The differential amplifier (shown in fig below) is also referred to as along-tailed pair. A differential amplifier contains 2 inputs namely, vIN1 and vIN2. This amplifier is used as amplifier in order to amplify the voltage difference between its inputs. When it does this, we say that it is operating in the differential mode.
To demonstrate this, 2 sinusoid signals that are 90 degree out of phase are fed to these two input terminals. The output signal is taken from the collector of Q2.At any instant:
vOUT = (vIN+ - vIN-) x differential-mode voltage gain
the Measurement figures on the graph (shown above) tells us that the differential voltage gain is nearly 129. If you feed the same signal to both of the inputs, you are operating the amplifier in common-mode. We obtain the result shown in diagram 9.13. The amplitude of the output signal is only half that of the input signal and it is inverted by the formula:
vOUT = vIN x common-mode gain
The common-mode gain is 20.5.
The differential amplifier has high differential mode gain but the common-mode gain very low. It is best fit for measuring voltage differences when the voltages themselves are subject to much larger change affecting both voltages in equal extents.
In the medical applications it is normal to measure very small differences of voltage between two probes connected to different places of the body surface. At the same time both probes are subject to comparatively large voltage changes resulting from electromagnetic fields from nearby mains supply cables and from other electrical equipment in the same room.The differential amplifier has the ability to amplify the small voltage differences between probes but is much less affected by the larger voltage swings that are affecting both probes. It has the ability to reject the common-mode signals. Below we have express its ability to do this by mean of calculating the common-mode rejection ratio:
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