In a differential pair, what is the role of the tail current source, and how does it affect common-mode rejection ratio (CMRR)?

• A. The tail current fixes the total differential current; a stable tail current helps CMRR by keeping common-mode changes from altering the total current.
• B. The tail current only biases one transistor and has no effect on CMRR.
• C. The tail current defines output impedance but does not affect CMRR.
• D. The tail current is used only for startup; CMRR decreases with stable tail current.

Answer: (A)

In a long-tailed differential pair, the tail current source sets the total current flowing through the two transistors and provides a high-impedance path at the common node. As the input voltage difference between the two bases changes, the current is steered from one transistor to the other, but the sum of the two collector currents stays nearly constant because the tail current source fixes it. When you apply a common-mode signal (both inputs move together), this fixed total current means the outputs won’t chase the common-mode change very much, so the device’s response to common-mode input is small. That is why a stable tail current helps achieve better common-mode rejection: the common-mode variations don’t translate into large output changes.

If the tail current source isn’t high impedance (not a good current source), common-mode changes can tug on the total current, causing both outputs to shift together and degrading CMRR. A robust tail current thus keeps the sum of the currents constant, suppressing the common-mode path while preserving strong differential behavior.