Post by Electronics Notes by Ian Poole

Understanding Transistor Schmitt Trigger Circuit Design The Schmitt trigger is a very useful circuit that has hysteresis i.e. the switch points from low to high and high to low occur at different voltages. The hysteresis of the Schmitt trigger is particularly useful because it enables circuits that might use a varying signal to switch in a more defined manner. It is less likely to switch up and down as the trigger voltage approaches. The Schmitt trigger is important because this type of electronic circuit design changes its output value at a certain value, but retains this until there is a sufficient reversal of the change to return the output to its original value. The circuit is said to have a hysteresis. As an example, when the input is below a lower threshold the output is low, then as the input moves higher, the output transitions to a higher value, but it then needs to move lower than the first transition point for the output to return to its original low value. It's possible to realise a Schmitt trigger using many approaches - one of these is using a simple two transistor design. This can be useful when one of these circuits needs to be built using discrete components. The circuit can be seen int he diagram. The transistor TR1 acts as a comparator having differential inputs with the base of TR1 providing the inverting input, and the emitter as the non-inverting input. The incoming waveform is applied to the base of TR1 via a resistor to ensure the base current is suitably limited by R1. The output voltage of the voltage divider is applied to the non-inverting input at the emitter of TR1. The voltage at the emitter determines the threshold of the comparator as the base must be more than 0.6 or 0.7 volts above the base to switch. The output from this comparator is then applied to a second transistor, TR2 through the resistor voltage divider network consisting of R4 and R5. It can be seen that the overall circuit consisting of two transistors switches over two aspects. It switches the upper legs of the voltage divider and also changes the threshold of the circuit. It is also possible to consider this Schmitt trigger circuit as a differential amplifier with series positive feedback. However there is also a small amount of negative feedback resulting from the resistor, R3. In order for the circuit to operate satisfactorily R2 ≥ R6. In this way less current flows through R3 from TR1. This results in voltage changes across R3 being much less when TR1 switches than when TR2 switches. It is possible to determine the threshold voltages for the high and low switching. VH = R3 / (R3 + R6) * Vcc The low switching threshold is: VL = R3 / (R3 + R2) * Vcc The Schmitt trigger can be used in many applications including level detection, digital to analogue conversion, line reception and many more areas. #schmitttrigger #circuitdesign #hardwaredesign #electronicsnotes #electronicsengineering #transistor #electroniccomponents