Schmitt trigger comparator

Description: The comparator features an output pull-up resistor (RL) and is configured to function as a Schmitt trigger using a single rail supply (Vcc). The feedback resistors (R1 and R2) establish upper and lower threshold levels (VT+ and VT-), respectively. The value of VT+ can be easily adjusted through appropriate resistor selection; however, there is limited independent control over VT- since it cannot exceed VCE(SAT).

In the provided schematic (Fig. 15-13B), a current source is created using transistors (Q1 and Q2) along with resistors (RE and Rb), generating a current (I) approximately equal to (VEB/RE), where VEB is approximately 0.65 V, the emitter-base voltage of Q1 and Q2. Fig. 15-13C illustrates the results of a practical test conducted with the circuit from Fig. 15-13B, detailing the following operational and component specifications: Vcc = 5 V; RL = 1 ohm; R1 = R2 = 10 ohms; RB = 3.6 ohms; RE = a combination of 1 ohm and a 10 ohm potentiometer; Q1 is a ZTX500 transistor, and Q2 is a 2TX500 transistor.

The described circuit operates as a Schmitt trigger, which is a type of comparator with hysteresis. This configuration is particularly useful in digital circuits for noise immunity. The pull-up resistor (RL) ensures that the output goes high when the input is above the upper threshold (VT+). The feedback provided by resistors R1 and R2 creates the necessary hysteresis, allowing the circuit to switch between states without bouncing around the threshold levels due to noise.

The choice of components is critical for the performance of this circuit. The transistors (Q1 and Q2) are selected based on their switching characteristics and VCE(SAT) values, which dictate the maximum voltage at which VT- can be set. The resistor values (R1, R2, RL, RE, and RB) must be calculated carefully to achieve the desired threshold levels and current output. For instance, the value of RE can be adjusted using a potentiometer to fine-tune the current source, allowing for precise control over the Schmitt trigger's operation.

This circuit can be applied in various applications, including signal conditioning, waveform shaping, and as a simple on-off control mechanism in digital systems. The design's flexibility in setting the threshold levels makes it suitable for adapting to different input signal characteristics.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).

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.