modified 9 years ago

LAoE 5L.1.5 Differential amplifier - Common mode gain with tail resistor and current source

05:23:36

CMMR calculation by "measurement" of
differential mode gain and common mode gain with 10K tail resistor and 15mA current source.

Left switch: switches between common mode signal and ground (quiescent point)

Bottom switch: switches between 10K tail resistor and current source.

With 10K tail resistor, CMMR=?
diff mode gain: 7.08/0.2=35.4
common mode gain: 0.1/0.2=0.5
CMMR=35.4/0.5=70.8
(37dB)

With current source, CMMR=?
diff mode gain=7.09/0.2=35.45
common mode gain=0.01/6=0.0017
(Here the signal was set to 6Vpp to have a visible effect of the very low common mode gain.)
CMMR=35.45/0.0017=21270
(86.6dB)

Ratio of CMMR between tail resistor and current source: 70.8/21270=3.33*10^-3
(-49.6dB)

published 9 years ago

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