008, 12:06pm

Title: How does cascoding helps in supply rejection?
Post by hchanda on Feb 14^th, 2008, 12:06pm

Hi all,

How does cascoding helps in supply rejection? Is it AC supply rejection or the DC supply rejection?
Can you please provide your analysis?

Supposing if the cascode current mirror providing current to an inverter (the current from the mirror can be varied) which controls the
delay of the inverter. Is this true that the delay of inverter is not susceptible to supply noise (both AC and DC)?

Also I notice that usually capacitors are placed on the bias nodes and supply for the cascode devices (connected to gates of transistors), this
means the gate nodes and supply voltage are AC coupled right, what is the purpose and how does it help?

Thanks
h

Title: Re: How does cascoding helps in supply rejection?
Post by thechopper on Feb 14^th, 2008, 5:14pm

Hi,

Cascoding basically boosts the output impedance of the current mirrors. This impedance is derived as the inverse of the slope of the IDS vs VDS characteristics of the current mirror (in saturation region). As such it is a small signal (AC) parameter and since is much larger than a non-cascoded mirror its supply rejection is much better. I guess when you say DC you are referring to large signal condition. In this case the answer is also yes, it improves the DC rejection too since the IDS vs VDS characteristic in the sat region are linear, and therefore constant slope.
So the rejection applies for both cases.

Hope this helps.
Tosei