Common-mode noise and why differential pairs are used
In single-ended circuits the load is directly or indirectly tied to VDD, which unavoidably introduces noise from the current source. This noise is referred to as common-mode interference. When that interference appears on the output signal it produces spurious components in the frequency domain. To reject this interference many designs use a differential pair.
Common-mode level and differential signals
The bias voltage for a differential signal is called the common-mode level. For the input it is the input common-mode level, and for the output it is the output common-mode level. The differential signal is the difference between the two node voltages, while the common-mode level is the component that is the same on both nodes relative to a fixed reference. The center fixed potential is denoted CM.
Tailed differential pair and tail current
In practical differential amplifiers the source nodes of the pair are commonly tied to a tail current source, for example:
When MOS transistors operate in saturation their drain currents vary with gate voltage. Using a constant tail current ensures that the currents flowing into the common node are equal for both branches. Because the two branches are symmetric, each branch carries half the tail current and those branch currents do not vary with Vin or the common-mode level. The tail current therefore suppresses the effect of common-mode variations on the transistors' operating points and on the output levels.
Input-output characteristic
By studying Vout1 - Vout2 as a function of Vin1 - Vin2 we obtain the differential input-output characteristic:
From the characteristic we see that when Vin1 - Vin2 = 0 the small-signal gain (the slope) is largest. Thus, with Vin1 and Vin2 biased at the same potential, small differential signals are amplified. The large-signal output swing limits are VDD (maximum) and VDD - RD*Iss (minimum). The small-signal output range is approximately -RD*Iss to +RD*Iss.
Summary
1. Differential amplifiers remove common-mode interference by subtracting inverted signals, while amplifying the differential component.
2. Adding a tail current makes the bias independent of common-mode level, simplifying bias design and improving output linearity.
3. The differential structure increases chip area and power consumption compared with single-ended solutions.
