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Design >> Analog Design >> gm boosting vs. W
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Message started by eng on Jun 5^th, 2008, 9:04pm

Title: gm boosting vs. W
Post by eng on Jun 5^th, 2008, 9:04pm

Suppose you have a common source nMOS with a cascode transistor and a resistor connected to VDD. Output is taken from resistor. gate of common source nMOS is connected to same size diode connected nMOS and a resistor. 1 mA is flowing thru resistor so does the diode connected. Input is capacitively coupled to common source nMOS. I want to increase gm of common source how far can I increase W?
What are the limitations increasing the W to increase gm? Ignore the capacitance of nMOS.
Please comment.

Title: Re: gm boosting vs. W
Post by vivkr on Jun 5^th, 2008, 11:05pm

For a given current value, the maximum possible Gm is fixed. This is the Gm of a BJT.

Gm/Id(MOS) < Gm/Id(BJT) = (q/kT) (At 300 K, this is around 38)

The MOS Gm/Id is typically constrained to be less than that of the BJT by a factor approx. 1.3 or so (think
of the MOS as a BJT but with a capacitively coupled input divided by the cap divider formed by
Cox and Cinversion.

That being said, you can use positive feedback to increase the Gm of the MOS. Please search the forum.
I remember having asked the same question and having received an answer a few months ago.

Regards
Vivek