• Different amplifier is a device which amplifies different between two s/g. $V_0 \alpha (v_1 – v_2)$

• Operation of the basic different amplifier in differential mode.

• For operation in the differential modes we need to have the two i/p s/g $VS_1$ and $VS_0$ of equal magnitude but opposite phase.

• i.e. $VS_1 = - VS_2$ and their magnitude should be equal. In order to get there s/g a centre tapped $X^mn$ is used.

• In positive half cycle of ‘$V_s$’ the i/p to $Q_1$ is the sinusoidal s/g and the o/p to $Q_2$ is negative sinusoidal s/g. this is due to center tapped $X^mer$

• o/p is equal to different between o/p of individual transistor.

• Its amplitude will be twice the amplitude of the s/g voltage obtained either collector to ground.

• The o/p voltage save form shown in figure is the w/f at collector of $Q_1$ w.r.t collector of $Q_2$

• In the positive half cycle of force at i/p of $Q_1$ is positive. Hence positive voltage is developed across $R_E$ this is because $Q_1$ acts as the emitter follower.

• At the same time $Q_2$ receiver a negative voltage at its i/p and hence produces negative voltage across RE due to emitter follower action.

• Thus equal and opposite s/g voltage appear across RE and cancel each other.

• Therefore, when we consider the effect of $Q_1$ and $Q_2$ simultaneously, no s/g voltage appears across RE and the s/g current flowing through RE is equal to zero.

Therefore, RE does not introduce negative F/B thus in the different mode, the s/g applied to the bases of the two transistor are equal to magnitude but opposite in sign.