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BASIC FET DIFFERENTIAL PAIR

Ch.11 , pg : 779 , 4th^ Edition

Department of Electrical Engineering

Differential-mode

and

common-mode input voltage

DC Transfer Characteristics

of MOSFET Differential pair Config.

vd

Differential mode input Voltage

As vs = v

As from figure

As Vd= VG1-VG Isn’t this For M^ contradiction? 1

For M 2

(2)

Differential mode input Voltage

vd

Similarly IQ and ID2 relation is as

When the differential Voltage becomes ZERO.

ID1=IQ/

And

ID2=IQ/

And also

We will have gm/2 for each MOSFET

AC equivalent circuit,

MOSFET differential amplifier

Ac equivalent circuit of the diff-amp configuration, showing only the differential voltage and signal currents as a function of the transistor trans-conductance gm

Differential- and Common-Mode Input

Impedances

Small Signal Equivalent

Assumption :

The two transistors are biased at the

same quiescent current,

and gm1 = gm2 ≡ gm.

Mathematical Analysis

CMRR (Common Mode Rejection Ratio) is a measure of the

quality of the amplifier and is=differential gain/common mode gain. Usually it´s expressed in dB.