Effects of the Resistance Parameters Used in a Schmitt Trigger Circuit on Its Temperature Stability and Switching Speed

摘要

The relationships between the magnitude of the resistance parameters used in a Schmitt trigger circuit and the temperature stability and switching speed of the circuit were investigated and correlated in an attempt to find a general approach for designing a Schmitt circuit. Expressions for the variations in the trigger-point voltages and output voltage swing due to a temperature rise were derived. In addition, expressions for the delay times associated with a forward bias on each transistor and also the switching time, t/sub s/, during regenerative operation of the circuit were developed using the hybrid-pi equivalent circuit. These equations were then used to investigate the characteristics of two circuits, one with small values of resistances and other with large values of resistances. Switching times and the variations in trigger points and output voltage swing due to a temperature rise were calculated as a function of the resistance parameters. The effects of resistance parameters on stability and speed were then correlated and a design approach for the Schmitt circuit was outlined. Large resistance parameters are preferable for good temperature stability, while small resistance parameters are preferable for high switching speed. Larger values of R/sub E/ improve both the temperature stability and the speed performance. A circuit with transistor Q sub 2 operated in a saturated state is generally easier to stabilize.