Question

In a differential amplifier, the noninverting input is 3cos(600t + 45°) mV and the inverting input is 4cos (600t - 45°) mV. The output is vo = 3cos (600t + 45°) + 4.004cos (600t - 45°) V. The CMRR is of? The unit must be in dB.

Answer 1

The input v1 in polar form is 3<45>.
The rectangular form is
v1 = 2.1213 + 2.1213i

The input v2 in polar form is 4<-45>.
The rectangular form is
v2 = 2.8284 - 2.8284i

vc=(v1+v2)/2=(2.1213 + 2.1213i+2.8284 - 2.8284i)/2= 2.4749 - 0.3536i

vd=v1-v2 = -0.7071 + 4.9497i

The output in polar form is v0= 3<45>+4.004<-45>

For 3<45>,

v01 = 2.1213 + 2.1213i

For 4.004<-45>

v02 = 2.8313 - 2.8313i

v0=v01+v02 = 4.9526 - 0.7099i

Ad=|v0/vd|= 1.0006

Ac=|v0/vc|= 2.0013

CMRR=20*log10(Ad/Ac)= -6.0206dB