Schmitt trigger comparator

The comparator has an output pull-up resistor RL and is connected up to operate as a Schmitt trigger using the single rail supply Vcc. The feedback resistors Rl and R2 give upper and lower threshold levels VT+, VWxw, respectively. VT+ is easily set by suitable resistor selection but there is little independent choice of VT_ because VT_ cannot exceed VCE(SAX).
Schmitt trigger comparator - schematic

In Fig. 15-13B current-source, comprising the transistors RE, Rb produces a current I ~: (VEB/RE), VEB ( — 0.65 V) being the emitter-base voltage of Ql and Q2. Fig. 15-13C shows the results of a practical test using the circuit of Fig. 15-13B, and the following operating and component data: Vcc = 5 V; RL = 1 ? ohm; Rj = R2 = 10 ? ohm; RB = 3.6 ? ohm; RE = 1 ? ohm + 10 ? ohm pot; Q1 = ZTX500; Q2 = 2TX500.

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