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Subject: Re: CMOS differential amplifier

Date: 12/12/00 at 10:44 PM
Posted by: cm
E-mail: tancming@yahoo.com
Message Posted:

In Reply to: CMOS differential amplifier posted by kristena louie on 12/12/00 at 2:59 AM:

Wthe differntial ampl w/l size is halved,

-differential-mode small-signal gain
experimentally no significance

-small-signal unity-gain bandwidth
experimentally no significance, the gain does not change until after the unity-gain bandwidth

-small-signal unity-gain phase margin
the phase margin will improve because of less parasitic capacitance, about 3deg-6deg

-output voltage linear range
this depends on both the transistors at the differential amp and at the output stage. changing the size of the differential amp does not affect the output voltage linear range.

-input common-mode voltage range
this depends on the transistors above and below the differential ampl, and also the backgate of the differential ampl, whether it is tied to source or to vdd(pmos diffamp). it does not depend on the size of the diff amp.

-input-referred 1/f noise voltage spectral density
would be worsened

-total input-referred RMS thermal noise?
would be worsened

More importantly is that the pair matching between the diff amp transistors will be less equal. The Voltage offset of the transistor will thus deteriorate when the size is halved.

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