how to use lower vdd devices at higher values

[arya.jagadeesh]

how to use say 1.2 v devices (transistors),in say 3v supply

 

[ashugtiwari]

Check the device datasheet. If the device is rated for 0-1.2V, there is no way u can run it at 3V, u need to use step down switching regulator or LDO regulator. But usually, transistors are rated from 0 to 5V at minimum. So, its ok to use it at either 1.2V or 3V.
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hope that helps.

 

[dick_freebird]

A device's "rating" has many facets, if you drill down into the
details. The foundry will call it out at its lowest limiting aspect.
It's quite likely that a FET has a higher Vgs and Vgd than its
long term reliable Vds at worst case HCI bias. So sometimes
(in some topologies) you can safely use a device beyond its
simple scare-the-innocent rating.

If you are using an SOI technology you can take advantage
of the infinite (for practical purposes) body-to-substrate,
hence body-body, voltage and use cascoding techniques to
divide the applied "off" voltage into bite-sized Vds portions.

The problem you run into, quickly (N>2) is that the higher
gates now require their own, and well phased, high-swing
gate drivers, and the elaborateness snowballs.

Or, you could use the "I/O" thick oxide transistors, which
might be capable as is. Or find a flow where this is true.

For small amounts of circuitry you may be better off with
an internal LDO and level shifting at the ins and outs.

 

[arya.jagadeesh]

when we use mos caps and when we use metal caps