Introduction To Differential Amplifier: Common Mode & Differential Mode
Introduction To Differential Amplifier: Common Mode & Differential Mode
Introduction To Differential Amplifier: Common Mode & Differential Mode
DIFFERENTIAL AMPLIFIER
Common Mode & Differential Mode
Significance of Differential Amplifier
The differential-pair or differential-amplifier configuration is the most widely
used building block In analog integrated-circuit design.
For instance. the input stage of every op amp is a differential amplifier. Also,
the BJT differential amplifier is the basis of a very-high-speed logic circuit
family called Emitter-Coupled Logic (ECL).
There are two reasons why differential amplifiers are so well suited for IC
fabrication:
1- The performance of the differential pair depends critically on the matching
between the two sides of the circuit. Integrated circuit fabrication is capable
of providing matched devices whose parameters track over wide ranges of
changes In environmental conditions.
Where
Operation with Common-Mode Input Voltage
The lowest value of VCM is determined by the need to allow for a sufficient
voltage across the current source I for It to operate properly.
If a voltage VCS is needed across the Current Source, then
PROOF:
VGS > Vt
VGS = Vt + VOV
VG – VS = Vt + VOV Where:
VG = VS+Vt + VOV VCS is the voltage across
VG = (VCS – VSS)+Vt + VOV Current Source ‘I’.
VCMmin = VCS – VSS+Vt + VOV
Operation with Differential Input Voltage
Ground the Gate of Q2.
Apply the signal Vid to the Gate of Q2.