Which configuration yields a high differential gain and high CMRR in a differential amplifier?

• A. A pair of transistors with emitters connected to ground
• B. A common-emitter amplifier with emitter degeneration
• C. A pair of matched transistors with one side unused
• D. A long-tailed pair with emitters tied to a current source

Answer: (D)

A long-tailed pair with emitters tied to a current source yields both high differential gain and high CMRR because the shared tail current source acts as a high-impedance path for common-mode signals while still allowing a strong, opposite change in each transistor’s current when a differential input is applied. With the emitters fed from a current source, the total emitter current stays nearly constant. When you apply a differential input, one transistor conducts more and the other less, so a large difference appears in the collector currents and across the load resistors, giving a strong differential output. At the same time, the high tail impedance means common-mode signals—changes that are the same on both bases—produce only small shifts in the emitter potential and thus only small output changes, resulting in excellent common-mode rejection.

If the emitters are simply tied to ground, the tail no longer presents a high impedance to common-mode variations, so common-mode signals can move the emitter potential and leak into the outputs, degrading CMRR and limiting differential gain. A single-ended or emitter-degenerated setup isn’t a true differential pair and won’t provide the same level of common-mode suppression or differential performance. A paired arrangement with one side unused loses symmetry, effectively acting more like a single-ended path, which also undermines CMRR.