Bipolar Schmitt Trigger

Emitter-coupled positive feedback creates different input thresholds for rising and falling transitions.

Circuit operation

Q1 and Q2 share RE. Q1 collector drives Q2 base through RB. When either transistor takes more current, the shared emitter voltage and the other transistor drive reinforce the transition. The triangle input sweeps both thresholds repeatedly.

Expected behavior

$$V_{T+}>V_{T-}$$ $$V_H=V_{T+}-V_{T-}$$

Measure the input voltage at each output transition. Rising and falling thresholds differ. The low output remains above ground because Q2 emitter is elevated by the shared emitter resistor; this circuit is not a rail-to-rail logic driver.

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 RE to 330 Ω; compare the shifted switching thresholds. 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.