Differential Amplifier: Common-Mode with Current Source

Replace the tail resistor with a current source and compare common-mode rejection.

Circuit operation

Both bases receive the same input as in the resistive-tail example. IT replaces the tail resistor with an ideal 1 mA current sink, keeping the total emitter current fixed.

Expected behavior

$$I_{E1}+I_{E2}=I_T$$ $$A_{CM}\longrightarrow0\quad(R_{tail}\longrightarrow\infty)$$

The two collector voltages remain close to 9.7 V while the emitters follow the common-mode input. Differential output stays near zero. The ideal current sink has unlimited compliance; a practical transistor sink has finite output resistance and a restricted voltage range.

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.