Schmitt Trigger SPICE Analysis

This is an analysis of the emitter-coupled Schmitt trigger circuit of Probs. 10.75 and 10.76 in Bobrow (see Fig. P10.75). A rising and falling 5V ramp is coupled to the input (base of Q1) via a 1K ohm resistor (source resistance). The input voltage, V(1), voltage on the base of Q1, V(6), voltage on the emitters, V(2), and the output voltage, V(4), are plotted as a function of time over a 200 ns interval. The analysis shows that the upper (low-to-high) threshold is approximately 2.6 V and the lower (high-to-low) threshold is approximately 1.7 V. The high output level is 5 V and the low output level is approximately 1.9 V.

The general behavior is as expected from the analysis following the text. Note how the emitter voltage changes to reinforce the transition (positive feedback). The upper threshold is somewhat higher than the text analysis would predict, but the lower threshold is in close agreement. The transistor model includes internal capacitances and other parameters which contribute to the finite bandwidth. The time scale is fast enough to show the rise and fall time of the output pulse and the delay relative to the input pulse crossing the thresholds. The rise and fall times appear to be of the order of 10 ns for this circuit.

SPICE input file:

SPICE V(t) output for Schmitt trigger. Click on image for full-sized view. Red=Vin, Blue=VB1, Green= VE, Light Blue=Vout (see SPICE input and circuit diagram in text).