Differential Amplifier: Common-Mode Input

Drive both bases together to expose the common-mode gain of a resistive-tail differential pair.

Circuit operation

Both bases receive the same 0.5 V peak sine wave around 2.5 V. Equal collector loads and a shared resistive tail make the two collector voltages move together. Vdifference measures their difference directly.

Expected behavior

$$I_T\approx\frac{V_{CM}-V_{BE}}{R_T}$$ $$A_{CM}\approx-\frac{R_C}{2R_T}$$

The individual collector voltages vary because common-mode input changes the tail current. The differential output stays near zero for this perfectly matched model. Real resistor and transistor mismatch would turn some common-mode signal into differential error.

Reading the results

The first voltage probe is green and the second is red. Additional voltage traces use distinct colors. Current probes, when present, start with blue and then purple. The chart opens with separate traces on a fixed time window; the table retains the same probe order. Zoom to extents includes the complete schematic.

The saved transient runs for 4.0 ms with a maximum step of 1.00 µs. Compare settled cycles after startup and use time-axis cursors to measure delays or periods.

Try a change

Set V1 peak amplitude to 0.8 V; compare common-mode response or nonlinear current steering. Restore the saved values after comparing the result.

Model limits

These are generic transistor models, not a specific manufactured part. The bipolar models use nominal forward current gain 100 with illustrative junction capacitances and transit times. Device mismatch, temperature drift, layout parasitics, breakdown and detailed noise are not represented. The schematic and component properties expose the values used.