4. Differential Amplifiers
4.2. Basic Differential Pair
Comparing to single-ended amplifiers, differential amplifiers provide common-mode noise rejection, supply noise rejection, and high voltage swing (twice that of single-ended swing).
The parameter common-mode rejection ratio (CMRR) in dB is defined as:
CMRR = 20log(Ad/Acm), where Ad is the differential gain, and Acm is common-mode gain.
Typically, a transistor with fixed bias acts as the current source for the differential pair. M3 should operate in saturation to ensure high impedance and CMRR.
Acm is calculated as
Acm ≈ gmRD/(1 + 2gmZ0) = (RD/2)/[1/(2gm) + Z0], where Z0 is the impedance looking into the drain of M3.
